Flat roof systems for BC strata and commercial buildings

There is no single best membrane, and any contractor who names one before looking at your roof is selling rather than advising. The roof decides it. Start with the deck: a combustible deck over occupied suites pushes toward hot-air welded single-ply, because an open propane torch on that assembly carries a smouldering-fire risk that is hard to justify. Then look at use and drainage. A roof walked constantly for mechanical service benefits from the puncture resistance of two torch-on asphalt plies. A roof that ponds favours single-ply, PVC especially, which is built to sit in standing water. If summer heat into top-floor units is already a complaint, a reflective single-ply surface helps. Both torch-on and single-ply typically last about 15 to 25 years on a Metro Vancouver roof when detailed and maintained well, so service life alone rarely settles the question.

Torch-on SBS and single-ply TPO or PVC both typically reach about 15 to 25 years on a Metro Vancouver low-slope roof when they are detailed properly and maintained. Where a roof lands in that range depends far more on the details and the upkeep than on the membrane type. A roof that gets an annual inspection, has its drains cleared, and gets small repairs at the first sign of a lifted lap or a suspect weld will reach the top of the range. A roof left alone until a suite leaks will not, and by then the interior repair often costs more than years of maintenance would have. The coastal climate is a factor too: Metro Vancouver sees well over 1,100 mm of rain in a typical year, so drainage that works is doing a lot of the work of keeping the roof alive.

Sometimes a recover is fine, and sometimes it buries a problem you will pay for later. Two things decide it. First, how many coverings are already on the roof: the general principle across Canadian building codes is a maximum of two total coverings on a low-slope roof, so the original plus one overlay. Once a roof carries two, it has to come off to the deck. Layer limits and how they are applied vary by building and jurisdiction, so confirm the specific requirement with your local building authority rather than assuming. Second, whether the assembly underneath is dry. Wet insulation left under a new membrane keeps feeding water into the deck and voids most warranties, and the RCABC RoofStar Guarantee requires the existing roof to be demonstrably dry and free of wet material. A test cut and a moisture survey answer both questions before anyone prices the work.

Because low-slope roofs almost never fail in the middle of the field. They fail at the details: laps and welds, and the flashings at parapets, drains, and penetrations, where the membrane has to turn a corner and seal against another material. A repair that patches the visible stain without tracing the actual entry point tends to fail again, because water rarely enters directly above where it shows up inside. It travels along the deck, through insulation, and down a structural element before appearing on a ceiling. Persistent leaks usually mean either the source was never found or the underlying problem is drainage rather than the membrane. If the roof ponds, laying new material over the same low spots does not fix anything. Our guide to how a building leak investigation works walks through the diagnostic sequence that finds the real source before a repair is scoped.

It can be, on the right deck and with a disciplined crew, but it needs to be taken seriously. Torch-on is hot work: a propane torch with an open flame melts the underside of the membrane so it welds to the layer below. The specific danger is a fire that starts smouldering under the membrane, in a combustible deck or in trapped dust, and does not show itself right away. It can flare up hours after the crew has packed up. WorkSafeBC treats torching as hot work, and a legitimate crew protects combustible material, keeps charged extinguishers on the deck, and holds a fire watch on site after the flame goes off. On a concrete deck with that discipline in place, the risk drops a long way. On a wood-frame deck over occupied suites, many strata councils and their consultants take the flame off the table entirely and go single-ply, which welds with hot air instead. Some insurers also add conditions or higher premiums for torch work, so check your policy before a torch-on bid gets far.

RCABC is the Roofing Contractors Association of British Columbia. It publishes roofing practice standards for the province and runs the RoofStar guarantee, which is backed by the association rather than by the individual roofing company. When a RoofStar-registered contractor installs an accepted assembly to the standard, the guarantee covers the roof even if that contractor later goes out of business, which a plain workmanship warranty cannot promise. For a strata that is real protection, because it does not depend on one firm still being around in fifteen years. RoofStar accepts SBS torch-on, TPO, and PVC assemblies, so it is available whichever path you take. Ask any bidder whether the job will carry a RoofStar guarantee, and confirm they are actually registered to provide the one they are quoting rather than taking it on trust.