Ponding water on the back roof, a blistered seam, a leak over the rear bedroom of a San Francisco flat? Send a few details and one vetted, licensed San Francisco roofer will tell you what the repair actually involves and what it should cost. No callback roulette, no five companies blowing up your phone.
Useful to include: the membrane type if you know it (torch-down, TPO, built-up), roughly how old the roof is, where the water shows up inside, and a photo from a window or the deck.
You’ll get a straight read on repair versus replacement. Not a script.
Most people in San Francisco don’t realize they own a flat roof until it leaks. The front of the building is the part you see from the street: a steep composition or architectural slope on the Victorian or Edwardian face. Behind it, over the rear rooms, the light well, and the deck, there is almost always a low-slope membrane roof. Two entirely different systems on one building, and the flat one is where the trouble lives.
It fails differently, it gets repaired differently, and a roofer who is excellent on a steep slope is not automatically competent on a membrane.
Ponding is the enemy
A “flat” roof isn’t flat. It’s built with a slight pitch, ideally a quarter inch per foot, so water runs to a drain or a scupper instead of sitting there. Over decades the framing sags, the insulation compresses, someone adds a deck, and the pitch that was designed in stops working.
Then water stands. The rule of thumb roofers use is that anything still holding water 48 hours after the rain stops is ponding, and ponding is what kills a membrane. It degrades the surface, it finds the seams, and in a city with a fog drip and a five-month rainy season it barely gets a chance to dry.
Which means a huge share of flat-roof repairs in the Sunset and the Richmond start not with the membrane at all, but with a clogged drain, a leaf-packed scupper, or a downspout that hasn’t moved water since 2019. Clear the drainage first. Sometimes that is the whole repair.
What actually leaks on a low-slope roof
Rarely the field of the membrane. Almost always the edges and the penetrations.
Seams open up where two sheets of torch-down mod-bit were lapped, especially where a previous repair used the wrong material or was done on a damp day. Parapet walls crack at the coping and let water in behind the membrane, which is a classic SF failure because so many buildings share a parapet with the neighbor. Flashing at the plumbing vents, the furnace flue, the skylight curb, and the point where the flat roof ties into the steep front slope all move over time and open. Blisters form where moisture got trapped under the membrane and then heated up.
And then there’s the deck. Half the flat roofs in this city have a walking deck built over the membrane, which means the leak is under a structure that must come apart before anyone can see it. That’s a cost factor and an honest contractor says so up front rather than discovering it on day two.
The stain is not the hole
Water gets in at one point, runs along the underside of the sheathing or down a joist, and drips out somewhere else entirely. On a flat roof with almost no pitch, it can travel a long way before gravity wins.
In a building of stacked flats it can also travel between units, which turns a roof leak into a conversation with your downstairs neighbor and, sometimes, with an HOA. So the first job is tracing, not patching. Anyone who arrives, looks at the ceiling stain, walks up, and smears mastic on the membrane directly above it has guessed.
Repair, or is it done?
Honest version: a torch-down mod-bit roof in San Francisco generally has 15 to 20 years in it. If yours is 12 and you have one open seam, repair it. If it’s 22, the surface is alligatored and brittle, and you’re patching the third leak in two winters, you are spending money to delay a decision you’ve already made.
Middle ground exists. A silicone or acrylic roof coating can buy real years on a membrane that’s tired but sound, and silicone in particular tolerates the ponding that would eat an acrylic. It cannot resurrect a roof with wet insulation underneath. If the substrate is saturated, coating over it just seals the water in.
And a word on timing. Between November and March, a roofer in a storm window is doing damage control, not a proper repair. Torch-down needs a dry, warm substrate to bond. Getting dried in during the rain and doing the real work in April is not a contractor stalling you. It’s the right call.
What flat roof repair costs in San Francisco
As of 2026. Access is a real line item in this city: no driveway, no parking, and material carried through the building or hoisted off the street.
- Emergency dry-in or tarp during a storm: $400 to $1,200.
- Seam, flashing, or blister repair: $600 to $2,000 depending on access and how much has to come apart.
- Drain and scupper clearing, ponding correction: $300 to $1,500.
- Deck removal to reach the membrane: added labor, quoted after someone has seen it.
- Silicone coating over a sound membrane: roughly $4 to $9 per square foot.
Common San Francisco flat roof questions
Why does water sit on my roof after it rains?
Because the drainage isn’t working or the pitch is gone. Start with the drain and the scuppers, which in the Sunset and the Richmond are usually packed with pine needles and grit. If they’re clear and it still ponds, the roof has sagged or settled and the fix involves tapered insulation to rebuild the slope, which is a bigger and more expensive conversation.
Can you repair a flat roof in the winter?
You can get it dried in and stop the water. A permanent torch-down or seam repair needs a dry substrate and reasonable temperatures, and neither exists during an atmospheric river. A roofer who tells you the real fix waits for a dry stretch is being straight with you.
The leak is over my downstairs neighbor’s unit. Whose problem is it?
Physically, the roof’s. Financially, it depends on your CC&Rs, because in most SF condo and TIC arrangements the roof is common area while the interior finishes are not. Get the leak stopped first and sort the accounting after. That’s practical guidance, not legal advice.
Related San Francisco roofing services
- Roof leak repair when you know there’s water inside but not where it’s coming from.
- Roof coating, the middle path for a tired but sound membrane.
- Roof replacement when the patching stops making sense.
Send your San Francisco roofing details
Membrane type, roof age, where the water lands inside. That’s most of the diagnosis. Describe it and you’ll get a straight answer, with no pressure to book.
If there’s a deck over the roof, say so. It changes the scope more than anything else on the list.
No cost, no obligation. A clear read from the roofer who’d actually do the work.