How to Install Cedar Shakes: Sheathing, Interlay Felt, Exposure, Keyways and Nails

Step by Step

1. 1Step 1: Confirm the slope, the rules and the product

   Shakes generally need a steeper roof than asphalt shingles, commonly not less than 4/12 for shakes and 3/12 for cedar shingles, but confirm the requirement for your product. On lower slopes, shakes are not suitable. Check with your municipality about fire ratings and any approved underlayments.

   Common shake lengths are 18 inches and 24 inches. The length sets the exposure. Buy about 10 to 15 percent extra for waste and trimming, since shakes split and vary in width.

2. 2Step 2: Prepare the sheathing and underlayment

   Spaced sheathing lets air circulate under the shakes so they dry quickly, and boards are commonly spaced on centre at the same distance as the shake exposure, so each shake is nailed to a board. Solid sheathing is also used, and often with an air space or ventilated batten system, which helps the wood dry. Follow the design for your roof and the manufacturer's advice. See how to install drip edge.

   Where ice and water protection is required at eaves and valleys, install it first, see how to install ice and water shield.

3. 3Step 3: Lay a doubled starter course

   The starter course is laid in two layers so the water at the eave has a thicker path to follow. Overhang the fascia or drip edge by about 1-1/2 inches so water drips clear of the wall. Offset the joints between the two layers by at least 1-1/2 inches. Use a chalk line to keep the butts straight.

4. 4Step 4: Space the shakes and offset the keyways

   The gap between shakes is called a keyway. It has to be wide enough for the wood to swell without buckling, and narrow enough that water does not run straight through. The Cedar Shake and Shingle Bureau manual gives 3/8 to 5/8 inch, and the BC Building Code gives 6 to 9 mm (about 1/4 to 3/8 inch) for shakes, so about 3/8 inch satisfies both. Keyways that line up in adjoining courses create a path for water, so each must be offset by at least 1-1/2 inches from the joints in the course below, and not line up with the next course either.

   Trim shakes as needed to keep the offset, and avoid slivers narrower than about 4 inches, which split and blow off (the BC Building Code sets a 100 mm minimum shake width). Mix widths as they come out of the bundle so keyways fall naturally out of line.

5. 5Step 5: Interlay each course with felt

   Interlay is what stops wind-driven rain from being blown up the slope between shakes. The felt sits over the top portion of the lower course and under the shakes above, with its lower edge set above the butt line at a distance equal to twice the weather exposure. For 24 inch shakes at 10 inch exposure, that is 20 inches above the butt. Felt is used with shakes, but not under tapersawn shakes or shingles with no air gap, which can trap moisture.

   Cover the felt with the next course, and never let it stick out where it would show or trap water.

6. 6Step 6: Maintain the correct exposure

   Exposure is the visible part of the shake. Maximum exposure limits exist for roofs because more exposure means less overlap and less protection. Common figures from the Cedar Shake and Shingle Bureau for roof slopes of 4/12 and up are 7-1/2 inches for 18 inch shakes and 10 inches for 24 inch shakes. Lower exposures are used on lower slopes. Confirm against the manufacturer and your local requirements.

   Snap a chalk line for each course, or use a straight board as a guide, so the butt line stays level.

7. 7Step 7: Nail each shake with two corrosion-resistant nails

   Place the nails so that the next course covers their heads. Driving them too high means they are not covered, and too low means they miss the boards. Use nails long enough to pass through the shake and into the sheathing, such as a 3d to 5d shake nail, with stainless recommended in very wet or salt-air conditions. Never use bright steel nails, which rust and stain. Do not overdrive, which splits the wood and lets the shake cup.

   Watch out: Do not face nail near the exposed edge. It splits the shake and leaves a leak path.

8. 8Step 8: Flash the details and finish the ridge and hips

   Valleys, chimneys, skylights and walls need metal flashing set in place as the courses rise, as with any roof. See step flashing and the lapping rule. At the ridge, alternate the laps of cedar ridge pieces, and face nail with two stainless or galvanized nails on each piece, with heads sealed. Ridge and hip pieces are laid starting at the end away from the prevailing wind, or as the instructions direct.

Keeping Cedar Healthy in the Fraser Valley

Spaced Versus Solid Sheathing

Common Mistakes

• Keyways that line up. They let water run straight through to the felt and deck. Offset them by at least 1-1/2 inches.
• Skipping the interlay felt, or using it under shingles. Shakes need the felt. Shingles without an air gap should not have it.
• Too much exposure. It reduces overlap and lets wind-driven rain in. Stay at or under the maximum for the length and slope.
• Ordinary steel nails. They rust and stain the wood and fail in a few years. Use hot-dip galvanized or stainless.
• Overdriving nails. It splits the wood and leaves the shake loose and cupped.
• Ignoring fire rules. Untreated wood roofing is restricted in many areas. Check before you buy.