A low-slope roof is covered with a membrane, not with shingles: the City's re-roofing standard allows asphalt shingles only on slopes of 2 in 12 or steeper. Below that, the choice is among four systems. Each must have a design slope of at least one-quarter inch per foot (IRC 2021, sections R905.9 to R905.13), so the new roof drains to its drain or scupper whichever you choose.
The four systems
| System | What it is | Code section | HUD useful life |
|---|---|---|---|
| Modified bitumen | Asphalt rolls modified with polymers, torched or adhered in overlapping layers | R905.11 | not listed separately |
| EPDM | Thermoset single-ply rubber sheet, adhered or fastened, seams taped | R905.12 | 15 years |
| TPO (and PVC) | Thermoplastic single-ply sheet, seams welded with hot air | R905.13 | 15 years |
| Built-up | Alternating felts and bitumen, often with gravel | R905.9 | 20 years with gravel, 10 without |
HUD's figures come from its useful-life table for capital needs assessments: a planning number, not a promise for any one roof. A manufacturer's warranty is a separate document with its own terms; ask for it in writing.
Modified bitumen
The City's Rowhouse Manual calls modified bitumen "a low tech, lower cost, reliable roofing system." It suits rowhomes because it handles parapets and penetrations well and many crews install it. Torch-applied sheets involve an open flame on the roof; self-adhered and cold-applied versions exist. Ask which one the estimate includes.
EPDM
EPDM comes in large sheets of rubber, so a small roof can be covered with few or no seams. The manual notes it is "a commercial roofing type sometimes used on houses." Black EPDM absorbs heat; white versions are made.
TPO
TPO is a white or light thermoplastic sheet whose seams are welded rather than glued. The light surface reflects sun, which the Rowhouse Manual recommends for the rooms under a flat roof.
Built-up
Built-up roofs were the traditional system and are still allowed; the code requires 1/4 inch per foot except for coal-tar built-up roofs, which may go to 1/8 inch. The EZ standard does not allow old gravel to be put back: "aggregate surface materials" are on its list of items that may not be reinstalled.
A surface layer is not a new roof
Silver paint and liquid surface products are sold for flat roofs. They can protect a sound membrane from sunlight, which the manual recommends, but they are upkeep, not a replacement. The City's permit and the contractor's certificate (TP_019_F) are for new roof coverings, and this line is for that work.
What decides the choice
- The existing layers. Two or more coverings, or a wet or deteriorated roof, mean a tear-off under the EZ standard, and then every system is open.
- What sits on the roof. A roof deck, solar racking or a condenser needs details the membrane maker allows; relocating rooftop equipment takes the job out of the EZ permit.
- Insulation. If insulation is altered it must reach R-49 under the standard, which can raise the roof edge at the parapet.
- The neighbors. The new membrane has to be joined to whatever is on the roofs next door.
Read next: rowhouse flat roofs and what a replacement includes.