Modified bitumen roof lap failure is when the side laps, end laps or T-joints of the membrane open up, and on self-adhered sheets, common on porches, garages and low additions, it is the usual way these roofs fail. The causes are installation below the manufacturer’s minimum temperature, dusty or damp surfaces, skipped or still-wet primer, laps that were never rolled hard, and mineral-surfaced end laps that needed heat or mastic. The fix is to clean, prime and reseal laps with the manufacturer’s materials, strip in failed laps, and replace the membrane when laps fail across the roof. This page is general information; your roofer and the membrane manufacturer make the final call.
On this page: Signs · Causes · Checks · Repairs · Basis · When to call
Signs of lap failure
- Side or end laps visibly lifting, especially at the low edge of each sheet.
- Small openings at T-joints where three layers meet.
- Wrinkles or “dog-legged” sheets that run crooked across the roof.
- Leaks inside that line up with a lap rather than a drain or wall.
- Smears of surface mastic over laps from earlier patch attempts.
Why self-adhered laps fail
- Too cold. Air, product and deck must be at or above the manufacturer’s minimum.
- Dirty or wet surface. Dust, oil, moisture or debris stop adhesion.
- Primer problems. No primer where required, or solvent primer still wet when the sheet went down.
- No rolling. Without hot asphalt to flow into gaps, laps and T-joints need a heavy roller.
- Granule-to-granule end laps that the manufacturer wanted heat-welded or set in compatible mastic.
- Sun-damaged rolls. UV on unwrapped rolls can weaken the adhesive before installation.
What you can check
- From a window above the roof, look for lifted lap edges and crooked sheets.
- Note where water shows up inside and whether it follows a lap line.
- Check whether water sits on the roof two days after rain. Ponding stresses laps; see flat roof ponding.
- Find the install date and season, since a winter install is a clue. On the Front Range, a sheet put down on a cold fall or winter day may never have reached the adhesive’s minimum temperature.
- Gather the patch history. Repeated mastic patches on the same lap tell a roofer the lap was never bonded, not just worn.
Repairs
| Repair | When it fits |
|---|---|
| Clean, prime per the manufacturer and reseal or roll the lap | A few laps lifting on otherwise sound sheet |
| Strip in failed laps and T-joints with compatible membrane | Several laps or joints are open |
| Correct drainage and then repair laps | Water sits on the laps |
| Replace the membrane | Laps fail across the roof |
Fixes that fail: surface mastic smeared over an open lap, coatings painted across failed laps, and roofing cement on a membrane its maker does not approve.
Code and industry basis
The 2024 IRC, section R905.11, requires modified bitumen roofs to have at least a 2 percent (1/4 inch per foot) slope and to be installed per the code and the manufacturer’s instructions. ARMA’s technical bulletin on self-adhesive modified bitumen membranes (April 2024) covers minimum application temperatures (typically about 40 to 50 F), clean and dry substrates, primers and solvent cautions, heavy rolling at laps and T-joints, heat welding or mastic at mineral-surfaced end laps, and storing rolls out of sun. For larger roofs, see commercial roof repair and low-slope systems compared.
When to call
Call when laps lift, when a porch or garage roof leaks along a straight line, or when a low roof installed in cold weather starts to wrinkle. Related guides: shingles on low-pitch roofs, single-ply seam failure and the roof problems guide.
Get the laps checked
Integrity Roofing and Painting is veteran-owned and family-run since 1990, an Owens Corning Platinum Preferred contractor with 650+ Google reviews. We photograph laps, T-joints and drainage and tell you whether a lap repair will hold. 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: ARMA, Self-Adhesive Modified Bituminous Roofing Membranes (April 2024); 2024 IRC Chapter 9, section R905.11 (UpCodes).
