Guide to Low-Slope Roofing
Learn the alphabet soup of materials, but first get the slope and underlying details right.
A good low-slope roof (3-in-12 pitch or lower according to the National Roofing Contractor’s Association, or NRCA)—rarely gets any attention. A bad one brings the kind of attention nobody wants. I see a lot of bad low-slope roofs and don’t know which is more astonishing: how little additional effort would have been required to build it right, or the extraordinary cost of correcting the roof and repairing the associated damage.
Karen L. Warseck, AIA, president of Building Diagnostics Associates, a Florida-based firm that specializes in identifying and fixing building-envelope problems, identifies the causes of roof failures in an Architect magazine article titled, “When It Leaks, It Pours”: “Normally we find that it’s about 60% to 70% construction, 20% to 25% design, and 10% materials.” In short, the roof is the wrong place to try to save money.
Though Warseck’s experience proves that the skill of the workers and the quality of the roof design are more important than the choice of materials, the first question I usually hear is, “What’s the best material?”
Well, the answer is, “It depends.” If there were one best roof for every application, we would have figured it out by now. Even standard asphalt roofing shingles can be used on roofs as low-sloped as 2.5-in-12, as long as the underlayment is installed in a double-coverage configuration. That simply means that there must be two layers of underlayment everywhere. So, instead of lapping a 36-in. underlayment the typical 2 in., the overlaps are 18 in. each.
For lower roof pitches, there are two broad categories of roofing: asphalt based and polymer based. But the best material choice depends on a variety of factors: location, weather, type of construction, intended use of the building, configuration of HVAC and rooftop equipment, anticipated foot traffic, foreseeable potential for…