“From torching onto damp concrete to missing corner fillets, these 5 oversights cause 90% of roof leaks. Fix them before finishing your slab.”
Field Insights from 500+ Site Inspections
Most roof leaks are not caused by defective waterproofing rolls. Over 90% of failures happen due to simple application shortcuts on site. Here are the top 5 mistakes our inspection team sees and how to correct them:
Mistake #1: Skipping the Corner Fillet (Chamfer)
Bending a thick 4mm bitumen membrane into a sharp 90-degree right angle where the floor slab meets the parapet wall stresses the material. Within months, thermal movements tear the seam. The Fix: Always cast a 50mm concrete angle fillet before applying primer.
Mistake #2: Applying Primer over Wet or Dusty Concrete
Dust acts as a bond breaker. When primer is painted over construction debris, it peels off like plastic wrap. The Fix: Sweep and vacuum the slab clean. Ensure concrete moisture is under 5%.
Mistake #3: Insufficient Lap Overlaps
Skimping on roll overlaps to save material creates guaranteed water entry channels. The Fix: Maintain strict 100mm side laps and 150mm end laps. Ensure a neat 5mm bitumen bead squeezes out along the entire seam edge during torching.
Mistake #4: Not Terminating the Parapet Wall Flashing
Leaving the membrane cut flush at the top of the parapet allows wind-driven rain to run behind the sheet. The Fix: Chase a 25mm groove (reglet) into the parapet wall, tuck the membrane inside, anchor with counterflashing, and seal with high-grade polyurethane sealant.
Mistake #5: Skipping the Water Ponding Test
Never cover a waterproofing layer with screed or tiles without flood testing first. Fixing a leak after tiling requires breaking expensive finishes. The Fix: Plug drains and pond water for 24 hours.
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