A flat roof is a layered assembly: a fire-rated slip sheet, then insulation board, then a TPO membrane with its seams welded shut. The choices that matter most are how the water drains off, how thick the membrane and insulation should be, and who is responsible where the flat section meets a skylight, a wall or the pitched part of the roof.
By Van Naik, owner · Updated September 24, 2026
A surprising number of Bay Area homes have a flat or low-slope section somewhere. Covered patios. Garage extensions. A room added in the '70s. Some mid-century and modern homes are flat from end to end. Shingles depend on gravity to move water down and off the roof, and a flat roof doesn't give them much gravity to work with. They can't go on anything flatter than 2/12, and a 2/12 or 3/12 slope needs 100% peel-and-stick underneath.
What TPO is
TPO (thermoplastic polyolefin) is a single-ply roofing membrane. It comes in wide rolls, gets laid across the roof, and the seams where two sheets overlap are welded together with hot air. Once they cool, the overlapping sheets are fused into one continuous surface. That's the whole idea: on a roof where water sits a while before it drains, you want one sheet with no gaps for water to find.
The surface is also light-colored and reflects a lot of the sun. On a flat roof that bakes in direct sun all afternoon, that helps keep the room underneath from heating up as much.
TPO vs. torch-down
We install TPO on flat roofs because it lasts longer. A TPO roof lasts 25 to 30 years. Torch-down lasts 10 to 15.
Torch-down, also called modified bitumen, is the other common flat roof system. It comes in rolls of asphalt-based membrane, and the roofer uses a propane torch to melt the asphalt at each overlap and bond the sheets together. Every seam comes down to how the torch was run that day. Too hot, and the seams get brittle faster. Too cold, and the seams curl.
TPO goes on differently. The membrane is mechanically fastened to the roof, and every seam and edge is welded together, so the whole roof becomes one continuous sheet.
What goes under the membrane
The membrane gets most of the attention, but the layers underneath do real work too. Here's what we install on every flat roof, from the deck up:
- FR10 slip sheet. A thin sheet laid over the deck that gives the roof a UL Class A fire rating, the highest there is.
- Half-inch insulation board. It does two jobs at once. It adds a layer of insulation over the living space, and it gives the membrane a perfectly flat, smooth surface to sit on.
- TPO membrane. Welded at every seam and at the edges.
From there, two upgrades are your call:
- Thicker insulation, 1" or 2". More insulating value over the room below. Worth considering if that space runs hot in summer or cold in winter.
- Tapered insulation board. Custom-built boards that are thicker at one end than the other, so they build slope into a flat surface and move water toward the drain. If your roof holds puddles for days after a rain, this is the fix.
Is the thicker membrane worth it, or should I spend the money on drainage?
This is the question that comes up most on flat roof visits, and it's a good one. You usually have a budget for one upgrade, maybe two.
TPO comes in different thicknesses. We typically install 60 mil, and a thicker 80 mil is available. Thicker membrane gives the roof more material to wear through over its life.
Water that sits is what wears a flat roof down, though. A puddle that hangs around for days after every storm keeps working on the same spot of membrane, season after season, whatever the thickness. So our rule of thumb goes like this. If your roof ponds, put the money into tapered insulation first. If it already drains well, the thicker membrane is the upgrade to look at. Skipping the 80 mil upgrade and putting that money into tapered boards is a sound strategy.
We'll give you a total for each option so you're comparing real numbers.
Who is responsible where the flat roof meets everything else
Flat roofs rarely leak in the middle. They leak at the edges and the connections: around a skylight, where the flat roof runs into a wall, where it meets the pitched roof, and at the drains or scuppers where the water leaves.
On a lot of houses, those connections are where two different contractors' work meets, and when a leak shows up there, you can end up with two contractors pointing fingers at each other. When we do the whole roof, we flash every one of those connections ourselves: the skylight curbs, the wall flashing, the transition to the shingled section and the drain edges. They're our work, so they're our responsibility.
If another trade is involved, like a skylight company or a solar installer, we'll tell you before the job starts who handles which piece and in what order.
Two warranties, and they cover different things
A flat roof comes with two separate warranties, and it helps to know which is which.
- The membrane manufacturer's warranty covers the TPO material itself. We install Mule-Hide TPO, and their warranty covers you for 20 years.
- Our 10-year workmanship warranty covers how we installed it: the welds, the flashing and the connections.
Ask for both in writing, from us or from anyone else you're talking to. If a bid mentions just one warranty, ask what the other one is.
Why a shingle company does TPO
Most of what we install is shingles, Owens Corning and CertainTeed. We install TPO because it's a good product for flat roofs, and we install it with our own crew. You get one company responsible for the whole roof, steep sections and flat.
Flat roof pricing depends on the size, the drainage and the upgrades you choose, so we don't list a standard rate. The permit is billed at exactly what the city charges.
Next step: book a visit
If you have a flat or low-slope section that's leaking, holding water, bubbling or cracking, book a free visit. We'll walk it, show you what we find, and give you a written total for each option worth considering. If the roof has years left in it, we'll tell you that too.
Call (925) 381-9162 or request a visit online.
About the author. Van Naik owns Affordable Roofing Systems, a family-owned roofer based in Contra Costa County and serving the Bay Area since 1989. If you've read older reviews that mention Kelly, Kelly is the previous owner. The crew in those reviews still does the work. CSLB #1145218 (C-39). Owens Corning Preferred Contractor and CertainTeed Master Craftsman.