How to fix a flat roof

Updated April 17, 2017

Exposure to rain, wind and extreme temperatures can take its toll on flat roofs over time. Flat roofs can crack or split to produce holes and an uneven surface that allows rainwater to pool and penetrate the building below. Always replace an extensively damaged or heavily worn flat roof. However, you can repair isolated cracks or holes with a flat roof patching kit from a DIY or hardware store.

Climb to the flat roof safely using a ladder or raised platform.

Examine the roof to assess its condition. If the surface of the flat roof surface is covered with stone chips, sweep these into a pile with a stiff broom so that you can better inspect the roof for damage. Walk the roof and identify any cracks or holes, particularly in areas where the ceiling below is damp or water damaged.

Buy a flat roof patching kit from a DIY or hardware store. Choose from a range of patching kits for felt, bitumen, rubber or lead flashed flat roofs. Patching kits contain filling compound, primer and waterproof roofing tape with a speciality adhesive for repairing cracks and holes. Most kits include enough material to carry out more than one repair, but check the packaging for details and buy sufficient patching kits to repair all holes and cracks.

Pick a dry day to carry out the flat roof repair. Avoid very hot, sunny days that could make the bitumen or felt roof surface sticky.

Sweep all stone chips from the roof surface, if necessary. Store them in a canvas or rubble bag and lower them down to the ground by attaching a rope to the bag handles and lifting them carefully down. Spray the roof with fungicide to kill mould growth. Wait for the roof to dry.

Fill all splits and cracks in the flat roof surface with the filler compound before patching with tape. Used the supplied applicator and then spread the compound flat over the crack with a putty knife. Wait for the compound to dry and cool.

Spread the treated area with the speciality primer supplied in the patching kit. Again, use the applicator if one was supplied. If not, use a 1-inch regular paintbrush.

Cut the patching tape to length with a sharp utility knife. Place the patch on the treated area and press firmly down to seal it into place. Run your finger around the edges of the patch to seal the edges securely. Wait for the repair to cure. Check the manufacturer's details for curing times.

Hoist the bag of stone chips back to the roof. Recover the surface with the chips and use a rake to spread them evenly.


Blisters in flat roofs can indicate water ingress through a tiny hole or crack. Dry these out with a hot air gun before patching in the same way as outlined here.


Get a friend or neighbour to hold the ladder while you climb to the roof. Carefully follow manufacturer's safety and usage instructions when using patching kits. Wear protective gloves while using the filler compound and primer.

Things You'll Need

  • Ladder or platform
  • Stiff brush
  • Canvas or rubble bag
  • Rope
  • Flat roof patching kit
  • Utility knife
  • Putty knives
  • 1-inch paintbrush
  • Rake
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About the Author

Adrian Grahams began writing professionally in 1989 after training as a newspaper reporter. His work has been published online and in various newspapers, including "The Cornish Times" and "The Sunday Independent." Grahams specializes in technology and communications. He holds a Bachelor of Science, postgraduate diplomas in journalism and website design and is studying for an MBA.