Flat roof repairs range from minor patching of isolated defects to comprehensive repairs of significant areas of damage. Understanding the type and extent of the defect determines the appropriate repair method.

Diagnosing the Problem

Before attempting any repair, identify exactly where water is entering. A leak inside the building may not be directly below the defect on the roof surface, as water can track along the deck before dripping through. Inspect the roof surface, the flashings at all edges and abutments, the drainage outlets, and any penetrations through the roof. The most common sources of leakage are flashing failure at upstands, cracked or split felt at joints, and blocked or deteriorated drainage outlets that allow water to pond and eventually penetrate.

Repairing Blisters in Felt

Blisters in felt are caused by trapped moisture or air expanding under the heat of the sun. Small blisters that have not split can be left if the felt is otherwise sound. Split blisters that have opened must be repaired: cut a cross through the blister, peel back the four flaps, dry the area thoroughly with a heat gun, apply bitumen adhesive, press the flaps back down, and cover with a felt patch bonded over the repair area extending 150mm beyond the split in all directions.

Patching Cracks and Splits

For isolated cracks and splits in the felt surface, a self-adhesive bituminous repair tape provides a quick and effective repair. Clean and dry the area, cut the tape to extend 100mm beyond the defect on all sides, press firmly into place, and roller to ensure full adhesion. For a more permanent repair on a three-layer felt roof, cut back to sound material and bond a new felt patch of the same specification using cold adhesive or torch-on technique.

Resealing Flashings

Failed mortar pointing or sealant at the top of upstand flashings is one of the most common causes of flat roof leaks and is one of the easiest to repair. Rake out failed mortar or sealant, clean the joint, and repoint with appropriate mortar or apply new sealant rated for outdoor use. Ensure water cannot track behind the flashing at the top edge.

Summary

Diagnose the defect location before repairing: inspect flashings, drainage, and the felt surface. Repair blisters by cutting, drying, and patching. Repair cracks with self-adhesive tape or a felt patch. Reseal failing flashings with new mortar or sealant. Use liquid waterproofing coatings as a life-extension treatment for aged but otherwise sound surfaces. Extensive surface failure requires full replacement.