Earlier this year one of our inspectors was on site with a client to discuss a fairly simple project, but the visit identified a problem with the plan to shingle the roof. The roof, a simple single-plane sloping structure (“barn dormer” in building lingo) had a slope of about two feet over eight feet, (a 3:12 pitch) and the builder wanted to use shingles.
Veteran contractors may already know the problem our inspector identified: the roof wasn’t steep enough for a simple shingle installation.
The National Building Code of Canada stipulates that shingles can be used without much in the way of specialized installation for any roof with a slope of 1 in 3 (a 4:12 pitch for builders). Shingles can be used on roofs as shallow as 1 in 6 (a 2:12 pitch), but with special requirements, most notably that the installation must be such that there are three layers of shingle over the entire roof, not two as well as special applications of cement and tar.
Ultimately, the builder chose a metal roof, which is Code-compliant for a 1:4 slope, and for the builder, a simpler and cheaper alternative.
Here’s the thing with shingles: the shallower the slope, the more susceptible they are to issues of wind-borne rain driving up-slope.
Of course, shingle roofs require what we call “eaves protection,” (see post here: http://snbsc-planning.com/roofing-tip-protect-those-eaves/) and what builder refer to an “ice and water shield” when they transition over a heated space to an unheated eave. This helps protect the roof from ice-dams.
For suggestions on how to tame ice-damming, see the post on drop-chord trusses here. (http://snbsc-planning.com/benefits-of-the-heel-drop-chord-truss/)