Torch-on, or SBS-modified bitumen, is the workhorse of flat and low-slope roofing on the coast - the system you will see protecting countless decks, garages, additions, and laneway homes across the Lower Mainland. It is built up in two layers of rubber-modified asphalt membrane, heat-fused with a torch into one continuous, fully bonded surface with no loose seams for water to work into. For roofs that sit nearly flat under months of Pacific Northwest rain, that fully adhered approach is hard to beat.
Why two plies matter in our climate
A flat roof has nowhere for water to run off quickly, so it has to be genuinely waterproof rather than just water-shedding like a steep shingle roof. Two-ply SBS gives you that through redundancy. The base sheet and the granulated cap sheet each form a complete waterproof layer, so there are two independent membranes between the weather and the space below. The SBS modifiers keep the asphalt flexible through winter cold snaps and stable through summer heat, and the mineral-surfaced cap resists UV and stands up to foot traffic.
Compared with single-ply systems, that second layer is the headline difference. TPO and EPDM are good products, but they rely on a single sheet seamed at the laps, and a flat roof here can sit in standing water for weeks at a stretch. With two torched plies you have built-in backup, and the system is straightforward to inspect, patch, and re-flash years down the road.
The details that actually keep water out
On a low-slope roof, the field membrane is rarely where leaks start - it is the transitions and penetrations. We torch the membrane tight into every detail and treat each one as its own small waterproofing job:
- Parapets and wall flashings carried up and capped so wind-driven rain cannot get behind them
- Drains and scuppers tied in with reinforcing plies and set low so water actually reaches them
- Curbs around skylights, vents, and mechanical units fully wrapped and sealed
- Slope-to-drain checked across the deck so water moves rather than ponds
Ponding is the enemy of any flat roof. Standing water adds weight, accelerates wear, and finds the smallest weakness over time. Where the existing structure allows, we build in tapered insulation or adjust the slope so water drains instead of sitting, and we make sure scuppers and overflow points are sized to handle a heavy coastal downpour. Keeping those drains clear of moss, needles, and debris is the single biggest factor in a torch-on roof reaching the upper end of its service life.
Decks, additions, and where torch-on fits
Because the granular cap takes foot traffic well, torch-on is a natural fit for walkable roof decks and rooftop patios, and it integrates cleanly with separate traffic-bearing coatings or pavers where a finished surface is wanted. It is equally at home on a detached garage, a flat-roofed addition, or the compact roof of an infill or laneway house where space is tight and the membrane has to be right the first time.
If you are weighing your options across the whole roof, our flat and low-slope roofing page covers how torch-on compares with the alternatives and where each one makes sense. And if your current flat roof is showing dark patches, blisters, or a stain on the ceiling below, a roof repair and leak investigation is often the faster, cheaper first step before committing to a full replacement.
Torch-on is a proven, repairable system, and when it is detailed properly it stays watertight through everything our winters throw at it. For a flat roof that needs to be genuinely waterproof through a Vancouver winter - not just for a season, but for decades - two-ply SBS remains one of the most dependable choices we install.