Cedar Shakes and Shingles

Cedar roofing is natural wood — made from either Western Red Cedar or Alaskan Yellow Cedar. The single decision that determines whether a cedar roof survives Montana freeze-thaw cycling is grade. The quality of the installed shakes or shingles will largely determine the longevity of a cedar roof. The highest quality shakes and shingles are 100% edge grain and free of knots and other growth defects, with no sapwood. Get that right and the rest is installation discipline.

Two terms get confused constantly. Here’s the plain difference: shingles are sawn on both sides for a smooth, tailored look, while shakes are thicker, offering a more rustic appearance through splitting or resawing. That distinction changes thickness, minimum slope, and whether you need felt interlay between courses.

One non-negotiable when buying: look for the Certi brand on the bundle label. Cedar shakes and shingles manufactured by Cedar Shake & Shingle Bureau members are the only products labeled with the “Certi” brand name. And don’t shortcut the ask — asking for “the Blue Label” or “number one blue label” is not specific enough; CSSB members’ products are the only ones with the Certi brand name on the label.

Grades, Sizes, and Types

Grade is where the longevity lives. For shingles, the hierarchy is clear. Number 1 Blue Label is the premium grade for roofs and sidewalls — 100% heartwood, 100% clear, 100% edge grain — available in 16”, 18” or 24” lengths. Below that, Number 2 Red Label is a good grade for many applications, requiring not less than 10” clear on 16” shingles, 11” clear on 18” shingles and 16” clear on 24” shingles, with flat grain and limited sapwood permitted. Number 3 Black Label is a utility grade for economy applications and secondary buildings, with not less than 6” clear on 16” and 18” shingles, 10” clear on 24” shingles.

Number 4 is not roofing. Number four grade shingles are a utility grade for undercoursing of double coursed sidewalls only. They are not a roofing material and they are not to be used as a starter course on roofs. No exceptions.

For shakes, the grade language shifts slightly. They’re available in Premium Grade (100% edge grain) or Number 1 Grade (up to 20% flat grain allowed in each bundle). The reason that matters: knots, flat grain, sapwood and other defects in the exposed portion of the shake or shingle are a clear indication of lower quality. Flat grain is what cups and curls when the weather swings.

Product	Type / Finish	Lengths	Butt Thickness
Certi-Split Handsplit & Resawn Shake	Split face, rustic; sawn back	18”, 24” (most common)	3/8” to 1”+
Certi-Sawn Tapersawn Shake	Sawn both sides, pronounced shadowline	18”, 24” (most common)	5/8” to 1-1/2”
Certigrade Shingle	Sawn both sides, tailored look	16” (Fivex), 18” (Perfection), 24” (Royal)	Gauged by shingle stack

The minimum roof slope on which Certi-label shakes are recommended is 4:12, and for Certi-label shingles, 3:12. Steeper is common and usually better for shedding.

Western Building Center’s cedar offering includes a #2 cedar shingle and an 18-inch grooved sidewall shingle — naming the kinds of material available, not where to find them on a given day.

Treated options. Two treatments exist, and you pick one — never both. Treated products use either a pressure-impregnated fire-retardant treatment or a preservative treatment, but not both. For fire compliance, Premium and Number 1 Grades of Certi-Split shakes, Certi-Sawn shakes and Number 1 Grades of Certigrade shingles are available pressure-impregnated with fire retardants, and Certi-Guard products may be used for Class A, B and C roof systems. For damp or tree-shaded sites, Certi-Last preservative-treated material is available by pressure impregnation process and is ideal in areas of high humidity. Sort out the paperwork before you buy: confirm labels required for warranty registration before purchasing product, and ensure warranty registration details are listed on the purchase order. Treated material also has its own fastener rules — contact the treatment company for treatment warranty information, accessory product requirements including recommended fastener types, and application details.

Installation Basics That Make or Break the Roof

Start with the deck. Shakes and shingles may be applied over spaced sheathing — usually 1x4 or 1x6 softwood boards, and shall not be less than 1x4 boards. For solid decks there’s only one tested sheet: solid sheathing is recommended for shakes and may be required in seismic regions or under treated shakes and shingles, is used in areas with wind-driven snow, and the only solid sheet sheathing tested with shakes and shingles is plywood. That matters here — the Flathead and Mission valleys both sit in active seismic zones, and wind-driven snow is a given.

Spacing between units keeps the roof from buckling. Spacing between units is critical to prevent buckling: 1/4 to 3/8 inch for shingles and 3/8 to 5/8 inch for shakes. Stagger your joints. Joints in any one course should be separated not less than 38mm from joints in adjacent courses, and in any three courses, no two joints should be in direct alignment.

Felt interlay is required on shakes — and only shakes. The CSSB recommends using an 18” wide strip of No. 30 ASTM D226 Type II or ASTM D4869 Type IV roofing felt laid over the top portion of the shakes and extending onto the sheathing. Position it right: the bottom edge of the felt should be positioned above the butt of the shake at a distance equal to twice the weather exposure. A 24” shake at 10” exposure puts the felt 20” above the butt. That felt earns its keep in this climate — the felt interlay acts as a baffle that prevents wind driven snow or other foreign material from entering the attic cavity during extreme weather conditions, and increases the roof’s insulating value.

Set your edges so water sheds clear: product should project 1 1/2” beyond the fascia and approximately 1” over the gable or rake end. Copper flashing works in most regions — copper flashing can be used with a cedar roof in most areas, with the exception of areas that are exposed to acid rain areas east of the great lakes. Not a concern in Montana.

What Kills a Cedar Roof Early

Cedar has to breathe. The failures trace back to the same handful of mistakes — sealing the underside so moisture can’t escape.

Don’t wrap the deck in a non-permeable membrane. The CSSB does not recommend the use of non permeable membrane as it can cause premature roof failure; do not cover the entire deck with non-permeable barrier including nonpermeable underlayment, as this could eventually lead to shakes/shingles cupping and curling.

Don’t foam the underside. CSSB does not recommend using spray foam underneath the plywood before installing cedar shakes or shingles on top of it; spray foam tends to create an air barrier and a non-ventilated base, which can lead to premature deterioration of the internal side and cause cup and curling issues. An adequately ventilated base is required to ensure the long life of the cedar roof.

The fix is airspace. The untreated shingles will last longer and the roof system will work better if you have an airspace directly below the shingles to allow them to breathe and dry out; it will also allow summer condensation to run out from under the shingles. A cedar roof that can dry between weather events is a cedar roof that lasts.

One thing not to worry about: color mismatch on repairs. Replacement sections and individual shakes or shingles will weather naturally to the same general gray color as the rest of the roof within six to nine months, on average.

Frequently Asked Questions

What’s the actual difference between a shake and a shingle? Shingles are sawn on both sides for a smooth, tailored look, while shakes are thicker and offer a more rustic appearance through splitting or resawing. Shakes are also thicker at the butt and require felt interlay between courses; shingles do not.

Which grade should go on a roof? For shingles, Number 1 Blue Label. No flat grain is allowed in Premium Grade cedar shakes or Number 1 Grade cedar shingles; Premium Grade shakes are the top shake grade, whereas Number 1 Grade shingles are the top shingle grade. Edge grain resists the cupping that wrecks lower grades.

Can Number 4 shingles be used as a starter course? No. Number four grade shingles are a utility grade for undercoursing of double coursed sidewalls only — not a roofing material, and not to be used as a starter course on roofs.

What’s the minimum roof slope for cedar? The minimum recommended slope is 4:12 for shakes and 3:12 for shingles; it is typical to see installations with slopes greater than these.

Fire treatment or preservative treatment — can a roof get both? No. Treated products use either a pressure-impregnated fire-retardant treatment or a preservative treatment, but not both. Fire-retardant Certi-Guard handles Class A, B, and C roof systems; Certi-Last preservative is the choice for high-humidity settings.

Does cedar need felt under every course? Shakes do. Felt interlayment is required when shakes are installed — cedar shake application is a system of shakes with interwoven felt. Shingles are not interlaid with felt the same way.

Is house wrap an acceptable substitute for roofing felt? No. The CSSB recommends ASTM D226 Type II or ASTM D4869 Type IV roofing felt, because house wrap typically allows moisture to flow behind the shingles where these ASTM standard roofing felts will not.

A final word on this climate: local jurisdiction requirements always govern. The contents of the CSSB manual are not intended to supersede local jurisdiction requirements or building code — always check with your local code jurisdiction for approval of any installation modifications. Snow load, seismic zone, and wildfire requirements vary across the valleys, so verify before you order.