Q: A client is considering pressure-treated cedar shingles for the roof on his new house. What are the pros and cons to using these shingles vs. regular cedar?

A: Chris Yerkes, a cedar-shingle installer certified by the Cedar Shake and Shingle Bureau (CSSB), and owner of Cedarworks, in Brewster, Mass., responds: In my 13-plus years installing cedar roofs, I have been asked to install regular cedar—both western red and Alaskan yellow—shingles and shakes, as well as pressure-treated (PT) cedar shingles. And both treated and untreated shingles can give you a long-lasting roof. But no matter what type of shingle you choose, the keys to the longevity of a wood roof are proper installation and quality materials.

Air circulation. In my experience, the No. 1 killer of wood roofs is rot from the inside out—usually a result of poor installation. When there isn’t enough air circulation behind the shingles to allow them to dry after a rain, the resulting moist environment is ideal for rot. When PT shingles first came out, many old timers thought that the treatment was a magic bullet that would keep the shingles from rotting, so they used PT shingles as a shortcut, eliminating the “breathing” requirement altogether. Treated shingles may slow the rotting process, but I’ve seen improperly installed PT shingles that failed way before their time. A properly installed red- or yellow-cedar roof should last 25 to 30 years, depending on site conditions and geographic location. Properly installed treated shingles may add five to 10 years to that number, all else being equal.

There are two basic approaches to roof shingle installation: skip sheathing, where the shingles are installed over a ladder-like series of wooden strips, and solid sheathing. With the more traditional skip-sheathing method, the underside of the shingle is exposed to air below the roof, whereas in a solid-sheathing application, a layer of ventilated underlayment material, such as Cedar Breather, lets air circulate underneath the shingles to let them dry out.

Fasteners. The second major factor for a successful installation is using the proper fasteners. I never use galvanized fasteners for any kind of roof shingle—only stainless steel. For most inland cedar roofing jobs, I use standard-grade 304 stainless steel coil nails. As an approved CSSB-warranty installer, I’m required to use a higher-grade 316 stainless steel nail on any roof installation within

11 miles of a body of salt water. I live on Cape Cod, so almost all my local installations are done with the 316-grade nail. Additionally, because of the preservatives in fire- and CCA-treated shingles, stainless steel is an absolute must for fastening in those applications.

Maintenance. Keeping the surface clean improves the longevity of any wood roof. This includes keeping valleys free of debris and keeping gutters clear. A more difficult problem for some roofs is the growth of moss and lichen, which live off moisture in the wood and act as sponges during rain events to prevent the wood from drying properly. Excessive moss growth can usually be seen on houses that are under large trees that shade the roof or on north-facing roofs, either of which can slow drying due to lack of full sun. Treated shingles are less prone to this growth, but we usually install a strip of copper along the ridge of every wood roof. Precipitation hits the copper and trace amounts dissolve and run down over the shingles, killing the moss along the way. Lead or zinc work as well, but copper is a nice aesthetic touch in addition to being practical.

Working with treated shingles is similar to working with regular cedar ones, but my crew members always wear gloves and take added precautions when working with treated cedar—as do smart carpenters who work with treated wood when building a deck. The price of cedar shingles can vary greatly depending on the market and availability. Pressure-treated-shingle prices vary as well, but they usually run 10% to 15% higher than prices for untreated cedar.