Flat Roofing

Flat Roofing Systems in San Francisco

Across San Francisco and the Peninsula, you find flat and low-slope roofs on row house additions, garages, mid-century and modern homes, and most commercial buildings. Because water does not run off quickly, these roofs depend on a continuous membrane and well-placed drains rather than on slope.

Ascender Roofing Solutions installs and repairs flat and low-slope roofing systems, with a focus on modified bitumen. We assess your building's specific needs (foot traffic, HVAC equipment, drainage patterns, insulation requirements) and recommend the system that will perform best over the long term.

Get a free estimate (415) 728-2724

Quick specs

Expected lifespan20 to 30 years
Typical SF replacement$8,000 to $30,000
Best for
Commercial buildingsLow-slope roofsModern home design

Key benefits

Maximizes usable space (rooftop decks, equipment, solar panels)
Multiple system options to match budget and performance needs
Excellent waterproofing when properly installed with correct drainage
Lower installation cost per square foot than sloped roof materials
Energy-efficient options available (reflective white membranes, cool roof compliant)
Easier and safer to inspect and maintain than steep-slope roofs

Things to consider

Proper drainage design is critical (ponding water is the leading cause of flat roof failure)
Requires experienced installers trained in the specific membrane system
More susceptible to leaks at seams, penetrations, and parapet walls if not detailed correctly
Regular maintenance (clearing drains, checking seams) extends life significantly

Flat roof leak repair

Most flat roof leaks start in a few places: seams, flashing at parapet walls and pipes, drains and scuppers, and low spots where water stands. We find the source before recommending anything. A small failure is usually a repair. A roof with widespread cracking, blistering or wet insulation is usually not worth patching again. Roof repair →

Standing water and drainage

Flat roofs need slope toward the drains, at least 1/4 inch per foot. Water still on the roof 48 hours after rain is ponding, and ponding shortens a roof's life. Common causes are clogged drains, settled framing and low spots left by earlier layers. Depending on the roof, the fix ranges from clearing and repairing drains to correcting drainage during a replacement.

Flat roof replacement

We remove the old roofing down to the deck, inspect and replace damaged decking, install the modified bitumen layers, and flash walls, penetrations and drains. Flat roof replacements in San Francisco typically run $8,000 to $30,000 depending on size, access, decking condition and drainage. The timeline is confirmed in your written proposal.

For how we price roofs in San Francisco, read the Roofing Cost Guide →

Roof coatings and overlays

Coatings. Reflective and waterproofing coatings can extend the life of a sound flat roof and help meet cool roof requirements. A coating does not fix a roof that is already failing.

Overlays. Some flat roofs can take a new layer over the existing one, if the roof is dry, sound and code allows another layer. A tear-off is often the better long-term choice, and the inspection decides.

Repair or replace?

  • Repair suits a roof with a few isolated problems and sound layers underneath
  • Replacement suits roofs with repeated leaks, widespread cracking or blistering, or wet insulation
  • A free inspection tells you which one your roof needs

Where we work on flat roofs

Row house additions, garages, flat sections below a sloped roof, and commercial and multi-unit buildings. Roofing for property managers →

Questions about flat roofing

How long does a flat roof last?+
Modified bitumen roofs often last 20 years or more when drains stay clear and the roof is inspected regularly. Drainage, sun exposure, foot traffic and installation quality all matter.
Why does my flat roof leak when it rains?+
Usually at a seam, flashing, drain or low spot, and often not where the stain appears inside. We trace the source before recommending a repair.
What causes standing water on a flat roof?+
Clogged drains, settled framing, low spots left by earlier layers, or poor original slope. Water that remains 48 hours after rain is considered ponding.
What is the best roofing system for a flat roof?+
For most San Francisco flat roofs we install modified bitumen, a multi-layer, flexible system with a long record of performance in Bay Area conditions.
Can my flat roof be repaired, or does it need replacing?+
It depends on how widespread the damage is. A few isolated problems can usually be repaired. Repeated leaks, widespread cracking or wet insulation usually mean replacement. Our inspections are free.
Is a roof coating worth it?+
On a sound flat roof, a coating can extend its life and improve reflectivity. On a roof that is already failing, it only delays the replacement.
Can you put a new roof over my old one?+
Sometimes, if the existing roof is dry, sound and code allows another layer. A tear-off is often the better long-term choice.
How much does flat roof replacement cost in San Francisco?+
Typically $8,000 to $30,000, depending on roof size, access, decking condition and drainage. Your written proposal gives the exact price.

Other roof types we install

Shingle RoofingModified Bitumen

Get a free flat roofing estimate

Call or request a free estimate online. We respond within one business day.

Request free estimate (415) 728-2724