Thermoplastic Roof System Installation Performance
Domain 2 tests execution and verification: welder setup, trial welds, seam technique, probing, flashing, repair, weather limits, and protection of finished work. The exam asks what you would accept, reject, or redo on site.
Setup and trial welds
- Confirm membrane is clean and dry, substrate and ambient temperatures are within the manufacturer's limits, and the welder is calibrated.
- Make a trial weld on the same membrane and thickness for each machine, operator, and significant weather change. Probe or peel it before production welding.
- Never weld over dew, rain, frost, dirt, or ponded water. Cold conditions may require preheating; solvent adhesives need ventilation and temperature control.
Welding and testing
- Center the nozzle in the seam, maintain steady speed, and roll immediately behind the welder. Too fast or too cool gives a weak weld; too hot scorches or embrittles the sheet.
- Check both seam edges with a rounded probe. A sound weld resists probing and shows a continuous weld bead; do not stab the membrane.
- Record trial welds and repairs as required by the project.
Flashing, repair, and protection
- Use preformed corners, proper termination bars, staggered fasteners, and compatible sealant.
- For a failed seam, cut out the defect, clean and dry the area, install a same-membrane patch or cover strip, weld, and re-probe. Do not merely cap a cold seam.
- Protect finished membrane from traffic and dropped tools; use walkway pads where specified.
Worked example
A 60-mil TPO seam lifts at the edge under a probe, and no weld bead is visible. The likely cause is insufficient heat, excessive speed, or contamination. The correct response is to mark the area, cut out the failed weld, clean and dry the sheet, install a patch or cover strip, re-weld, and probe. Re-rolling the cold seam will not fuse it; adding sealant is not a structural repair.
Traps
- Changing welder settings without a new trial weld.
- Probing with a sharp tool and creating leaks.
- Welding dirty or wet membrane.
- Repairing thermoplastic with EPDM adhesive or patch.
- Failing to protect seams from traffic before closeout.