Flat Roof Leak Repair in San Francisco: Causes and When a Repair Is Enough

San Francisco has more flat roofs than most people realize. Many Victorians and Edwardians look pitched from the sidewalk but are flat behind the front parapet. Row houses in the Sunset and Richmond, mixed-use buildings in the Mission and SoMa, and newer infill buildings across the city often have low-slope roofs too. We cover the Victorian side of this in more detail in our guide to Victorian and Edwardian roofs.

Flat roofs are not bad roofs. They just depend on two things working all the time: the membrane has to stay watertight, and water has to leave the roof quickly. When either one slips, water finds the weakest point and follows it inside — often a long way from where it first got in.

Our weather plays a part. Long dry summers with strong sun on the south and west sides of a building dry out and shrink roofing materials. Fog keeps surfaces damp for hours most mornings in neighborhoods near the ocean. Then the winter storms arrive, sometimes with a lot of rain in a short time, and every weakness shows up at once.

Ponding means water that sits on the roof long after the rain stops. A common rule of thumb in the roofing industry is that water still standing about 48 hours after rain is a problem worth looking at. Standing water adds weight, speeds up wear on the membrane, and keeps pressure on any small opening in a seam or patch.

What ponding looks like

• Dirt rings or dark stains on the roof surface where water has dried
• Low spots near the middle of the roof or in front of drains
• Moss, algae or debris collecting in the same spot every season

Can ponding be repaired?

Often, yes. If the low spot is small, we can sometimes build it up with tapered material or add a drain or scupper to give the water a way out. If the whole roof sags, that can point to deflection in the framing below, and a new membrane by itself won't solve it. That is one of the cases where we'd explain why a bigger fix is needed rather than patching over it.

Every flat roof is made of rolls or sheets joined together. On torch-down and modified bitumen roofs, the laps are heat-fused. On TPO, the seams are hot-air welded. Over time, sun, movement and age can open those joints. Seams near parapet walls and at the edges of the roof see the most stress.

From inside, a seam leak often shows up as a stain that grows slowly with each storm, rather than a sudden drip. On the roof, we look for lifted edges, fishmouths (small open wrinkles at the end of a lap), and cracks along the joint.

Seam failures are usually repairable. On a torch-down roof, we can cut out the bad section and torch a new patch over it. On TPO, a failed seam can often be cleaned and re-welded or covered with a welded patch. If you're weighing which system to use on a future roof, our honest comparison of torch-down vs TPO covers how each one handles repairs.

On a flat roof, drains do the work that slope does on a pitched roof. Leaves, sand, bird nests, trash blown up from the street and even old tennis balls can block them. Once a drain is clogged, water backs up, rises above the flashing at the drain bowl, and finds its way in.

This is the cheapest flat roof leak to prevent. Clearing drains, scuppers and downspouts before the rainy season — and after big windstorms — takes care of a large share of the leaks we see. Our fall roof prep checklist has the full list.

If a drain bowl itself is cracked or the flashing around it has failed, that is a small, specific repair. We handle these through our skylight and drain repair service.

Flashing is the material that seals the roof where it meets something else — a parapet wall, a vent pipe, an HVAC curb, a chimney or a skylight. It is the most detailed part of any roof and the most common place for leaks to start.

• Parapet walls: the flashing turns up the wall and is capped or tucked under a coping. When the coping or the top of the flashing fails, water gets behind it.
• Vent pipes and penetrations: pipe boots crack in the sun, and sealant around them dries out.
• Skylights: the curb flashing, the seal between the glass and frame, or both can let water in. See why skylights leak and how to fix them.

Flashing repairs are normally targeted repairs. We replace the failed flashing, tie it properly into the existing membrane, and leave the rest of the roof alone.

We believe in repair before replace. If a roof has years of life left and the problem is in one place, a repair is the honest answer. Here is roughly how we think about it.

A repair is usually enough when

• The leak traces back to one seam, drain, flashing detail or penetration
• The rest of the membrane is still flexible and well-bonded
• The insulation and decking under the leak are dry or only slightly damp

A larger repair or replacement may make sense when

• There are leaks in several unrelated places at once
• The membrane is cracking, blistering or brittle across the whole roof
• Wet insulation or soft decking has spread well past the leak
• The roof has already been patched many times and the patches are failing

If replacement is the right call, we'll show you why with photos. You can read more about how we approach full replacements on our re-roofing page.

Put a bucket under the drip, move valuables, and take photos of the ceiling and any water. Don't go on the roof in the rain. If it's safe, check from a window or the street whether a drain or downspout is visibly overflowing. Then call a roofer. Our step-by-step guide on what to do when your roof leaks during rain covers this in detail.

Ace Roofing & Construction has been fixing flat roofs across San Francisco since 1996. We respond next day for active leaks, and our flat roofing team works on homes and buildings in SoMa, the Mission, Bayview and every other SF neighborhood.