Seam Failure

Seam failure is the loss of a watertight bond at the laps where sheets of roofing meet. Because almost every low-slope membrane is made from many sheets joined in the field, the seams are often the weakest part of the system. Seams are also a concern on metal roofs and on taped or lapped underlayments.

Single-ply membranes

Thermoplastic membranes such as TPO and PVC are joined by hot-air welding, which fuses the two sheets together. Weld quality depends on temperature, travel speed, pressure, cleanliness and conditions such as wind, sun and the substrate below. Cold welds, scorched welds, contamination, wrinkles and fishmouths can all leave weak spots. Manufacturers typically call for test welds through the day and for every seam to be probed at the end of each work day.

EPDM sheets are joined with seam tape or adhesive. Problems arise from poor cleaning or priming, insufficient rolling, or ageing of older liquid-adhesive seams. Shrinkage of the membrane can also put seams under stress.

Bituminous and other systems

In built-up and modified bitumen roofs, laps can open where bitumen was too cool, where there was too little of it, or where the roof has moved. On metal roofs, standing seams can open if not fully formed, and lapped seams on exposed-fastener panels may leak where sealant tape has failed.

Detection

Visual signs include open edges, fishmouths and wrinkles at laps, loose flaps and dirt lines in the lap. A seam probe, a blunt hooked tool drawn along the edge of a welded lap, finds unbonded sections. Destructive cut tests and peel tests check bond strength. On a finished roof, leaks traced to seams can be confirmed with electronic leak detection where the system allows.

Repair

Repairs follow the membrane manufacturer's procedures: thermoplastic seams can often be re-welded or covered with a welded patch after cleaning; EPDM seams are cleaned, primed and covered with cover tape. Widespread seam failure can indicate a systemic installation or product problem and may justify a professional survey.