Cedar Shake Calculator

Cedar shakes are split rather than sawn, so they carry an irregular face and a butt thickness of roughly 1/2 to 3/4 inch on medium shakes and up to about 1-1/4 inch on heavies, against 3/8 to 1/2 inch for a sawn shingle. Coverage therefore depends on three things at once: shake length, exposure, and whether the surface is a roof or a wall. Roof exposure is limited to about one third of the length less half an inch, giving 7-1/2 inches for 18 inch shakes and 10 inches for 24 inch shakes. Sidewalls, which shed water without ponding, may run to roughly half the length. Courses are wall height in inches divided by exposure, and coverage is quoted in squares of 100 square feet, with about four bundles to the square on roofs and five at the tighter wall exposures. Waste runs near 10 percent on plain walls and 15 percent on roofs, where hips, valleys, and starter courses force cutting. A cedar roof weighs roughly 350 to 450 pounds per square, which matters when re-covering existing framing. Two constraints are decisive. Shakes need air movement beneath them, which is why they go over spaced sheathing or a ventilated mat and why an 18 inch felt interlayment strip is woven between courses. And untreated cedar rates Class C for fire, so pressure-impregnated fire-retardant grades are required in many jurisdictions and often demanded by insurers.

Split cedar roofing is far older than the industry that sells it. Peoples of the Pacific Northwest used western red cedar, Thuja plicata, for planks, walls, and roofing long before European settlement, exploiting a wood whose natural extractives, the thujaplicins, resist decay, fungi, and insects. Settlers rived shakes by hand with froe and mallet, and the product stayed a local craft until Pacific coast mills began producing shingles and shakes for national distribution in the late nineteenth century. Standardization arrived with the trade bureau that still grades the material, which established the Blue Label, Red Label, and Black Label system separating premium 100 percent heartwood, edge grain stock from lower utility grades, and which publishes the exposure and interlayment rules that building codes reference. The material's fortunes then turned on fire. Repeated wildland-urban conflagrations, most visibly the Oakland Hills firestorm of October 1991, identified untreated wood roofs as a mechanism of ember-driven spread, and California and many western jurisdictions responded by banning or restricting them. Manufacturers answered with pressure-applied fire-retardant treatments reaching Class A or Class B ratings, while composite polymer and fiber cement imitations captured much of the appearance market. Cedar shakes consequently occupy a narrower niche today than in 1980, valued mainly for historic restoration and high-end custom work rather than as a general-purpose roof covering.