A mobile reference tool for contractors and architects. This mobile field guide is a quick reference to information published in NSA's Slate Roofs Design and Installation Manual.
Nailing: Set the nail in the countersink in the top surface of the slate. Do not over-nail or under-nail. Slate shingles must hang on their nails.
Special Consideration: Gypsum and Concrete Roof Decks: Use smooth shank nails, not ring shank. Stainless steel nails may be needed to avoid bending.
Nails: 11-gauge, smooth shank, copper wire slating nails are used most frequently.
Special Consideration: Gauge: Use 10-gauge nails for larger slates, hardwood roof decks, and where longer nails are required (e.g., hips).
Number of Nails: Use 2 nails per slate.
Special Consideration: 4 Nails: Use 4 nails per slate for slates measuring ≥ 3/4" in thickness and ≥ 20" in length. Place the 2 additional nails 2" above the regular nail holes.
Nail Length: Generally twice the thickness of the slate plus 1".
Special Consideration: Flashings: Do not nail slates through flashings. Move the slating nails up, place one nail above the other on one side of the slate, or use copper wires secured to the roof deck upslope of the flashing.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 51-52. Purchase Slate Roofs here.
Headlap: The vertical length of the upper portion of a slate that is covered by the butt of the slate two courses above. Amount of headlap is determined by roof slope.
Standard Headlap: North American quarries use 3-inches as the standard headlap to calculate the number of pieces of slate in a square (100 square feet of roof area).
Special Consideration: Required Headlap: The amount of headlap is now prescribed by the International Building Code. See figure below for guidance on recommended headlap for various roof slopes.
Exposure: The vertical length of the visible portion of a slate when lapped by the successive course.
Special Consideration: Calculating Exposure: The exposure is used for laying out the roof and chalking/marking horizontal course lines. Exposure depends on headlap, and can be calculated for any slate by subtracting the headlap (H) from the length (L), and dividing by 2.
Exposure = (L - H) ÷ 2
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 10-12, 20-21, 39-42, 54, 56-58, 83-86, 160-163, 229, 244, 247 and 268. Purchase Slate Roofs here.
Offset: The distance between vertical joints in adjacent courses should be minimum 3". This will prevent leaks due to lateral movement of water.
Special Considerations: Maintaining Offset: When laying single width slates, offset is automatically achieved by setting the edge of each slate at the center of the course below; the alternating vertical joints will align up the roof slope. When installing random width slates, offset is maintained as each slate is selected for installation.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 40, 42, 44 and 83. Purchase Slate Roofs here.
Best Practice: Roof Layout and Chalking of lines are critical to the roofs appearance and function.
Special Consideration: Before starting, consider the slate size, pattern or style, the recommended exposure, and head lap. Some chalk is more permanent then others and can stain finished slate; choose a color for the chalk-line that will not leave long-lasting marks.
Best Practice: Check the eave with a string line to determine if it is straight and level - then snap a line for the starter course.
Special Consideration: Most roof eaves will not be completely straight, and adjustments will need to be made over several courses. However a base eave line needs to be made to continue with the layout.
Basic Practice: For standard width slate a vertical line will need to be snapped perpendicular to the eave line, and a second line parallel to the first, using a measurement of 1/2 the width of the slate.
Special Consideration: The 3-4-5 method works well to establish a perpendicular line. Also check that dormer sides and gables are parallel to the vertical layout line. Adjustments may be needed.
Basic Practice: The first course line can be snapped above the starter line.
Special Consideration: Be sure to allow for the overhang needed.
Basic Practice: Horizontal lines can now be marked by using the determined exposure up from the top of the first course line.
Special Consideration: Multiple lines are usually chalked all at once to save time. It's important at this stage to verify that the lines end up parallel with the ridge. Plan the approach for the finishing course. Adjust the layout by increasing the exposure by no more than a 1/4" over several courses.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 58-62. Purchase Slate Roofs here.
Replacement Slates: When replacing broken slates, the replacement slate should match the existing in type, color, weathering characteristics, size, shape, and texture.
Special Consideration: These can be new slate, reclaimed slate, or slates taken from attic stock left over from the original construction.
Repair Steps:
Special Consideration:
The bib typically measures 3" x 8" and is bent into a concave shape to help prevent it from sliding out.
Materials: Copper nails and 16 oz. copper bibs are most commonly used.
Special Consideration: The objective is to use materials with service lives equal to or greater than that of the remaining service life of the slate itself. Thus, on an older roof galvanized nails and sheet metal may be acceptable.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 213-221. Purchase Slate Roofs here.
Replacement Slates: When replacing broken slates, the replacement slate should match the existing in type, color, weathering characteristics, size, shape, and texture.
Special Consideration: These can be new slate, reclaimed slate, or slates taken from an attic stock left over from the original construction.
Repair Steps:
Special Consideration:
Materials: Copper and stainless steel slate hooks are most commonly used.
Special Considerations: Galvanized steel slate hooks tend to rust and stain the roof, but may be appropriate where only a few years of service life are required.
Size: Standard slate hooks are 3" long and are made to accommodate 1/4-inch-thick slate laid with a 2" or 3" headlap.
Special Considerations: Custom made slate hooks can be used to accommodate thicker slates, but it is often more practical to use the nail and bib method instead.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 213-221. Purchase Slate Roofs here.
Purpose: To provide for replacement of flange gutter, built-in gutter, low-sloped roof, replacement of deteriorated decking, etc.
Special Consideration: Be sure to number each slate to make re-installation easier.
Repair Steps - "Saw-Tooth" Method:
Special Consideration:
Alternate Method: If full courses are being removed (instead of using the "Saw-Tooth" method), see the Manual for fastening of top row of slate to avoid skewing.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 220-221. Purchase Slate Roofs here.
The table below provides the number of slate shingles per 100 Square feet of roof area for three different headlaps (see Slate Roof Basics: Headlap and Exposure Notes for required headlap).
In North America, slate is sold by the square (quantity of slate required to cover 100 square feet of roof area), assuming a 3" headlap.
To calculate the number of squares of slate required at a 2" or 4" headlap, use the following formula: roof area in square feet / 100 x pieces per 100 sq ft at a 2" or 4" headlap from the chart / pieces per square (i.e., at a 3" headlap) from the chart.
| Pieces per 100 Square Feet | |||
| Headlap | |||
| Size | 3" | 2" | 4" |
| 24 x 14 | 98 | 94 | 103 |
| 24 x 12 | 115 | 109 | 120 |
| 22 x 14 | 108 | 103 | 114 |
| 22 x 12 | 126 | 120 | 133 |
| 22 x 11 | 137 | 131 | 146 |
| 20 x 14 | 121 | 114 | 129 |
| 20 x 12 | 141 | 133 | 150 |
| 20 x 11 | 154 | 146 | 164 |
| 20 x 10 | 170 | 160 | 180 |
| 18 x 14 | 137 | 129 | 147 |
| 18 x 12 | 160 | 150 | 171 |
| 18 x 11 | 175 | 164 | 187 |
| 18 x 10 | 192 | 180 | 206 |
| 18 x 9 | 213 | 200 | 229 |
| 16 x 14 | 158 | 147 | 171 |
| 16 x 12 | 185 | 171 | 200 |
| 16 x 11 | 201 | 187 | 218 |
| 16 x 10 | 222 | 206 | 240 |
| 16 x 9 | 246 | 229 | 267 |
| 16 x 8 | 277 | 257 | 300 |
| 14 x 12 | 218 | 200 | 240 |
| 14 x 11 | 238 | 218 | 262 |
| 14 x 10 | 262 | 240 | 288 |
| 14 x 9 | 290 | 267 | 320 |
| 14 x 8 | 327 | 300 | 360 |
| 14 x 7 | 374 | 343 | 411 |
| 12 x 10 | 320 | 288 | 360 |
| 12 x 9 | 355 | 320 | 400 |
| 12 x 8 | 400 | 360 | 450 |
| 12 x 7 | 457 | 411 | 514 |
| 12 x 6 | 534 | 480 | 600 |
| 10 x 8 | 516 | 450 | 600 |
| 10 x 7 | 588 | 514 | 686 |
| 10 x 6 | 685 | 600 | 800 |
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 10-15. Purchase Slate Roofs here.
Purpose: Ice dam protection membranes help protect the roof system from water infiltration due to ice damming. They are generally required by building codes where the average daily temperature in January is 25° F or less, or where there is a possibility of ice damming.
Special Consideration: Covering the ice dam protection membrane with roof underlayment (e.g., #30 felt) will help protect the membrane from UV degradation and prevent the slate shingles from sticking to the membrane, which can damage the membrane and make future slate repair work difficult.
Materials: Ice dam protection membrane should be a minimum of 40 mils thick and comply with ASTM D 1970, Standard Specification for Self-Adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection.
It may be necessary to prime the roof deck with a manufacturer supplied / recommended primer to help ensure proper bond, especially in cold weather.
Special Considerations: High-temperature (HT) grades of membrane are recommended for use below metals, such as valleys, gutter liners, and gussets.
Installation: Ice dam protection membranes typically extend up-slope a minimum of 2'-0" beyond the interior face of the exterior wall, measured in a horizontal plane, or as required by code. End laps are typically 4" to 6". Side laps 3" to 4" in the direction to shed water. Refer to manufacturer's instructions.
For added protection, turn the membrane down the fascia, behind the metal drip edge.
At vertical walls, ice dam protection membrane turns up the wall a minimum of 4" to 8" and laps under the wall underlayment system.
Special Consideration: Self-adhering polymer modified bitumen sheets are vapor retarders. Use caution and be aware of the risks of unintended condensation issues when using ice dam protection membranes beyond the eave.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 66, 69, 71-72, and 113. Purchase Slate Roofs here.
Purpose: Underlayments protect the roof during installation, handle water that may migrate under the slate and flashings, cushion the slate, smooth out the roof deck, and are required by building codes.
Special Consideration: See Ice Dam Protection Membranes for description and locations where climate conditions warrant and building codes require them.
Materials: Various asphalt-saturated and asphalt coated felts with organic or inorganic reinforcing mats have been used for many years. Consider their wrinkling & aging characteristics.
Some polymer modified bitumen base sheets reinforced with fiberglass or polyester mat may be used for good weathering, aging and strength attributes. These do not currently comply with the International Building Code as they do not meet ASTM D 226 or ASTM D 4869 (the ASTM standards for felt underlayments). Check for ICC-ES Evaluation reports, use in conjunction with code-compliant underlayments, and/or verify their use with code officials having jurisdiction before using modified bitumen base sheets.
Self-adhering polymer modified bitumen sheets, when adhered to the deck, are vapor retarders and waterproof the roof deck. The sheets may become permanently adhered to wood and concrete roof decks. To avoid having the heads of the slates sink-in and stick to the sheets, making future slate repair more difficult, consider using a felt or synthetic cap sheet over the self-adhering polymer modified bitumen sheet.
Some synthetic and composite sheets of polyethylene, polypropylene, recycled rubber and similar components are being evaluated by building code authorities for acceptance as alternative underlayments. They have various strengths, weathering, and vapor and air transmission qualities. Synthetic sheets currently need an ICC-ES Evaluation Report and need to be used as described in the Report to be code compliant.
Underlayment materials are evolving.
Special Considerations: For some buildings the vapor permeability of the underlayments is of concern and referring to the designer for guidelines is recommended.
Single or Double Layer: A single layer of #30 felt underlayment is the minimum recommendation for slate shingles. A double layer of #30 felt is recommended for roof slopes between 4:12 and 8:12, for slates measuring 3/8" or more in thickness, and for slates smaller than about 12" x 9".
Other combinations are possible; see, for example, The NRCA Roofing Manual, Steep-Slope Roof Systems, 2013.
Methods: The materials and methods should reflect the design life of the roof, the construction sequence and the risks of water getting under the slate shingles and flashings.
Lap sizes, fastener types and spacing, and critical area details should take into account roof slope, climate, water amounts, wind speeds, building code, and manufacturer's requirements.
Felt underlayments are typically secured with galvanized nails or nails and caps in sufficient quantity to prevent blow-off. They are laid with 6" end laps which should be staggered a minimum of 36".
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 64-69. Purchase Slate Roofs here.
Woven Valley Underlayment: Whether a single or double layer of underlayment is to be woven across the valley depends on the number of layers of underlayment being installed in the field of the roof (see Underlayments: Single or Double Layer).
The underlayments must cross the valley and extend far enough up the opposite slope such that their lower outside corner is completely covered by the next course of underlayment coming from the opposite direction.
Special Consideration: If ice dam protection membrane is being installed at the roof eave (see Ice Dam Protection Membranes), the membrane should be installed in the valley at least to the same height as that at the eave.
With woven valley underlayments, it is not possible to lap the roof underlayment on top of the metal valley flashing or to strip-in the edges of the valley flashing with ice dam protection membrane.
Overlapping Valley Underlayment: In this type of system, a single or double layer of underlayment is installed longitudinally, at the center of the valley. After the open valley flashing is installed, roof underlayments on the roof slopes adjacent to the valley are laid to lap on top of the valley flashing and trimmed to just above where the slate line will be.
The top corner of each course of underlayment is typically trimmed to help direct stray water toward the center of the valley.
Special Considerations: Overlapping valley underlayment is not recommended for closed valleys.
As with woven valleys, ice dam protection membrane may be placed the full length of the valley, or just at the eave. For added protection in severe climates, the edges of the open valley flashing may be covered with a strip of ice dam protection membrane. The membrane should be adhered directly to the valley flashing and to a width of similar underlying material laid at the center of the valley.
Materials: See Underlayments and Ice dam Protection Membrane.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 76-78. Purchase Slate Roofs here.
Purpose: The butt end of each slate is raised by the course below to create the field angle of the slates. Therefore an eave cant is needed under the starter course slate to raise the butt end to the same angle as the other courses.
Special Consideration: Each course should conform to the field angle of the course below to give support and strength to the slate and minimize infiltration of wind-blown rain and snow.
Material: The cant is wood lath, beveled wood strip, formed metal or other method of raising the starter course.
Special Consideration: Drainage: Allow for incidental drainage of water from behind the cant.
Location: The cant is best located under the eave flashing, drip edge or gutter apron.
Special Consideration: If the cant is placed on top of the underlayment, 4' lengths with gaps between will allow for drainage.
Thickness: The cant thickness is similar to the thickness of the first course of slate.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 70-75. Purchase Slate Roofs here.
Required: Ice dam protection membranes are used at the eave as required by building code, and anywhere ice and snow dams can occur. They are usually self-adhering modified bitumen membranes.
Special Consideration: Location: Ice dam protection membranes are usually installed to at least 24" to 36" inside of the heated wall line.
Installation: Ice dam protection membranes are installed according to the manufacturers’ instructions. Some are adhered directly to the roof and onto the fascia to be covered by the drip edge. Some are adhered to the top of the drip edge or gutter apron. Some are then covered with the loose laid field underlayment.
Stripping In: A strip about 12" wide may be used to strip in the metal drip edge where the drip edge is laid on top of the ice dam protection membrane.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 70-75. Purchase Slate Roofs here.
Purpose: The drip edge helps guide water shedding off the edge of the roof. It protects and creates a neat edge for roof deck or fascia.
Special Consideration: Required: A drip edge may be a building code requirement, and is needed where the roof deck edge is plywood or end grain lumber and where the deck edge is irregular or gapped.
Material: Corrosion resistant metal is used.
Installation: The metal is usually side lapped 3" and fastened at the top edge every 12" to 24". The underlayments or ice dam protection membrane should shed water on top of the drip edge.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 70-75. Purchase Slate Roofs here.
Purpose: Underlayments help shed water beyond the roof eave edge. Underlayments are required by building code.
Special Consideration: Quality underlayments should be used with lower roof slopes and wetter roof areas. A long term service life for the underlayment is needed for a reliable weather-tight roof system.
Material: Asphalt saturated felts, polymer modified bitumen sheets, and synthetic sheets are used.
Special Consideration: Drainage: See "Underlayments" section of this Field Guide.
Installation: The underlayment should shed water on top of the eave drip edge metal, gutter apron or other eave flashing.
Various Methods: Single ply, multiple ply, and adhered membranes are used.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 70-75. Purchase Slate Roofs here.
Beginning: Installation of slate shingles begins with laying the starter course on top of cant, drip edge and underlayment. The starter course is then covered with the first course, resulting in 2 layers of slate shingles at the eave edge.
Special Consideration: The starter slate sheds water from the side joints of the first course. The starter course along with the cant creates the field angle of the slate shingles.
Size: The minimum vertical dimension of the starter course is the exposed length of the first course plus the appropriate headlap of 3" or 4". The thickness is the same thickness as the first course.
Installation: The underlayment should shed water on top of the eave drip edge metal, gutter apron or other eave flashing.
Special Consideration: Starter slates are laid with the beveled edge up or down depending on local practice.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 70-75. Purchase Slate Roofs here.
Gutter Types: A half-round hanging gutter is illustrated in the Detail. For other gutter types, see pages 89-91 and 93-96 of the Manual.
Gutter Hangers: Hangers (shanks and circles) should be spaced between 16" and 36" on center, depending on anticipated snow and ice loads.
Special Consideration: In severe snow climates, hangers should be fastened to the roof rafters rather than the roof deck for added strength.
Hanger Installation: Shanks are secured to wood roof decks with a minimum of 2 screws of compatible metal. Cants for the slate shingles are set between the shanks. Circles are secured to the shanks with round head machine screws and nuts.
Special Consideration: Added insurance against rotation of the circles can be had by drilling out the next best pair of aligned holes in the circle and shank, and then inserting a second screw and nut.
Drip Edge: The drip edge flashing should extend up the roof slope 3" to 4" and be secured with large head roofing nails.
Gutter Installation: Minimum slope is generally between 1/16" and 1/8" per foot and usually not less than 1" in 20 feet.
The outer edge of the gutter should be set at, or just below, the plane of the roof slope to help avoid damage during snow slides.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 88, 89, and 92. Purchase Slate Roofs here.
Purpose: The rake edge of a slate roof is built to protect the underlayment, metal flashing, roof deck, rafter, and trim boards.
Special Consideration: The edge of the slate shingles should be straight.
Overhang: A 1-inch overhang is typical.
Special Consideration: More overhang results in more wind uplift pressure on the rake edge shingles but provides more protection. Heavier slate can be extended further.
Flashing: Metal rake edge flashing or wood molding is used to cover the edge of the roof deck. Non-corrosive metal, 1 1⁄2 inches by 3 inches is typical.
Special Consideration:
Slate Width: A minimum width of 6 inches is used.
Special Consideration: On a uniform width roof an accurate layout is needed at the right side and left side rake edges so the vertical joints are at the mid-point of the slate in the course below and the vertical joints are in a straight line through the full rafter length. Adjustments may have to be made to the widths of the slates as the rake edge is approached to maintain the 6 inch minimum width in the rake edge slate. See approach slate methods.
Nailing: The location of the rake side nail is moved because of the slate overhang, the metal rake edge flashing or the stability and holding power of the nailing deck.
Special Consideration: Sealant can be used to additionally secure the rake edge slate.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 79-81. Purchase Slate Roofs here.
Purpose: Several methods can be used to adjust the width of slate as they approach a rake edge, hip, valley and other edges.
Special Considerations: The recommended minimum width for the edge slate and the minimum offset of 3 inches for the vertical joints are maintained.
Random Width Methods: In a random width slate roof, wider and narrower slates are used in each course to maintain the minimum width and offset as the slate approach the edge.
Uniform Width Methods: In a uniform width slate roof, wider or narrower slates are also used, except that the vertical joints are kept near the mid-point in the slate below and are maintained in a straight line. See Drawings for a few examples.
Another method is to slightly adjust the width of the slate over several feet by trimming up to 1/4 inch off a series of slate or using slightly wider slates.
Slight but uniform adjustments to the width of the gap in the vertical joints can also help. Tight to 3/8 inch gaps can be used. The slight adjustment should not be noticeable from a distance.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, page 79. Purchase Slate Roofs here.
Appearance: To create a uniform slate roof by utilizing slates of a single length, width and thickness that maintain horizontal course and alternating vertical joint (bond) lines.
Special Consideration:
Jointing: Adjacent slates in a course may be laid slightly touching or with a small gap ±1⁄4 inch to enhance aesthetics, ensure adequate slate widths at details, or to accommodate offset of vertical joints.
Materials: Slates of one length, width and thickness. eg: 18 inch long x 10 inch wide x 1⁄4 inch thick (wider slates can be used at valleys & hips).
Details: Try to avoid using slates narrower than 6" at rakes and other details. This can be achieved by subtly increasing the joint spacing between slates or by employing slightly narrower slates in courses approaching the detail, or by installing wider slates at the detail. In all cases, attention should be given to maintaining the required offset and alternating vertical joint lines. Where unavoidable, narrow slates may be bedded in sealant to prevent blow off.
Additional Information: See Approach Slate Methods.
Layout: The field should be laid out to both horizontal course chalk lines and frequent vertical joint chalk lines. Layout should accommodate the appropriate exposed length and width of slates in each course and maintain alternating vertical joint lines at rakes, valleys, walls, dormers, roof penetrations and other roof details.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 10-12, 20-21, 39-42, 54, 56-58, 83-86, 160-163, 229, 244, 247 and 268. Purchase Slate Roofs here.
Appearance: A random width slate roof utilizes slates in a variety of widths that maintain horizontal course lines and random vertical joint (bond) lines.
Special Considerations:
Jointing: Adjacent slates in a course may be laid tight or with a small gap ±1⁄4 inch to enhance aesthetics, ensure adequate slate widths at details, or to accommodate offset of vertical joints.
Materials: Slates ranging in width from 1⁄2 the slate’s length and up to 14 inches wide, e.g., 18 inch long x 9, 10, 11, 12 and 14 inch wide. A random width roof can consist of as little as two or as many as seven widths.
Details: Try to avoid using slates narrower than 6” at rakes and other details. See Notes under Uniform Width Slate Methods.
Layout: The field should be laid out to horizontal course chalk lines dictated by the appropriate exposed length of slates in each course. The minimum offset of 3 inches should be maintained as slates are laid in each course, while also minimizing the vertical alignment of joints with nearby slates in courses below.
Blending: The number of pieces of slate of each width available is used to determine the blend of widths used as the slate is installed. This serves to best utilize the slate available, while creating an aesthetically pleasing roof and avoiding repetitive patterns
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 44, 56-60, 83-84, 86, and 249. Purchase Slate Roofs here.
Appearance: A graduated slate roof traditionally utilizes slates of a variety of sizes to create a roof of diminishing slate lengths, widths and thicknesses.Courses of longer/thicker/wider slates are installed at the eaves, with shorter/thinner/narrower slates placed in courses as installation progresses up slope. See manual for other variations of graduated slate methods.
Special Considerations:
Jointing: Adjacent slates in a course may be laid tight or with a small gap (approx. ±1⁄4 inch) to enhance aesthetics, ensure adequate slate widths at details, or to accommodate alignment of vertical bond lines.
Materials: Depending on roof size and design, slates in three or more consecutive standard slate lengths and of random widths are employed. Thicknesses can commonly range from 1" to 3/16" in at least three standard thickness graduations.
Details: Try to avoid using slates narrower than 6" at rakes and other details. See Notes under Uniform Width Slate Methods.
Layout: The field should be laid out to horizontal course lines in the number of courses as determined by the available quantity of each length and thickness of slate.3-inch minimum offset should be maintained as slates are laid in each course, while also minimizing the vertical alignment of joints with nearby slates in courses below.
It is good practice to gradually shorten the exposure (example by 1⁄4 inch) in a few courses as a length change is approached to provide a smooth transition between slate lengths.
Blending: The number of pieces of slate of each length, width and thickness delivered to the site is used to determine the roof layout and the blend of sizes used as slate is installed. This serves to best utilize the slate available, while creating an aesthetically pleasing roof and avoiding repetitive patterns.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 45-47, 83-84, and 87. Purchase Slate Roofs here.
Valley Underlayments: See Woven Valley Underlayment and Overlapping Valley Underlayment.
Valley Flashing Width: A common width for closed valley flashings is 18" (9" each side of valley centerline). In addition to rectangular flashings, chevron and diamond shaped flashings may also be used.
Special Considerations: Consider using wider valley flashings when the adjacent roof slopes are not very steep. Consider increasing the width of the flashings near the bottom end of the valley should the valley be very long or snow loads severe.
Valley Flashing Length: As a general rule, the length of each flashing is equal to the exposure of the slate, plus the headlap, plus another 1 to 2 inches. Each flashing should be long enough to lap the flashing below at least 3" (more if slope and climate conditions warrant).
Installation: Closed valley flashings are interwoven with each course of slate. The butt ends of the slates in each course on either side of the valley centerline should align. The upper corner of the slate shingle laid along the valley line is often clipped to allow the slate to lay correctly and to direct moisture toward the valley centerline.
Special Consideration: See Slate Roofs, 2010 Edition, for aligning the butt ends of the slates when the roof slopes either side of the valley centerline are different.
Slate Nailing: Avoid nailing thru the valley flashings. Consider the following techniques:
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 97-100. Purchase Slate Roofs here.
Underlayments: The roof underlayment may be woven at the valley or laid as a single strip centered on the valley. See Woven Valley Underlayment and Overlapping Valley Underlayment.
Special Considerations: Use ice dam protection membrane directly on the roof deck where site and climate conditions warrant. Lay open valley flashings greater than 8- to 10-feet in length on rosin paper to help prevent sticking to the roof underlayment.
Valley Width: Minimum valley pan width is 16"; 4" exposed and 4" below the slate shingles each side of the valley centerline.
Special Considerations: Local climate, roof slope(s), adjacent tree canopy, total water volume, and valley length must all be considered and may warrant increasing the valley width.
Slating: Do not puncture the valley flashing with the slating nails. Use wider slates adjacent to valleys, place two nails on the upslope side of the slate, or hang the slates from copper wires to help with nailing away from the valley flashing.
Special Consideration:
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 97-102. Purchase Slate Roofs here.
Layout: The detail drawing shows a typical pan layout at the peak of an open valley. Other layouts are possible.
Special Considerations: If a metal ridge flashing is used, it can be used to flash the peak of the valley as well (See Detail 14-B in the Manual.).
Seam Type: A lapped, riveted, and soldered seam is shown as this may be the easier to form and solder.
Special Considerations: Valley peak flashings are usually fabricated in the field as the angles must be precise in order for the flashing to lay flat on the roof deck.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 99 and 102. Purchase Slate Roofs here.
Underlayment: Roof underlayment should turn up the wall.
Special Consideration: Wall underlayment should lap over base-flashings. Siding or cap-flashing must lap over base-flashings, and should end ± 1" above roof surface.
Placement: Interweave step-flashings with each course of slate.
Size: Minimum step-flashing size is 4" x 4". The length is the exposure + headlap + extra 1"-2" for nailing.
Special Consideration: Increase size of step-flashing to accommodate thicker slates or lower-sloped roofs.
Nailing: Fasten step-flashing in outer corner, away from the wall.
Special Consideration: Do not nail slates through step-flashings. If needed, place 2 nails at the edge of the slate away from wall.
First step-flashing: First step-flashing should be under the starter slate, and should "kick-out" to avoid directing water behind siding.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 108-109, and 113. Purchase Slate Roofs here.
Purpose: The purpose of the side wall diverter/kick-out flashing is to divert rainwater to the outside of the wall covering and/or into the gutter where a roof eave terminates at a vertical wall. The diverter helps to prevent rainwater from traveling behind the wall covering or from running down the exterior face of the wall.
Diverter Type: The type of diverter flashing employed depends largely on the type of construction at the side wall. For any given side wall construction, several diverter flashing variations are likely possible. One type of diverter flashing for clapboard siding is shown in the detail drawing. See the Manual for other options and other wall constructions.
Flashing Length: It is important that the bottom base (step) flashing extends to, or slightly beyond, the slate roof edge so that moisture will not enter behind the flashing. This typically can be achieved by forming the diverter flashing from a longer length of base flashing.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 111-112, and 120. Purchase Slate Roofs here.
Basic Construction: Metal counterflashings cover, or cap, the top of the vertical legs of the base flashings and are integrated into the wall covering or wall surface. The detail shown is for a unit masonry wall and is the ideal manner of installation, especially for new construction.
Special Consideration: Wall coverings can sometimes serve as the counterflashing.
Lap: Counterflashings typically lap each other 3 inches. In the area of lap, fill the joint into which the counterflashing is set completely with mortar.
Special Consideration: It is prudent to lap counterflashings more than 3 inches when the height of the individual counterflashings exceeds 10 to 12 inches and where the climate demands.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 109-111, 114. Purchase Slate Roofs here.
Basic Construction: Metal counterflashings cover, or cap, the top of the vertical legs of the base flashings and are integrated into the wall covering or wall surface. The detail shown is for a unit masonry wall and is commonly used in repair and replacement work where the wall is already built and no thru-wall flashing is present. The detail relies on the integrity of the masonry wall and its ability to shed moisture.
Special Consideration: Wall coverings can sometimes serve as the counterflashing.
Raggles: Raggles (the joints or saw-cuts into which the counterflashings are set) should be a minimum of 1-1/2 inches deep. Raggles may be filled with mortar, backer rod and sealant, or mortar, followed by backer rod and sealant.
Special Consideration: If sealant is used, weep holes should be provided as required to allow moisture to drain from the joint.
Special Consideration: The raggle flange of the counterflashing should be completed and all seams soldered watertight at inside and outside corners.
Lap: Counterflashings typically lap each other 3 inches. In the area of lap, the raggle should be completely filled.
Special Consideration: It is prudent to lap counterflashings more than 3 inches when the height of the individual counterflashings exceeds 10 to 12 inches and where the climate demands.
Method of Securing: Lead wedges, spaced at 8 to 12 inches on center (2 per counterflashing, minimum) are typically used to secure the counterflashings in the raggle. Set the lead wedges back from the face of the joint a sufficient depth to allow for adequate coverage by the mortar/sealant used to fill the raggle.
Reglets: Reglets (receivers) may be used in combination with counterflashings. The primary advantage of reglets is that the portion of the reglet within the wall does not need to be removed in order to replace the base and counterflashings. For different types of receivers, see the Manual.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 110-111, 122. Purchase Slate Roofs here.
Purpose: Provide a watertight interface where the top edge of a sloped roof meets a wall. Intercept water running down face of the wall as well as water that penetrates behind the wall cladding.
Special Consideration: Exposure: As the field courses approach the headwall, the exposure of each course of slate can be reduced slightly to result in a straight appearance at the wall and the finishing course having approximately the same exposure as the field of the roof.
Laps: Adjacent lengths of apron flashing are typically overlapped 4"-6" without rivets or solder.
Special Consideration: Finishing Course: The finishing course can be set vertically or horizontally.
Headlap: Lap the apron over the slates sufficiently to maintain the required headlap.
Special Consideration: Counterflashing: If there is no metal counterflashing in the wall, the wall cladding can lap over the vertical leg of the apron to serve as the counterflashing. Siding should terminate approximately 3/4"above the roof surface.
Protection From Wind-Driven Rain: The vertical leg of the apron should be lapped by the wall underlayment 2" minimum.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 123-130. Purchase Slate Roofs here.
Purpose: Provide a watertight transition where the apron turns a corner; typical at dormers, skylights, chimneys, etc.
Underlayment: Underlayment (building wrap) at wall must lap over the top of the apron.
Special Consideration: Variations: there are other acceptable techniques of forming a corner, but overlap and soldering must be achieved as shown.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 123-130. Purchase Slate Roofs here.
New Construction: In new construction, the height below the window sill should be minimum of 4". The flashing of the rough opening should include end-dams and soldered joints.
Creating a pocket window can provide additional flashing height if needed (see Slate Roofs pages 139-141).
Rehabilitation Work: If the flashing cannot be installed beneath the sill, the sill may be clad with metal. Cladding should be absolutely watertight to prevent rotting of wood sill.
Additional Information: For related information, see "Headwall Flashing" and "Apron Outside Corner".
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 131-135. Purchase Slate Roofs here.
Basic Construction: Metal vent pipe boots are generally more durable than pre-manufactured neoprene boots, as the neoprene tends to become brittle over time. Neoprene boots are not recommended for use on slate roofs.
Base Plate Size: The bottom edge of the base plate should lap over the slates below sufficiently to maintain the required headlap. The sides of the base plate should extend below the slate to maintain the required offset and provide for a 1/2" to 1"-wide drainage channel. The upper edge of the base plate should extend upslope far enough to be lapped by the slates at least 6".
Special Consideration: When installing the slate shingles, place the nails to avoid puncturing the base plate.
Base Plate - Sleeve Connection: To help ensure the connection remains watertight, the seam should be thoroughly soldered.
Special Consideration: For added protection against condensation and leakage, a self-adhering membrane target patch can be installed around the base of the pipe prior to installation of the roof underlayment.
Caps: Copper or lead caps can be snipped and soldered, or crimpled, and rolled into the pipe. Copper and stainless steel caps are often fabricated from 3 separate pieces—an inner flange, an outer flange, and a top—soldered together.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 182-185. Purchase Slate Roofs here.
Basic Construction: A one-piece metal cricket flashing is illustrated in the detail drawing. A two-piece metal cricket, with one soldered seam at the ridge of the cricket, is also common.
Special Consideration: If ice-damming is anticipated, snow guards can be installed to prevent sliding snow from building up in the cricket.
Drainage Paths: Slates located adjacent to the top outside corners of the chimney should be trimmed to permit water to drain out of the cricket. The width of the drainage paths - typically not less than about 2" - should be adequate to handle the anticipated flow of rainwater from the roof area above.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 188-189. Purchase Slate Roofs here.
Removing Existing Slates: If replacing an existing low-sloped roof, see "Removing Slates at Eave" for method of removing slates located above the low-slope roof.
Flashing Height: Membrane should extend up the slope as needed to protect against build-up of sliding snow, or other factors.
Manufacturer Requirements: When installing low-sloped membrane, comply with manufacturer instructions for base tie-in and top termination.
Nailing: When installing slates above the low-slope roof, slating nails should be placed to avoid puncturing the membrane. The can-strip may be hung with copper wire which is also nailed above the top edge of the membrane.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 196-199. Purchase Slate Roofs here.
Maintaining Headlap: The metal edging at the low-sloped roof should lap over the slate to maintain proper exposure and headlap (similar to "Metal Ridge Detail").
Special Considerations: Run-off from the low-sloped roof may cause staining and discoloration of the slates. This can be avoided by sloping the low-slope roof to interior drains.
Manufacturer Requirements: Comply with low-slope membrane manufacturer's requirements for tie-in of metal edging to the membrane.
Also, comply with applicable requirements for wind-uplift.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 177. Purchase Slate Roofs here.
Size: In general, the long dimension of a saddle hip slate should be about twice the exposure of the field slates and the short dimension about equal to the exposure of the field slates, rounded up to the next standard slate size.
Special Considerations: For a neater appearance and tighter joint, the long edge of the hip slate is trimmed to create a reverse bevel, allowing the pairs of hip slates to come together at the hip apex in a miter.
Installation: A properly detailed and installed hip starts with planning the approach slates to the hip.
Hip lines are typically highly visible. Work to snapped chalk lines and keep the apex of the hip formed by the hip slates true and straight.
Special Consideration: When possible, saddle hip slates are laid so that their butt ends align with the butts of the field slate courses.
Flashing: Soft metal hip flashings are typically interwoven with each pair of hip slates in a course in order to provide for a more watertight joint at the apex of the hip.
Special Consideration: On roof slopes between 4:12 and 7:12 it is often beneficial to incorporate flashings into the hip slates.
Nailing: Saddle hip slates are secured with at least 3 nails each, set as far apart as possible in a triangular pattern. The butts of the hip slates may also be bedded in dabs of adhesive or trowel grade flashing cement. Longer nails than used for the field slates are often required for hip slates in order to achieve adequate penetration into the substrate.
Special Consideration: If 4 nails are used, they should be set as far apart as possible in a diamond or zigzag pattern. Consider using 5 or more nails per slate for larger and thicker slates.
Nailers: Wood nailers provide support for the hip slates in an even plane, flush with the top surface of the field slates. The use of dry, high-quality wood nailers at hips will help prevent shrinkage, twisting, and warping of the nailers.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 142-145, 148-149. Purchase Slate Roofs here.
Size: The length of a mitered hip slate should be equal to the length of the field slates. The width of a mitered hip slate (and associated approach slates) should be as wide as possible to permit adequate space for nailing and to avoid small triangular slates.
Special Considerations: In finer work, the long edge of the hip slate is trimmed to create a reverse bevel, allowing the pairs of hip slates to come together at the hip apex in a miter.
Installation: A properly detailed and installed hip takes time and patience to install, and starts with planning the approach slates to the hip.
Mitered hip slates are trimmed to conform to the apex of the hip and laid in the same plane as, and with their butts aligned with, the field slates.
Hip lines are typically highly visible. Work to snapped chalk lines to ensure alignment of field courses on both side of the hip. Keep the apex of the hip formed by the hip slates true and straight.
Special Consideration: It is common practice to install mitered hips prior to running out the field slates on each side of the hip.
Flashing: Soft metal hip flashings can be interwoven with each pair of hip slates in a course in order to provide for a more watertight joint at the apex of the hip.
Special Consideration: On roof slopes between 4:12 and 7:12 it is often best to incorporate flashings into the hip slates.
Nailing: Mitered hip slates are secured with at least 3 nails each, set as far apart as possible in a triangular pattern. The butts of the hip slates may also be bedded in dabs of adhesive or trowel grade flashing cement (this tends to work best when flashings are not interwoven with the hip slates). In all cases, nails should be so placed as to be covered by the next course.
Special Consideration: If 4 nails are used, they should be set in an elongated diamond or zigzag pattern. Consider using 5 or more nails per slate for larger and thicker slates.
Special Consideration: Where exposed nail heads are unavoidable, they should be covered with sealant.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 142-146, 150-151. Purchase Slate Roofs here.
Size: In general, the long dimension of a saddle ridge slate is about twice the exposure of the field slates (but this can be varied to achieve different aesthetics), and the short dimension sufficient to maintain the required headlap over the second-to-last field slate. Both dimensions are typically rounded up to the next standard slate size.
Special Considerations: A saddle ridge will blend well with a saddle hip, but may not provide the desired appearance when combined with other types of hip treatments.
Special Considerations: For a neater appearance and a tighter joint, the long edge of the ridge slate is trimmed to create a reverse bevel, allowing the pairs of ridge slates to come together at the ridge apex in a miter.
Installation: A properly detailed and installed ridge starts with planning the approach slates to the ridge. Field slate below the ridge should be laid-out such that the exposure of the last, or finishing, course equals, or is slightly less than, that of adjacent field courses.
Saddle ridges are typically laid with an exposure equal to half of their length. Headlap is not required. As installation of the ridge approaches an end point, adjustments (slight decreases) in the exposures of several pairs of ridge slates can be made to ensure that the last pair of ridge slates has the desired exposure.
Special Consideration: Work to snapped chalk lines and keep the apex of the ridge formed by the ridge slates true and straight.
Flashing: Soft metal flashings are typically interwoven with each pair of ridge slates in order to provide for a more watertight joint at the apex of the ridge. Alternatively, concealed flashings can be omitted and the ridge slates set in a 2-inch wide band of flashing cement.
Special Consideration: On roof slopes between 4:12 and 7:12 it is often beneficial to incorporate flashings into the ridge slates.
Nailing: Ridge slates are normally supplied unpunched to allow for custom punching in the field. Saddle ridge slates are secured with at least 3 nails each, set as far apart as possible in a triangular pattern. The butts of the ridge slates may also be bedded in dabs of adhesive or trowel grade flashing cement. Longer nails than used for the field slates are often required for ridge slates in order to achieve adequate penetration into the substrate.
Special Consideration: If 4 nails are used, they should be set as far apart as possible in a diamond or zigzag pattern. Consider using 5 or more nails per slate for larger and thicker slates.
Nailers: Wood nailers provide support for the ridge slates in an even plane, flush with the top surface of the field slates. The use of dry, high-quality wood nailers at ridges will help prevent shrinkage, twisting, and warping of the nailers.
A wood field nailer is needed when the length of the finishing course of slate is significantly less than the length of the last full-length course below. The field nailer supports the head of the slate so that when it is fastened, its butt end will not stick up.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 158-163, 166-167. Purchase Slate Roofs here.
Construction: Metal ridge and hip flashings are most often constructed of 16 oz. cold-rolled copper, although other metals could also be used.
The roof flange of these flashings must be securely fastened to prevent blow-off. Beyond the minimum width to achieve the required headlap, the width of the roof flange of ridge and hip flashings is largely governed by aesthetics and the flashing's ability to resist wind uplift. It is also critical that the roof flange be wide enough to cover the heads of the nails used to secure the slates located adjacent to the ridge and hips.
Special Considerations: A roll at the apex of the ridge/hip helps to reduce the apparent overall size of the flashing and provides a place to secure the flashing that is less susceptible to water infiltration.
Approach Slates at Ridge: The exposure of the finishing (top) course of field slates should be as close as possible to the exposure of the field slates, or perhaps 1” to 4” less (how much less depends on the size and exposure of the field slates).
The exposure of the finishing slates is determined by a well-planned approach to the ridge – adjusting (decreasing) the exposure of the field slates slightly, over several courses.
Special Consideration: If the second-to-last course is near the length of a full slate, it may be possible to omit the wood field nailer shown in the detail drawing.
Clamps: Clamps should be rigid (1/8” thick x 1” wide clamps are common) and constructed of the same metal as the ridge/hip flashing or a galvanically compatible metal.
Screws used to secure the clamps should be the same metal as the clamps, or a compatible metal. Holes for the screws in the flashing should be slightly oversized to permit thermal movement. Washers should be placed between the screw heads and flashing to help prevent tear-out.
Clamps should be equally spaced along each 8- to 10-foot length of ridge/hip flashing. Clamps should not be placed at the laps between adjacent lengths of flashing in order to better accommodate thermal movement.
Special Consideration: If required, expansion clips can be placed at transverse seams to help seat the overlying and underlying flashings to each other.
Ridge/Hip Interface: Where metal ridge and hip flashings intersect, they should be mitered, riveted, and soldered watertight. To relieve some of the stress on the seams, the length of the ridge flashing and each of the hip flashings emanating from the point of intersection may be kept to 3 or 4 feet.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 174-181. Purchase Slate Roofs here.
The National Slate Association, its officers, directors and committee members, assume no responsibility whatsoever for the application, or misapplication, modification, or adaptation of the information contained herein for the design, specification, or construction of any specific roof system or portion thereof, or for any other purpose. The National Slate Association expressly disclaims all liability for damages of any sort, whether direct, indirect, or consequential, arising out of the use, reference to, or reliance on this Mobile Field Guide or any of its contents. The National Slate Association makes no warranty, express or implied, as to any particular roof system or this Mobile Field Guide and specifically disclaims any warranties of merchantability or fitness for a particular purpose.
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