Single-ply membrane roofs (TPO, PVC and EPDM) most often leak at seams: laps that were cold-welded or burned, oxidized sheets that were not cleaned before welding, T-joints without the patches the manufacturer requires, and adhesive or tape seams that aged and let go. The signs are open laps, fish-mouths and splits along a seam line. The fix is to clean and re-weld or re-tape the seam with the membrane maker’s materials, add cover strips where needed, and replace the membrane only when seams fail across the roof. This page is general information; your roofer and the membrane manufacturer make the final call.
On this page: Signs · Causes · Owner checks · Repairs · Basis · When to call
Signs of seam failure
- A lap edge you can see lifting, or a “fish-mouth” opening where the top sheet puckers away.
- Splits in the sheet right beside a weld line.
- Dirt lines that follow a seam, showing water runs along it.
- Interior leaks that follow the line of a seam rather than a drain or penetration.
- Patches over patches along the same joint.
Why seams fail
- Cold welds. The welder ran too cool or too fast, so the sheets never fused.
- Burned welds. Too hot or too slow scorches the sheet beside the seam.
- Dirty or weathered sheet. Oxidized TPO that was not cleaned before welding.
- Missing T-joint patches where three sheets meet, if the manufacturer requires them.
- Aging tape and adhesive on EPDM laps.
- Ponding. Water sitting on seams speeds wear; see flat roof ponding.
What an owner can check
- From a window or roof hatch, without walking the roof, look for lifted edges, puckers and dirt lines along seams.
- Note where and when it leaks inside, including wind direction.
- Check drains and scuppers for clogs, which raise water over seams.
- Keep records of repairs and the membrane brand, since warranties depend on them.
Repairs
| Repair | When it fits |
|---|---|
| Clean and re-weld a thermoplastic seam (TPO, PVC) following the maker’s prep | A short run of open or cold-welded seam |
| Clean, prime and re-tape an EPDM seam | Tape or adhesive lap has let go |
| Install cover strips or patches over long seam runs and T-joints | Several seams are weak |
| Replace the membrane | Seam failure is widespread or insulation is soaked |
Fixes that fail: asphalt mastic or general caulk on a thermoplastic seam, coatings over open laps, and patching only the spot above the drip.
Code and industry basis
The 2024 IRC, section R905.12, covers single-ply roofing (EPDM, TPO and PVC), requires at least a 2 percent (1/4 inch per foot) slope for drainage, and says installation follows the code and the manufacturer’s instructions. Stephen Sharp’s Professional Roofing article on TPO (June 2002) describes TPO’s narrow welding window, cold and burned welds, careful seam probing, and the cleaning steps weathered sheet needs before welding. On larger buildings, see our commercial roof repair and low-slope systems compared pages.
When to call
Call when you see an open lap, when a leak follows a seam line, or before a warranty runs out so the manufacturer can be involved. Related guides: modified bitumen lap failures, ponding water and the roof problems guide.
Get the seams checked
Integrity Roofing and Painting is veteran-owned and family-run since 1990, an Owens Corning Platinum Preferred contractor with 650+ Google reviews. We probe and photograph seams near the leak and upslope of it, and tell you whether this is a repair or a replacement conversation. Colorado Springs area: (719) 488-2800 or a free inspection. Denver metro: (303) 209-0152 or a free Denver inspection. Austin: (512) 610-5897 or a free Austin inspection. Same-business-day reply.
Sources: 2024 IRC Chapter 9, section R905.12 (UpCodes); Professional Roofing (NRCA), Sharp, TPOs (2002).
