A field tool for contractors and architects, this mobile 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.
Nails: 11-gauge, smooth shank, copper wire slating nails are used most frequently.
Number of Nails: Use 2 nails per slate.
Nail Length: Generally twice the thickness of the slate plus 1”.
Gypsum and Concrete Roof Decks: Use smooth shank nails, not ring shank. Stainless steel nails may be needed to avoid bending.
Gauge: Use 10-gauge nails for larger slates, hardwood roof decks, and where longer nails are required (e.g., hips).
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.
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.
Hundreds of years of experience has shown that the method fastening the slate can make a signifi- cant difference in the reliability and longevity of the roof. We offer several suggestions 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).
Exposure: The vertical length of the visible portion of a slate when lapped by the successive course.
Required Headlap: The amount of headlap is now prescribed by the International Building Code. See figure below for guidance on recommended headlaps for various roof slopes.
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.
Offset: The distance between vertical joints in adjacent courses should be minimum 3”. This will prevent leaks due to lateral movement of water.
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.
Hundreds of years of experience has shown that the method fastening the slate can make a signifi- cant difference in the reliability and longevity of the roof. We offer several suggestions here.
Replacement Slates: When replacing broken slates, the replacement slate should match the existing in type, color, weathering characteristics, size, shape, and texture.
Repair Steps:
Materials: Copper nails and 16 oz. copper bibs are most commonly used.
Replacement Slates: These can be new slate, reclaimed slate, or slates taken from an attic stock left over from the original construction.
Details:
Materials: 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.
Replacement Slates: When replacing broken slates, the replacement slate should match the existing in type, color, weathering characteristics, size, shape, and texture.
Repair Steps:
Materials: Copper and stainless steel slate hooks are most commonly used.
Size: Standard slate hooks are 3” long and are made to accommodate 1/4-inch-thick slate laid with a 2” or 3” headlap.
Replacement Slates: These can be new slate, reclaimed slate, or slates taken from an attic stock left over from the original construction.
Details:
Materials: 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.
Custom Size: 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.
Hundreds of years of experience has shown that the method fastening the slate can make a signifi- cant difference in the reliability and longevity of the roof. We offer several suggestions 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.
Material: The cant is wood lath, a beveled wood strip, formed metal or other method of raising the starter course.
Location: The cant is best located under the eave flashing, drip edge or gutter apron Thickness: The cant thickness is similar to the thickness of the first course of slate.
Purpose: 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.
Drainage: Allow for incidental drainage of water from behind the cant.
Location: If the cant is placed on top of the underlayment, 4' lengths with gaps between will allow for drainage.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 70-75.
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.
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.
Location: Ice dam protection membranes are usually installed to at least 24” to 36” inside of the heated wall line.
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.
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.
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.
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.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 70-75.
Purpose: Underlayments help shed water beyond the roof eave edge. Underlayments are required by building code.
Material: Asphalt saturated felts, polymer modified bitumen sheets, and synthetic sheets are used. Installation: The underlayment should shed water on top of the eave drip edge metal, gutter apron or other eave flashing.
Quality: 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: See Underlayments, page 7 of this Field Guide.
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.
Beginning: Installation of the 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.
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 side joints of the starter slate offset the side joints of the first course by 3”. Nail holes are located about 1_” from the top edge.
Purpose: 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.
Installation: 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.
Hundreds of years of experience has shown that the method fastening the slate can make a signifi- cant difference in the reliability and longevity of the roof. We offer several suggestions 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.
Laps: Adjacent lengths of apron flashing are typically overlapped 4”-6” without rivets or solder.
Headlap: Lap the apron over the slates sufficiently to maintain the required headlap.
Protection From Wind-Driven Rain: The vertical leg of the apron should be lapped by the wall underlayment 2” minimum.
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.
Finishing Course: The finishing course can be set vertically or horizontally.
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 _” above the roof surface.
For additional information see Slate Roofs: Design and Installation Manual, 2010 Edition, pages 123-130.
Purpose: Provide a watertight transition where the apron turns a corner; typical at dormers, skylights, chimneys, etc.
Joints: Flashing corners should be soldered watertight.
Underlayment: Underlayment (building wrap) at wall must lap over the top of the apron.
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.