How to fix a leak on a flat roof

If a flat roof shows signs of age-related deterioration, patching various leaks may be an ongoing project. If this is the case, it may be time to call in a professional to replace the entire roof. On the other hand, if a leak is caused by either a lifted section of roofing felt, a tear or crack in the material, or a faulty section of galvanised flashing, you can successfully repair the roof yourself.

Examine your roof carefully. Although the roof may appear flat, it has a gradual slope to assist run-off. Also, because water travels, the leak may not be directly above the room with a dripping ceiling or damp walls. Sweep roofing chips to one side with a stiff-bristle broom. Look for blisters, cracks and tears or lifted seams in the roofing felt. Mark any suspect areas with a piece of chalk. Inspect all galvanised metal flashing around the perimeter, vent pipes and chimneys for raised or damaged sections. Look for evidence of dried-up standing water. These areas are indicated by concentric rings of dirt left behind when pooled water evaporates.

Sweep the surface of a blister with a stiff-bristle broom to clear it of debris and roofing chips. Wash the area with a mixture of household detergent and water to clean away mould and dirt. Press the surface lightly and examine for cracks or tears. If the felt appears sound, leave it undisturbed for now; you can always come back later if the leak persists.

Cut a shallow "X" across the blister with a sharp utility knife. Cut deep enough to part the surface layer without damaging the under layer. Carefully peel the segments back. Spray the area with fungicide. Allow adequate time for the chemical to kill any mould. Dry the area thoroughly with a heat gun. Keep the nozzle moving to prevent the felt from melting.

Apply a liberal amount of bitumen adhesive to the underside of the surface layer and the top of the under layer with a 2-inch paintbrush. Fold the four segments down and press into place. Run a small rubber roller over the repair to squeeze out air bubbles and to ensure full surface contact.

Cut out a square of new roofing felt with a pair of heavy scissors. The patch should overlap the blistered area by 2 inches on all sides. Round off the corners of the patch. Apply a thick layer of bitumen cement to the roof surface with a 2-inch paintbrush and carefully position the patch over the repair. Using the rubber roller, roll lengthwise and crosswise, but don't press hard enough to squeeze the adhesive out. Coat the edges of the patch with a 2-inch-wide overlapping layer of bitumen adhesive. Sprinkle roofing chips over the repair.

Lift the edges of a tear or split carefully without tearing the felt further. Sweep out any loose debris with a 1/2-inch paintbrush. Spray fungicide into the damaged area and allow enough time for the chemical to work. Carefully dry the underside with a heat gun to avoid melting the felt. Use the paintbrush to coat the underside liberally with bitumen adhesive. Press the surface down with the rubber roller.

Apply a strip of felt coated with bitumen adhesive over the repair. Overlap all sides by 2 inches. Roll the patch gently to squeeze out air bubbles and to ensure full adhesion. Coat the edges of the strip with another 2-inch-wide layer of bitumen. Sprinkle roofing chips over the repair.

Lift any loose overlapping seam. Clean out debris with the 1/2-inch paintbrush and treat with fungicide. Apply bitumen adhesive to the underside and flatten with the rubber roller. Place a thick plank over the repair and weigh it down with a couple of buckets filled with water to prevent the seam from curling back while the adhesive cures. Leave overnight. Apply a 2-inch overlapping patch of felt to the seam. Coat the seam with another 2-inch-wide layer of bitumen and sprinkle with roofing chips.

Apply a wide strip of commercially available self-adhesive roof repair tape to any damaged or suspect areas of galvanised flashing along the roof edge. Follow the manufacturer's instructions carefully. Roughen the surface of the metal with a strip of coarse emery cloth and clean the area thoroughly before applying the repair tape.

Apply long strips of repair tape. Have a helper stretch the other end of the tape to ensure perfect alignment. Do not kink or crease the tape during application. Overlap joints and separate horizontal strips by at least half an inch. Start at the lowest point on the roof and work upward when applying overlapping layers. Press the surface and edges down firmly with the rubber roller to ensure perfect adhesion. Repeat around damaged vent pipes and chimney flashing.

Sweep identified water-pooling areas thoroughly with a stiff-bristle broom. Apply a liberal coating of fungicide and allow enough time to dry. Following the manufacturer's directions, pour enough bitumen or mastic-based roofing compound into depressions to cover the pooling area. Allow time for the material to settle and then spread roofing chips over the area.


Wear latex gloves, kneepads and eye protection while carrying out repairs. Various repair kits containing adhesives and reinforced repair tape are available through major home improvement outlets and over the Internet. Sprinkle fresh roofing chips over bitumen adhesive repairs for added protection. Although flat-roof leaks can travel a considerable distance before showing up, pay special attention to areas directly over damp interior walls or wet patches on the ceiling. Do not attempt repairs on a damp roof. Do your repairs after week or two of sunny, dry weather.

Things You'll Need

  • Stiff-bristle broom
  • Household detergent
  • Buckets
  • Utility knife
  • Spray fungicide
  • Heat gun
  • Bitumen adhesive
  • 2-inch paintbrush
  • Small rubber roller
  • Pair of heavy scissors
  • 1/2-inch paintbrush
  • Thick plank
  • Self-adhesive roof repair tape
  • Coarse emery cloth
  • Bitumen or mastic-based roofing compound
  • Roofing chips
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About the Author

After graduating from the University of the Witwatersrand and qualifying as an aircraft engineer, Ian Kelly joined a Kitchen remodeling company and qualified as a Certified Kitchen Designer (CKD). Kelly then established an organization specializing in home improvement, including repair and maintenance of household appliances, garden equipment and lawn mowers.