Roofing services
Flat roof replacement: membrane, drainage and seams
Flat roof replacement removes a failed low-slope covering and installs a new membrane system: EPDM rubber, TPO or PVC single-ply, modified bitumen, or built-up tar and gravel.
Written by HyreRoof Research Desk Primary-source research and fact checking
The short answer
Flat roof replacement removes a failed low-slope covering and installs a new membrane system: EPDM rubber, TPO or PVC single-ply, modified bitumen, or built-up tar and gravel. On a flat roof the membrane matters less than the drainage under it and the seams across it, because standing water finds every weak lap.
What is different about a flat roof?
The model code asks for a design slope of at least a quarter inch per foot on membrane roofs, so a replacement is the moment to fix slope with tapered insulation or crickets.
TPO and PVC seams are heat-welded; EPDM seams are taped. Ask how the seams will be made and tested.
Code bars a new layer over a wet roof or over two existing layers.
Low-slope work uses different tools and details than shingles. See low-slope roof.
What should be checked before a flat roof is quoted?
Ponding marks, dirt rings and low spots after rain. See roof ponding.
How many, where, and whether they sit at the low points.
A core cut or moisture survey decides whether recover is even allowed.
Two or more existing applications rule out another layer on top.
Where the flat roof meets a wall is where most leaks start.
Curbs, vents and condensers each need their own flashing detail.
What happens during a flat roof replacement?
Wet insulation and old plies come off. A soft deck is replaced at a per-sheet or per-foot rate agreed beforehand.
Tapered insulation, crickets behind curbs and chimneys, or added drains move water off instead of letting it sit.
Fastened or adhered to the deck. The cover board gives the membrane a firm, even base.
Sheets are rolled out and seamed by welding, tape or torch, depending on the system.
Membrane turns up walls and around every pipe and curb, terminated with metal bars or counterflashing.
Edge metal goes on, seams are probed, and drains are run before the crew leaves.
Which membrane fits which roof?
Five systems cover nearly every residential flat or near-flat section. Each has a different seam, a different failure and a different repair.
| Membrane | What it means for you |
|---|---|
| EPDM (rubber) | A thermoset sheet, usually black or white, seamed with pressure-sensitive tape. Large sheets mean few seams; repairs are patch-and-prime. Seam adhesion and flashing laps are where it fails. |
| TPO | A thermoplastic sheet whose seams are melted together with a hot-air welder. Light colors reflect heat. Weld quality depends on the crew and the day’s temperature, so ask how seams are probed. |
| PVC | Also heat-welded. Holds up around kitchen exhaust and some chemical exposure. Costs more to buy and fewer crews install it. |
| Modified bitumen | Asphalt sheets in layers, applied by torch, hot asphalt, cold adhesive or self-adhesion. Torch application is an open flame on your roof; see torch-down roofing before you agree to it. |
| Built-up (tar and gravel) | Alternating plies of felt and bitumen under gravel. Heavy and durable, but leaks are hard to trace through the gravel, and tear-off is slow and messy. |
Membrane categories as described by the Single Ply Roofing Industry and the IRC 2021 R905 sections cited below.
What it costs
We publish no price for flat roof replacement yet: sourced ranges are pending until we hold at least 12 dated quotes. These are the lines that move a flat roof bid.
Build your own number with the roof cost calculator or line up bids in the estimate comparison tool, and see what a roofing price is built from.
| Factor | Why it changes the number |
|---|---|
| Tear-off and wet insulation | Built-up and gravel roofs are slow to remove; saturated insulation adds disposal. |
| Drainage correction | Tapered insulation and crickets are material and labor a straight re-cover does not include. |
| Membrane and thickness | Different systems and sheet thicknesses carry different material prices and crew rates. |
| Penetrations and walls | Each curb, pipe and parapet run is a hand-flashed detail. |
| Access | Hoisting material to a second-storey deck or over a building adds time. |
HyreRoof publishes no national average price for this work. Regional labor, disposal and material rates move it far enough that a single figure would mislead.
What to ask before you sign
If a recover, how did you confirm the existing roof is dry?
Name the tapered insulation, crickets or drains.
Get the product name on the contract.
Welded, taped or torched, and probed how?
If yes, what is the fire watch after the torch work ends?
Material and workmanship separately, with lengths.
Where this goes wrong
Code bars recover over a water-soaked roof, and trapped water rots the deck from below.
A new sheet over the same low spots repeats the old failure.
Old termination bars and counterflashing are where a new flat roof often leaks first.
Your inquiry
Tell us about the work
Nothing goes to a roofer unless you agree.
A person reads it and replies by email, free and with no obligation. Matching is not live yet, so this is not a quote. If water is coming in now, call a local roofer directly and check their license first.
Find a Roofer
One short form: your address and how to reach you. A person at HyreRoof reads it and replies.
Questions
Is EPDM or TPO better for a house?
Can a new flat roof go over the old one?
Why does my flat roof hold water?
Do I need to be on the roof to judge the work?
Written and audited by
HyreRoof Research Desk
Primary-source research, data analysis and fact checking
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Sources & retrieval dates
Information on this page is general and may vary by location, building code, roof design, insurance policy and your own circumstances. It is not insurance, legal or engineering advice.
HyreRoof is not a roofing contractor: we do not repair, replace or inspect roofs, and we do not supervise or warrant roofing work. This page explains the work so you can judge a quote and a contractor.