“Flat roof” is a category, not a single product. Two of the most common options you’ll hear about are TPO and modified bitumen. Both can perform well—if the system matches the roof design and the install quality is high.
This guide compares the two systems in practical terms: seams, durability, repairs, and how each handles Bay Area sun, fog, and winter rain.
TPO at a glance
- What it is: a single‑ply membrane with heat‑welded seams.
- Strength: seams can be very strong when welded correctly; reflective surfaces can reduce heat gain.
- Consider: details around penetrations and drains are critical; workmanship matters.
Modified bitumen at a glance
- What it is: an asphalt-based membrane system, often installed in multiple layers.
- Strength: robust puncture resistance and proven performance when properly installed.
- Consider: seam detailing and transitions require skill; maintenance still matters.
How to choose: the questions that matter
1) Does your roof have ponding risk?
Roofs that hold water need special attention: drainage improvements, tapered insulation, and careful seam/penetration detailing. Material choice helps, but design fixes are often the bigger win.
2) How many penetrations and details are on the roof?
Skylights, vents, HVAC curbs, and rail posts are common leak sources. Choose the system your installer can detail cleanly and consistently.
3) Will you need frequent access?
Patio decks and rooftop access can increase wear. Consider protection layers and walk pads where appropriate.
Maintenance and repairability
No system is “set and forget.” Plan for periodic checks—especially after storms—so small seam issues don’t become interior water damage.
What we recommend for Bay Area homeowners
Start with the roof design: slope, drains, and penetrations. Then choose the membrane system that fits your roof’s use and exposure. If you need a professional assessment, see our flat roof services or request an estimate.