Roof types · Flat and low-slope roofing
Flat and low-slope roofs
Vancouver Specials, mid-century ranchers, additions, garages, carports and most strata and commercial buildings have flat or low-slope roofs, and they need a membrane rather than shingles. Two-ply torch-on SBS is the local standard, with TPO and EPDM single-ply membranes common on larger roofs and budget jobs. A flat roof here lives or dies on drainage, so the quote should talk about slope and drains as much as the membrane.
- Installed
- $7 to $16 per sq ft
- Lifespan here
- About 15 to 30 years depending on the membrane
- Upkeep
- Moderate
- Upkeep cost
- Twice-yearly drain clearing and a look at the seams and flashing
Common brands and products: IKO, Soprema and Firestone SBS membranes; Carlisle, Firestone and GAF TPO and EPDM; Duro-Last
Pros
What it does well
- The only sensible option for a flat roof
- Torch-on is well understood by Lower Mainland roofers and durable
- Single-ply membranes go down fast on large roofs
- Tapered insulation can fix ponding for good
- Easy to inspect and repair
Cons
What to weigh up
- Shorter life than a pitched roof in most cases
- Everything depends on drainage; ponding water finds every seam
- Torch-on involves open flame and smell during installation
- Flashing at parapets, drains and rooftop units is where leaks start
- Dark membranes get hot in summer
In our climate
Flat roofs under a metre of rain
A flat roof here spends six months under water unless it drains well. The roofs that last have a real slope to the drains, usually built with tapered insulation, drains and scuppers that are cleared twice a year, and membrane flashing that runs well up every wall and parapet. The ones that fail have low spots that pond, drains that clog with needles, and flashing that stops short.
Two-ply torch-on SBS has been the local standard for decades because the two layers forgive small flaws and the granulated cap sheet protects the bitumen from UV. TPO is lighter, white and reflective, and welded rather than torched; EPDM is the budget rubber membrane. On a house, the choice usually comes down to the roofer's experience and whether open flame near wood siding is acceptable.
Rough budget
2,000 sq ft of flat roof
- Installed, typical range
- $18,000 to $32,000
- Per square foot
- $7 to $16 per sq ft
- Same roof in asphalt shingles
- $11,000 to $19,000
Before tear-off extras, plywood repairs, permits and GST. Steep or cut-up roofs cost more.
Price your own roof →Questions
Flat and low-slope roofing questions
How long does a torch-on roof last?
A two-ply torch-on SBS roof that drains well typically lasts 20 to 30 years. Ponding water, blocked drains and failed flashing shorten it considerably.
Why does water pool on my flat roof?
Low spots in the deck or insulation, sagging framing, or drains that are too few or clogged. Tapered insulation and extra drains fix it for good; a coating or patch doesn't.
Is torch-on dangerous to install?
It uses an open flame, so it needs an experienced crew, fire watch and care near wood siding and vents. TPO and EPDM are installed without flame and are sometimes chosen for that reason on houses.
Can a flat roof be converted to a pitched roof?
Yes, but it's a structural change that needs a building permit everywhere and an engineer. It's a renovation project rather than a roofing job.
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