Quality with FransThe common defects
| Defect | What you see | Likely cause | How to check | Fix |
|---|---|---|---|---|
| Blisters | Raised domes that move underfoot | Moisture or air trapped under or between layers, expanding in the sun | Tap test; cut one to see if it goes down to the substrate | Cut, dry, re-bond and patch; small stable ones can be monitored |
| Poor lap bond | Lifting edges; a trowel slides into the lap | Too little heat, dirty or gritty lap, lap too narrow | Probe the laps; look for a continuous bitumen bead | Re-heat and re-seal, or strip over |
| Fishmouths and wrinkles | Open "mouth" at a lap edge, creases | Sheet not relaxed or not aligned before torching | Walk every lap | Cut out and patch before the next layer |
| Upstand failure | Wet base of parapet, open or slipping flashing | Upstand too low, no termination bar or chase, poor corners | Measure the height above the finished surface | Rebuild to at least 150 mm above the finished surface |
| Ponding | Water still standing about 48 hours after rain, dirt rings, algae | Falls too flat, back-falls, deck deflection | Level or straightedge survey | Re-screed to falls or add outlets |
| Outlet failure | Leak at the drain | No reinforcing piece, membrane not lapped into the outlet, laps against the flow | Lift the clamp ring and look | Add a reinforcing piece; lap with the flow |
| Burnt membrane | Charred or shiny surface, exposed reinforcement | Flame held still or too close | Visual check, brittle spots | Patch or replace; heat-damaged reinforcement means the sheet can't be used |
| Trade damage | Punctures, scuffs, crushed granules | No walkways or protection while other trades work | Walk the roof after each trade | Patch; fit walkway pads and protection boards |
Laps and falls: check the data sheet, not memory
Lap widths are not universal. Manufacturers' guides I checked range from about 60 mm side / 100 mm end (Sika South Africa) to 75/100 mm, 90/150 mm and 102/152 mm for other products. The product data sheet for the membrane on your site is the one that counts. Most guides also ask for a small continuous bitumen bead at the lap edge, although one Canadian industry guide notes a lap can be fully bonded without a visible bead, so probe it as well.
For falls, UK guidance (BS 6229 as summarised by industry bodies) sets a minimum finished fall of 1:80, and recommends designing steeper (around 1:40 to 1:60) to allow for building tolerances. Other countries use slightly different numbers, so follow your specification. Upstands are commonly required to be at least 150 mm above the finished roof surface, with lower heights only allowed at specially detailed accessible thresholds.
Why this matters for QA/QC
Torch-on is a workmanship product. The membrane can be first class, but a cold lap, a 100 mm upstand or a back-fall will still leak, and usually only after the screed, tiles or plant are on top and the contractor has left site. That is when repairs get expensive and the arguments start.
The good news is that these faults are visible at the right stage. Your hold points should be: substrate and primer before the first sheet, laps and details during laying, and a water test or integrity test before anything covers the membrane. On a water (flood) test, be careful: guidance differs. Some waterproofing manufacturers recommend at least 24 hours of shallow flooding (around 50 mm at the high point), but the US roofing association warns against flood-testing whole roof systems because of the load on the structure and drains. Check the structural design and the manufacturer's instructions first, and agree the test method in the ITP.
Check on your site tomorrow
- Run a trowel along 10 random laps. Look for a continuous bead and no gaps.
- Measure five parapet upstands. They should be at least 150 mm above the finished surface (or your spec).
- After rain or a hose test, mark any puddles still there the next day and check the falls.
- Lift the clamp ring at each outlet: is the membrane lapped in, with laps facing the drain?
- Check that walkway pads and protection boards are down where other trades are working.
The 31-point checklist I use before every pour: bar sizes, spacing, cover and laps against the approved drawing.
Download the checklist
Sources: Sika SA: Torch-on application manual · IKO: APP application manual · Johns Manville: End lap bulletin · Roofing Canada: Torch-welding guidelines · LRWA: Guidance Note 7 (falls) · IKO: BS 6229 guidance · NHBC Standards 7.1 · NRC Canada: Roof blisters · Professional Roofing (NRCA): Flood testing · Ardex: Flood testing bulletin · IIBEC: Walkways. Summary in my own words; your specification and the membrane manufacturer's instructions apply.