Cedar Shake Roof Repair: Split Shakes, Moss, Treatments and When to Stop Repairing

Cedar shakes are split or sawn from western red cedar and laid in overlapping courses, usually two nails per shake. The wood sheds water, then has to dry. Problems start when it cannot: on north slopes, under mature trees, and anywhere snow sits for weeks, the wood stays wet, moss and algae take hold, and the shakes break down from the bottom up.

Freeze-thaw cycles add to that. As shakes wet and dry, they swell and shrink unevenly, which is what splits them along the grain and makes them cup or curl at the edges. Sun-exposed slopes weather faster on the surface, while shaded slopes rot. A Michigan cedar roof often has both problems on different faces of the same house.

Not every split needs a new shake. Shimming slides a piece of metal, bent at the corners, under a split shake to cover the crack from below. The CSSB calls it effective for the odd shake, but says no more than 20 percent of a roof area should be shimmed, for both performance and appearance. It also suggests that if a 100 square foot section has more than about 25 hail impact splits, repair is probably not the right path.

Plenty of companies sell cedar roof treatments. The CSSB's care guide says to use only products labeled for cedar roofs (or with a letter from the manufacturer saying so) that are water repellents, UV inhibitors or EPA-registered wood preservatives, with a safety data sheet and a performance guarantee. It says not to use products that make outrageous claims such as 10 years of effectiveness, that make fire-retardant claims, that are sealants, waterproofers or plasticizers, or that contain unfortified linseed oil, diesel fuel or crankcase oil.

Fire-retardant cedar is made at the factory. The CSSB's Certi-Guard label identifies shakes and shingles that have been pressure-impregnated with fire retardants, and those products can be used in Class A, B and C roof systems. A spray applied to an existing untreated roof does not turn it into one of those systems. If your roof is already pressure-treated, the CSSB says to contact the treatment company before applying any finish or cleaning agent.

For repairs on a treated roof, the replacement shakes should carry the same treatment, and the fasteners should be Type 316 stainless as the code requires for treated shakes. Ask your roofer to confirm both, and keep the label or invoice in your house records.

Cedar roofs in Michigan fall under the wood shake and wood shingle sections of the 2015 Michigan Residential Code, and several of those rules turn on Michigan's own climate entries.

Start with the eaves. Michigan's version of the code's climate table, adopted as Mich. Admin. Code R 408.30516, answers yes to ice barrier underlayment statewide. Section R905.8.3.1 says that where required, ice barriers on wood shake roofs shall comply with Section R905.1.2, which runs the barrier from the lowest edge of the roof to at least 24 inches inside the exterior wall line. Section R905.8.1.1 adds that where the average daily temperature in January is 25 degrees or less, solid sheathing is required on the part of the roof that needs the ice barrier. Federal 1991 to 2020 normals for the Grand Rapids airport put the January average high at 31.0 degrees and the average low at 18.6, an average daily temperature of about 24.8. So around Grand Rapids, rebuilding a cedar eave over old spaced boards generally means solid sheathing and an ice barrier under that first stretch of roof. Ask your building department how it applies the rule to your address.

Slope is the second rule. Section R905.8.2 allows wood shakes only on slopes of 3 in 12 or steeper. A low porch or addition that was finished in cedar to match the house is outside what the code allows for shakes, and repairs there tend to be short lived.

Fire classification is the third. Section R902.1 requires a listed Class A, B or C roof covering where a jurisdiction designates it by law, or where the edge of the roof is less than 3 feet from a lot line. On tight city lots that can matter for a section rebuild, and Section R902.2 says fire-retardant-treated shakes must be pressure impregnated to the AWPA C1 standard with each bundle marked by the manufacturer. A topical spray does not meet that.

Last, overlays. Section R908.3.1 permits metal panels, metal shingles and concrete or clay tile over existing wood shakes only when applied under Section R908.4, which requires covering the old shakes with gypsum board, mineral fiber, glass fiber or another approved material where the new roof would create a combustible concealed space. Confirm any of these with your local building department before work starts.