Flat and low-slope roofs built to drain in Vancouver, WA

Shingles cannot keep water out on a roof that is nearly flat. Below a certain pitch, rain moves too slowly and wind pushes it uphill under the shingle laps. These roofs need a continuous membrane instead. Around Vancouver we see them on mid-century ranches with flat sections over carports, modern homes with low-slope rooflines, porch roofs tied into a second story, and garage tops that double as decks.

Our crews install and repair the common membrane systems: single-ply TPO and PVC, EPDM rubber, and modified bitumen. Each has strengths. The right one depends on how much foot traffic the roof gets, how it drains, what it ties into, and whether the roof is visible from upstairs windows. Torch-applied systems have their own page, since that work carries its own safety steps.

Why flat roofs leak

The main enemy of a flat roof is standing water. A low spot that holds an inch of water after rain stresses seams, grows algae, collects fir needles and turns to ice on cold nights. Over time the membrane wears faster in that spot than anywhere else. Ponding usually comes from sagging framing, a deck that was built dead level, or drains and scuppers clogged with needles and leaves. Fixing the cause matters more than patching the symptom.

The second common source is the edges. Where a flat roof meets a wall, a parapet or a pitched roof above it, the membrane has to turn up and be flashed. Those transitions are where shrinking rubber pulls away, sealant cracks and wind lifts loose corners. Penetrations such as vent pipes, skylight curbs and HVAC stands are the third weak point, and each needs its own detail rather than a smear of caulk.

Flat roof systems we install

Choosing a membrane is about matching it to the roof's use. A deck over a garage needs a surface that can take furniture and foot traffic under protective pavers. A hidden roof over a back addition may only need a reliable rubber sheet. We explain the tradeoffs for each roof and price more than one option where it makes sense, so you can weigh cost against expected service life and upkeep.

• TPO single-ply, heat-welded seams, white or gray
• PVC membrane for decks and chemical resistance
• EPDM rubber, fully adhered with taped seams
• Self-adhered modified bitumen, no open flame
• Tapered insulation to build slope toward drains
• New scuppers, drains and edge metal

Fixing drainage before replacing the membrane

Putting new membrane over a roof that ponds just repeats the problem. Where the deck is flat or sagging, we install tapered insulation boards cut to create a gentle fall toward the drains. On small roofs we sometimes add crickets, which are small raised triangles that divert water around a curb or toward a scupper. When framing has sagged badly, we open the deck and sister the rafters before covering it.

We also look at where the water goes after it leaves the roof. Scuppers that dump onto a lower roof, or downspouts that empty against the foundation, cause trouble of their own. Larger scuppers with conductor heads, or a new downspout run, are often part of the job. Owners are sometimes surprised that the drainage plan matters more to roof life than the brand of membrane. A well-drained roof dries out between storms, and that alone adds years.

Repair or replace a flat roof

A flat roof with one leaking seam or a single punctured spot is a good repair candidate. We clean the area, patch with matching membrane and heat-weld or adhere it in place. Replacement makes sense when the membrane is cracked across the field, seams are failing in several places, the insulation below is wet, or the roof has been coated and patched so many times that leaks cannot be traced. We use a moisture probe to check for wet insulation before deciding.