We have a 20-year-old Cape Cod with a cedar shake roof (white cedar, I believe). Although the shingles on the north side of the house are in fair condition, some of the shingles on the south side of the house are curled, split or have holes in them. When I took up some of the more seriously damaged shingles I saw they were nailed to furring strips that had been nailed directly into exterior grade plywood. There is no tarpaper or other backing over the plywood. Is not having tarpaper going to cause a real problem over time, or is it an accepted practice?
This sounds like an unhealthy marriage of old and new building techniques. As far as I know, it's not an accepted building practice.
Years ago, before the advent of plywood, it was common for carpenters to leave gaps between planks of roof sheathing—hence the term "skip sheathing." Attics were not stuffed with insulation so the spaces between boards promoted air circulation and allowed the backs of the singles to dry out.
A roof built this way was less likely to trap moisture, and so shingles lasted longer than if they had been nailed over solid sheathing. If you take a peek inside an old New England barn, this is what you're likely to see. So we know it was effective.
Plywood sheathing needs a weather barrier. When plywood and modern roofing materials came along, builders began adding a protective layer of tarpaper over the sheathing. This extra layer picked up any water that got past the singles.
Today, builders still use tarpaper or one of several high-performance equivalents. Ice & Water Shield made by Grace Construction Products is one of the best known (www.graceathome.com). It's a self-adhering membrane that sticks tenaciously and seals itself around nails.
Whatever the builder or roofer may choose, there should be some type of underlayment between the plywood sheathing and the shingles. Roof shingles should never be nailed to unprotected sheathing.
Your roof sounds like half a good idea (what our onetime mechanic Walter sagely called a "lose-lose situation"). Water getting past the shingles can't dry to the inside of the building. And without a layer of tarpaper or other underlayment, the plywood is exposed to more moisture than it was designed to handle.
New underlayment just for wood shingles. It's not uncommon for shingles with a southern exposure to show signs of wear first. After all, they're taking the brunt of the sun's UV radiation.
It may be time to remove at least the shingles on that side of the roof and see what's inside. It wouldn't be a huge surprise to find some rot, and stripping back the roof will give you a chance to repair the damage.
If you re-roof with cedar, consider using a special underlayment designed for wood shingles. One such product is Cedar Breather, manufactured by Benjamin Obdyke (www.benjaminobdyke.com). It's a quarter-inch thick mesh that aids air circulation and helps shingles last longer--just like skip sheathing did.
But don't forget the underlayment. Your roof sheathing should be protected by a layer of 30-lb. tarpaper before the singles go on.
