Slate Systems Standard

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Slate Systems Standard

Division B - Standards

Standard for Slate Roof Systems

How to use this Standard

The fourteen (14) Parts of this Standard contain the requirements, guiding principles, recommendations, and reference materials, necessary to design and construct a project that will qualify for a RoofStar 5-Year Guarantee, RoofStar 10-Year Guarantee, or RoofStar 15-Year Guarantee. Guarantee requirements for a RoofStar 15-Year Guarantee are identified in various Parts. All RoofStar 15-Year Guarantee requirements must be read together with the general requirements for each Part in this Standard.

Readers are advised to review relevant materials that can be accessed through the hyperlinks embedded in the body of text. Part titles shown in blue indicate hyperlinks to more relevant material that the reader is advised to consult.

Refer to Structure and Organization of RPM and Standards in Division A of this Manual for more information about this Standard.

© RCABC 2023
No reproduction of this Standard, in whole or in part, is lawful without the expressed permission of the RCABC.

Note to Reader
This section of the RCABC Roofing Practices Manual is to provide general information on slate roofing. The information included is intended as a general guide for specifiers. RoofStar Guarantees are NOT available for slate roof systems.


This section of the RCABC Roofing Practices Manual is to provide general information on slate roofing. The information included is intended as a general guide for specifiers. This material has not been submitted for consideration under the RoofStar Guarantee Program.

Slate is a naturally occurring stone material which is formed of compressed sedimentary particles of clay and silt built up on ancient sea beds. The layering of the materials gives slate a natural cleavage which allow it to be split into thin layers. Slate also has a natural grain at right angles to the cleave. Slate colours vary from region to region due to slight mineral composition differences. Colours include black, green, grey, purple and red, or ribboned with various combinations of these colours. Combinations of colour can be used to enhance the aesthetic appeal.


Roofing slate is tested in accordance with ASTM test method to determine the following:


  • Modulus of rupture (breakage) 9,000 lb / square inch minimum
  • Weathering resistance (water absorption) to resist freeze thaw damage and delamination. Maximum absorption - .002%
  • Depth of softening (scratch test)

S-1 rated for 75 to 100 year service
S-2 rated for 40 to 60 year service
S-3 rated for 10 to 25 year service
Slate is cleaved by hand and sight-measured for nominal thickness. Thickness variances are considered natural. Minimum nominal thickness for roofing slate is 4 mm (3/16").


3.1 Deck Requirements

Slate roofing needs a minimum slope of 1:3 (4" in 12") to achieve required runoff. Slate can be installed over 16 mm (5/8") plywood or 24 mm x 100 mm (1" x 4") strapping.

3.2 Underlayment and Eave Protection

Slate should have ice dam protection at the eaves and an underlayment of #30 asphalt saturated felt laid perpendicular to the slope with end laps min. 100 mm (4") and 100 mm (4") laps on successive courses. Valleys should be run vertically with field rolls overlapping at least 300 mm (12"). Hips and ridges should be overlapped 300 mm (12") in both directions.

3.3 Slate

Slate is available in three nominal thicknesses:

Standards 4 mm - 6 mm (3/16" - 1/4")

Quarters 6 mm - 8 mm (1/4" - 3/8")

Heavies 8 mm - 12 mm (3/8" - 1/2")

The different thicknesses of slates should not be mixed. Using slates of different thickness in a course will result in an uneven substrate for slates of the succeeding course, which may result in breakage. Slates for a given roof area should be mixed before application to ensure an even colour blend.

The roof deck should be loaded with slates required for that area before application begins to prevent excessive traffic from damaging slates already laid. Starter courses must be doubled or be underlaid with specially cut starter slates.

Slate is installed in a 2 ply shingle fashion with a minimum head lap of 75 mm (3") for roofs with a 1:2 (6"/12") and greater slope or 100 mm (4") for roofs with slopes between 1:3 (4"/12") and 1:2 (6"/12"). Slates should be staggered on successive rows with more than 75 mm (3") difference. Slate can be either uniform width or random width. Most slate comes with two pre-punched holes for nailing and should be applied with copper nails, considering the expected life of a slate roof can be up to 100 years. Nails should be driven just flush so as not to crack the slate over or under.

Additional holes can be made by use of a zax. The holes should be struck on the underside so the spalling will create a countersink for the nail head.

3.4 Valleys

Valleys on a slate roof can be done in several ways including swept, laced, mitred and open. The recommended style for B.C. is open valleys, due to ease of installation and better drainage of debris. It is recommended that copper, lead or stainless steel be used to match the longevity of slate.

Valley metal should be a minimum of 600 mm (24") wide and be centre crimped. Successive pieces should lap a minimum 150 mm (6"). Valley metal sections should be drag load fastened and sealed between laps with a compatible caulking. Slates should extend a minimum of 150 mm (6") into the valley with the cut line being tapered 1% from bottom to top.

3.5 Hips & Ridges


Hips on a slate roof can be either mitred, saddled or have a metal flashing.

Saddled hips require a lathe nailer on both sides thick enough to support the hip tiles from the field tiles. The width should allow nailing and keep a 75 mm (3") lap over field tiles. Nail heads should be covered with elastic cement between successive tiles.

On mitred hips, field slates are cut flush at the apex to match slates on the other side. The unexposed portion of these slates is covered with a metal flashing which should extend 50 mm up from the tiles being covered.


Ridges have several traditional styles including saddled or a metal flashing. Ridges are installed after the field slates are installed flush with each other on opposite sides of the ridge.

  • Saddle Ridges
The slates are run horizontally and overlapping with the nails being driven between slates of the last course. Elastic cement should be applied in overlaps and joints on top of the ridge.
  • Metal Ridge Cap
Metal used for a ridge cap should be non-corrosive to last the life of a slate roof. It is installed on a ridge board and must cover the last course of slate down to its normal exposure. Attachment is provided by hidden clips.

3.6 Rake Edges

Under construction

3.7 Flashings: Protrusions, Walls and Curbs

Flashings should be made of non-corrosive metals to match the longevity of slate.

  • Step Flashings (Soakers)
Step flashings are used where a long vertical detail (chimneys, walls, skylights) runs parallel to the slope. Step flashings run 100 mm (4") up the vertical and 100 mm (4") on to the slate, covering the unexposed portion, then hidden by the next course of slate. Step flashings should be 75 mm (3") longer than the slate exposure.
  • Back Pans
Back pans are used up slope from chimneys and skylights, etc. They are used to divert the water rushing against vertical to the sides then down roof. Back pans should extend at least 600 mm (24") up under the up roof slates and 100 mm (4") up the vertical. Back pans should extend at least 75 mm (3") out either side of the vertical.
  • Apron Flashing
Apron Flashing is installed on the down roof side of chimneys, skylights, etc. They are used to direct water running off an object on to the down roof slates. Apron flashing should run a minimum 100 mm (4") up the vertical and extend past the exposure line of the previous course of slates.
  • Pipe Flashings
Lead pipe flashings work well with slate. Pipe flashings are laid over preceding courses and under successive courses of slate.

For information on qualified RCABC contractors installing slate roofing, please call RCABC at: (604) 882-9734 or email

3.8 Nailing Requirements

  • Slate nails should be copper or stainless steel, although copper is recommended. Standard thickness slate may be applied with 38 mm (1 ½") nails, with 50 mm (2") for hips and ridges. Thicker slate requires longer and heavier gauge nails. Add 25 mm (1") to twice the thickness. Nail holes are machine punched or drilled.
  • Minimum shank diameter should be 3 mm (1/8") or No. 11 Stubs gauge but thick slate requires longer length and larger diameter. Slope, weight of slate, wind loads and roof sheathing should also be considered when determining shank gauge.

Slate Size

  • To relieve uniformity of shadow line, architects have continued to specify one length and random widths.

Length Standard Widths Exposure with Standard
75 mm (3") Headlap
600 mm (24") 400 - 350 - 300 mm 263 mm
(16" - 14" - 12") (10 ½")
550 mm (22") 350 - 300 - 275 mm 238 mm
(14" - 12" - 11") (9 ½")
500 mm (20") 350 - 300 - 275 - 250 mm 213 mm
(14" - 12" - 11" - 10") (8 ½")
450 mm (18") 350 - 300 - 275 - 250 - 225 mm 188 mm
(14" - 12" - 11" - 10" - 9") (7 ½")
400 mm (16") 350 - 300 - 275 - 250 - 225 - 200 mm 163 mm
(14" - 12" - 11" - 10" - 9" - 8") (6 ½")
350 mm (14") 300 - 275 - 250 - 225 - 200 - 175 mm 138 mm
(12" - 11" - 10" - 9" - 8" - 7") (5 ½")
300 mm (12") 300 -250 - 225 - 200 - 175 - 150 mm 113 mm
(12" - 10" - 9" - 8" - 7" - 6") (4 ½")
250 mm (10") 250 - 225 - 200 - 175 - 150 mm 88 mm
(10" - 9" - 8" - 7" - 6") (3 ½")

Standard Smooth Texture 318 - 386 kg / Sq (700 to 850 lb / Sq)
6 - 10 mm (1/4" - 3/8") Rough Texture 454 kg / Sq (1000 lb / Sq)
10 - 13 mm (3/8" - 1/2") Rough Texture 680 kg / Sq (1500 lb / Sq)
13 - 19 mm (1/2" - 3/4") Rough Texture 1040 kg / Sq (2300 lb / Sq)
19 - 25 mm (3/4" - 1") Rough Texture 1450 kg / Sq (3200 lb / Sq)



© RCABC 2023
No reproduction of this material, in whole or in part, is lawful without the expressed permission of the RCABC Guarantee Corp.