Flat roofing · Metro Vancouver & the Fraser Valley
Flat roofing in Metro Vancouver: low slope, not level, and why that decides everything
A flat roof is not flat. It is a low-slope roof built with a deliberate fall towards a drain or a scupper, and almost every flat roofing failure in this region begins when that fall stops working. Water that cannot leave a roof will eventually find a way into it.
We build flat roofs as two-ply SBS torch-on membrane, and we detail the drainage as carefully as the membrane, because in the Lower Mainland the drainage is the part that fails.
Definitions
What counts as a flat roof.
In roofing, "flat" means low slope, not level. The working line is roughly two units of rise in twelve of run: above it, a roof sheds water quickly enough for shingles or panels to be lapped like scales and rely on gravity. Below it, water lingers, wind drives it sideways and uphill, and lapped materials stop being watertight.
That is why a flat roof is a membrane rather than an assembly of overlapping pieces: there are no laps for water to work back under, because the surface is continuous.
Most flat roofs are built to fall about a quarter-inch to the foot towards their outlets. It is a small slope and it is easy to lose. Forty years of settlement, a joist sagged under a rooftop unit, an addition framed level because it was quicker — any of these leaves a low spot that collects rather than drains.
Five kinds of flat roof turn up over and over on Lower Mainland houses: the garage or carport roof, the roof over a rear addition, the sun deck, the mid-century house with a low-slope roof throughout, and the small flat section behind a pitched main roof that most owners forget exists until it leaks.
The local failure mode
Ponding is what kills flat roofs here.
Every region has a characteristic way of destroying flat roofs. Here it is ponding, and it is ponding because of how our rain arrives.
Environment and Climate Change Canada's published climate normals put annual precipitation near 1,160 mm at Richmond, around 1,505 at Abbotsford, 1,537 at Vancouver Harbour, 1,668 at Chilliwack Airport and above 2,500 at the Capilano station in North Vancouver. What those figures do not show is the shape of it: long, low-intensity rain across five months rather than short violent storms.
A flat roof in that pattern is never asked to survive a downpour and then dry out in the sun. It is asked to stay wet, on and off, from October to March.
Standing water does not punch through a sound membrane. What it does is test every detail continuously. A lap at a low spot, a seal at a vent curb, the flashing where the roof meets a wall, the collar on a plumbing stack — each of those is a joint, and a joint that spends five months underwater will find its weakness in a way a joint that gets wet and dries never does.
Ponding also accelerates everything else. It holds debris in place, so needles and moss rot down into a mat instead of washing off, and it adds weight exactly where the structure has already sagged, which deepens the pond.
A useful test: walk your flat roof two days after rain stops. Anything still wet is a low spot, and every low spot is a countdown.
Drainage
Drains, scuppers, and the things that block them.
A flat roof has one job it cannot do by itself: get rid of water. Everything that makes that happen is hardware, and hardware is maintainable, which makes drainage the cheapest thing you can look after and the most expensive thing to ignore.
- Internal drains sit in the field of the roof and take water down through the building. They are efficient and they are the easiest thing on a roof to block, because everything that lands on the roof eventually migrates to the lowest point, which is exactly where the drain is. A strainer dome over the drain is not optional.
- Scuppers are openings through a parapet wall that let water off the edge into a downspout or a collector box. They are less prone to blocking than internal drains and far more prone to being undersized, especially on older additions where the parapet was built first and the drainage was worked out afterwards.
- Overflows are the second outlet, set slightly higher, that stops a blocked primary drain turning a roof into a tank. Plenty of flat roofs here have none, and on a roof with one internal drain and no overflow, a blocked strainer is a structural problem rather than a roofing one.
- Gutters and downspouts at the edge of a low-slope roof are defeated by the same fir needles and cedar debris that block everything else.
Most of the flat-roof leaks we are called to are not membrane failures at all. They are a blocked drain, a cracked seal at a vent curb, or a wall flashing that was never lapped high enough. All three are cheap to fix and expensive to leave.
Where the fall itself has been lost, clearing the drains is not enough and tapered insulation rebuilds the slope so the low spot empties. That is a framing-level correction and it belongs on the quote as its own line, not buried inside a membrane price.
What we install
Two-ply SBS torch-on, and nothing else.
Every flat roof we build is a two-ply SBS modified bitumen torch-on system: an IKO TP-180 base sheet fully bonded to the prepared deck, then a TP-250 granulated cap sheet torched over it with staggered laps, with no mechanical fasteners anywhere through the membrane.
We chose one system and stuck to it deliberately. A roofer who installs four flat systems details each of them a little less fluently than a roofer who installs one, and years later a repair to a system nobody on site has handled recently is a repair that does not last. Everything we put down, we can also patch in the same material, bonded the same way.
We install two-ply SBS torch-on and do not install TPO or other single-ply membranes. If you have a quote for one and want the comparison before you decide, it is set out on the torch-on page below.
The full torch-on explanation
How a two-ply membrane is actually built, ply by ply, why SBS suits a climate that swings between wet and freezing, what it does better than the alternatives, and what decides whether it lasts twenty years or thirty — that is a page of its own rather than a paragraph here.
Read the full guide to torch-on roofing
Flat and torch-on work carries a five-year workmanship warranty, which is the standard for flat membrane systems and shorter than the ten years we carry on shingle, metal, cedar and composite. The manufacturer's product warranty is separate, longer, and on its own terms.
The decision
Flat roof repair, or flat roof replacement?
On a membrane the line is clearer than it is on shingles, because a membrane either is or is not still doing its job across the field. What you are deciding is whether the problem is at a detail or in the system.
Flat roof repair is the right answer when
- The leak traces to a drain, a scupper, a vent curb, a skylight kerb or a wall flashing
- The membrane away from the problem is well bonded, flexible and intact
- There is one failure, or several failures that share one cause
- The deck underneath is sound
- The roof is well short of the twenty to thirty years a two-ply membrane runs here
A torch-on repair is bonded in the same way the original membrane was, which is why patches made with roofing cement almost never hold. Cement shrinks, cracks and hides the joint from the next person who looks at it.
Flat roof replacement is the cheaper answer when
- Blisters, open laps or crazing appear across the field rather than at one point
- The membrane has gone hard and no longer moves with the building
- Water is already in the deck, so the plywood has to come off regardless
- This is the third repair in two years on the same roof
- The fall has been lost badly enough that tapered insulation is needed anyway
A flat roof replacement is a tear-off to the deck, sheathing repaired or replaced where it has softened, then base sheet, cap sheet and every flashing detail rebuilt. It is a smaller job than a whole-house re-roof and it is the moment to correct the drainage, because the roof will not be open again for twenty years.
Over living space
Decks, suites and flat roofs with a room underneath.
A flat roof over a garage forgives a lot. A flat roof over a bedroom forgives nothing, and the difference is the cavity.
Under a pitched roof there is an attic: a ventilated, inspectable space where a slow leak stains the plywood long before it reaches a ceiling. Over a laneway suite, a kitchen addition or a converted roof deck there is usually no such space. Membrane, deck, insulation, vapour barrier and drywall are pressed together, so the first sign of water is often a ruined ceiling and insulation that has been wet for months.
Two consequences follow. The first is that these roofs are worth inspecting on a schedule rather than on symptoms, with particular attention to the perimeter, the wall transition and every penetration. The second is that where a deck sits over living space, the waterproofing and the walking surface have to be two separate things.
A membrane needs to be complete, continuous and inspectable in its own right before anything goes on top of it, and whatever goes on top — pavers on pedestals, a floating duckboard, sleepers and decking — has to be liftable. A deck screwed down through a membrane, or a topping poured directly onto it, converts a half-day repair into a demolition project. We see it regularly, usually on a deck built by somebody other than a roofer.
The other recurring problem on these roofs is the door threshold. A patio door onto a roof deck sits too close to the membrane on most houses, and wind-driven rain backs up under it. It needs proper height above the finished surface and a flashed, drained sill.
Where we work
Flat roofing near you, from West Vancouver to Chilliwack.
Flat roofs are not evenly distributed across this region. The North Shore, East Vancouver, Burnaby and New Westminster are thick with mid-century low-slope houses and rear additions; Richmond and Delta have flat garage and carport roofs on almost every post-war street; the Fraser Valley has more laneway and suite conversions going on than anywhere.
Most Lower Mainland municipalities require a permit for a full re-roof, and the requirements differ between them — the City of Vancouver publishes its process on its building permits pages. Where a permit is required for your flat roof, it is included in the quoted price.
We repair and replace flat roofs across Vancouver, Burnaby, Surrey, Richmond, Coquitlam, Port Coquitlam, Port Moody, North and West Vancouver, New Westminster, Langley, Delta, Tsawwassen, White Rock, Maple Ridge, Pitt Meadows, Abbotsford, Mission, Chilliwack and Anmore. We are WorkSafeBC registered, carry five million dollars of liability insurance, are GAF certified, and belong to the ICBA and the BBB.
Questions
Flat roofing, plainly answered.
What counts as a flat roof?
Anything too low to shed water the way a pitched roof does, which in practice means a slope below about two units of rise in twelve of run. A flat roof is not level and was never meant to be. It is built with a deliberate fall towards a drain or a scupper, usually a quarter-inch to the foot, and that small fall is the entire drainage strategy of the roof.
Why do flat roofs fail in Vancouver?
Ponding, nearly always. Our rain arrives in long, low-intensity spells between October and March, so a flat roof here is not asked to shed a storm and dry out; it is asked to sit under water on and off for months. Wherever the fall has been lost to settlement, a sagging joist or a badly placed unit, water stands, and standing water finds every weakness in the detailing it is sitting on.
What flat roofing system do you install?
Two-ply SBS modified bitumen torch-on: an IKO TP-180 base sheet with a TP-250 granulated cap sheet, fully bonded, with no mechanical fasteners through the membrane. We do not install TPO or other single-ply systems, so that every flat roof we put down is detailed the same way and repaired the same way years later.
How do I know if my flat roof needs repair or replacement?
Look at where the problem is. A failure at a drain, a scupper, a vent curb or a wall flashing on a membrane that is otherwise sound and well bonded is a repair, and usually a small one. A membrane with blisters across the field, open laps in several places, cracking or crazing in the cap sheet, or water already in the deck is failing as a system, and patching it buys months rather than years.
How long should a flat roof last here?
Twenty to thirty years for a well-detailed two-ply membrane, and drainage decides where in that range a particular roof lands. A roof that empties quickly after every storm runs at the top of it. A roof with a drain that has been half-blocked with fir needles since 2019 runs at the bottom.
Can a flat roof have a deck or a patio on it?
Yes, and it is very common here on roof decks, sun decks and the flat roofs over additions. The rule that matters is that the waterproofing has to be complete and inspectable on its own before anything is put on top of it, and the walking surface has to be removable. A deck built directly onto a membrane in a way that cannot be lifted turns a small repair into a demolition job.
What can be done about a flat roof that ponds?
Three things, in increasing order of cost: clear and if necessary relocate the drainage so water has a route out; add a scupper or an overflow where the roof has only one outlet; or rebuild the fall with tapered insulation so the low spot drains. Tapered insulation is a separate line on a quote rather than something folded into a membrane price, because it is a framing-level correction, not a roofing product.
Do you work on flat roofs over suites and living space?
Yes, and those roofs get treated differently. A flat roof over a laneway suite, a basement suite's entry, or a bedroom has no attic between the membrane and the ceiling, so there is no forgiving cavity where a slow leak can show itself early. Those roofs are inspected more carefully at the penetrations and the perimeter, because by the time water appears on a ceiling below, it has already been in the assembly for a while.
What warranty comes with a flat roof?
Five years on the workmanship of a torch-on or flat membrane installation. That is the standard for flat systems and is deliberately shorter than the ten years we carry on shingle, metal, cedar and composite work. The membrane manufacturer's own product warranty is a separate document with its own term and conditions, registered in your name, and the two figures are never combined.
Can flat roofing be installed in the rain?
No. Torch-on has to be bonded to a dry surface, so flat roof installation is scheduled around the forecast rather than the calendar. An active leak can be stopped temporarily in wet weather and often has to be, but the permanent membrane work waits for a dry window. Anyone promising a permanent flat roof installation during a November downpour is promising something the material cannot do.
Next
Find out what your flat roof would cost.
Type your address and we measure the roof from satellite imagery, then price a two-ply torch-on replacement against what is up there now, with the drainage work named separately if your roof needs it. The number is on the screen before we ask you for anything.