A flat roof is never quite flat. It slopes a little, and water has to find the drain. Failures can start where it does not. Replacing a low-slope roof means reading the layers that are there, choosing a membrane, and rebuilding the details around drains, edges and rooftop equipment.
Choosing a membrane
Single-ply options include TPO and PVC, with seams welded by heat. Modified bitumen builds in layers and goes down by torch or cold adhesive. EPDM is rubber sheet. Built-up roofing alternates felt and asphalt. Spray foam cures from a liquid.
We compare these on the roof's use, its drainage, how much foot traffic it sees and the cool roof values for the slope. Each product comes with a CRRC listing and a maker's data sheet, and we put both in front of you.
Cool roof values on a low slope
State low-slope values apply in zones 4 and 6 through 15 when the job replaces over 50 percent of the roof. SRI 75 is the alternative to aged reflectance 0.63 with emittance 0.75. An insulation trade-off table is another route (California Energy Commission, 2022).
In the City of Los Angeles, the LADWP sheet lists 0.65 and 0.85, or SRI 78, for low slopes (LADWP, 2022). The rule is not required where solar is installed or for work on 50 percent of the roof or less.
What a flat roof replacement includes
- Core cut to confirm layers and insulation
- Old membrane removed where the scope calls for it
- Deck and insulation repaired
- New membrane welded or applied to the maker's instructions
- Drains, scuppers and crickets rebuilt for flow
- Edge metal, curbs and flashing finished with the roof
Signs a flat roof needs replacing
- Puddles still there two days after rain
- Open seams, splits or blistering in the membrane
- Ceiling stains below the low slope
- A coating that keeps failing in the same places
Layers, recover and core cuts
Public Works (2025) sets three conditions for covering an old roof in County jurisdiction: one layer, adequate framing, a deck that is sound and dry. A core cut is a small sample through the roof. It reveals the layer count and any wet insulation.
Wet insulation means tear-off, not recover. You get the core result before the scope is final.
Drainage, ponding and coatings
Water that sits on a membrane ages it. Crickets, small raised wedges, push water toward drains and scuppers. Scuppers, openings in the parapet, carry it off the roof. We measure the fall toward every drain and scupper, and rebuild any that sit too high or clog.
Coatings are a different category. Express permits at LADBS include a line for an approved coating applied to an existing roof (LADBS, 2026). A coating can extend a sound roof. It does not replace a roof whose layers are failing.
Flat roof questions
What does the single-layer rule mean for a flat reroof?
County jurisdiction allows recover over an old roof with a single layer, adequate framing and a sound, dry deck (Los Angeles County Public Works, 2025). A core cut shows the layers. Two layers mean a tear-off.
When does a coating fall short of a replacement?
A coating sits on top of the roof, so it only helps when the layers beneath are sound and dry. Wet insulation, open seams or a failing deck call for replacement. LADBS lists an approved coating on an existing roof as an express permit item (LADBS, 2026).
Is torch down roofing illegal?
Torch-down is open-flame work, so fire safety governs how it is done. Cold-applied and self-adhered sheets give a flame-free route. Ask your insurer and the permit office about the method before you choose it.
Does a low-slope reroof have to meet cool roof values?
Yes, where the rule applies. For zones 4 and 6 through 15 the state asks for aged reflectance 0.63 and emittance 0.75, with SRI 75 as the alternative, when the job replaces over 50 percent (California Energy Commission, 2022). The City of Los Angeles uses 0.65 and 0.85, or SRI 78 (LADWP, 2022).
How do crickets and scuppers stop ponding?
Crickets are small raised wedges that tilt water toward a drain, and scuppers are openings in a parapet that let it out. Together they keep water from sitting on the membrane. We check the slope to each outlet before the new roof goes on.