How Shake Roofing Works:
All roofing materials I can think of are comprised of 2 major components. (1) Some type of waterproof material. (2) Something to protect that material from the sun’s rays (UV).
With shake roofing, there is what is known as a “shake liner” which is 2 layers of 18″ or preferably 22″ organic roofing felt that is installed first & lapped over each other half way or 11 inches, then the cedar shakes laced into them. In this case the lower half of the felt is actually not waterproof (due to nail holes from the shake on top of it) but acts as a vapor barrier. The upper half is water proof. It is the shakes themselves that protect the felt from sun & wind.
After 5-6 years when shakes dry out completely & become too brittle they began to split & crack, sometimes causing the split to align with the shake below or above. According to CSSB (California Shingle & Shake Bureau) the minimum safe distance for the offset on these cracks is 1.5 inches. When the offset is less than 1.5 inches the felt & or nails from the adjacent shake become exposed to the sun & rain.
Once the “felt exposure” is created it is a short time (a matter of months) before the sun burns through the felt, leaving the attic area exposed to the weather.
Felt exposures are the primary cause of shake roof leaks. Now that you have a good understanding of this, in the next post we will show how to properly fix a felt exposure.