Flat & Low-Slope Roofing

Flat and low-slope roofs are common on Calgary infills, garages, additions, porches and older homes, and they fail differently from shingles. Water sits on them instead of running off, so seams, drains and the edges where the membrane turns up a wall do the work that slope does on a pitched roof.

The main systems are two-ply SBS modified bitumen (often called torch-on), and single-ply membranes such as TPO and EPDM. SBS is heavy and tough and handles foot traffic well. TPO is a white, heat-welded membrane that reflects sun. EPDM is a rubber membrane that stays flexible in deep cold. Each can last decades when it is detailed properly at drains, scuppers and parapets, and each fails early when it is not.

Ponding water that stays more than a couple of days after rain is the warning sign to act on. In Calgary it freezes and thaws through the winter and chinook cycles, opening seams and pushing water into the insulation below. Replacement is also the time to check the insulation and slope, since wet insulation and a flat deck are why many low-slope roofs leak again.

A like-for-like membrane replacement does not usually need a permit under the provincial re-roofing exemption, but adding insulation that changes the structure, building up slope with new framing, or converting to a rooftop deck is structural work and does. Commercial and multi-family roofs follow separate permit and fee rules.