How to Stop a Leak on a Flat Roof

Updated February 21, 2017

The roof of your home is the first line of protection from precipitation. Most roofs are slanted, which allows the water to easily rinse off. However, if there is a portion of your home which has a flat roof, the water may begin to pool, which can cause leaks. If you have a leaky flat roof, you will need to take steps to repair it before it causes more damage to your home.

Sweep off any debris on the roof to give yourself a clean area to work in. The cement will not bond to the roof if there are is any dust or dirt in the way of the patch.

Locate any cracks or openings in the existing roof. Widen the cracks slightly (1 to 2 inches) if the crack is minute to give yourself a nice working area.

Place the roofing cement into the crack with the putty knife. Press the cement as far into the roof crack as you can in order to completely fill it and create a good seal.

Continue to add cement to the crack until you can no longer fit any into the opening. Build up the cement around the crack until there is a thick layer which extends six inches past the opening of the crack on either side.

Place a strip of fibreglass reinforcing fabric over the cement. Press it down to ensure a tight seal.

Add another layer of the roofing cement to the crack, completely covering the fibreglass. Extend the cement out six inches past the end of the crack like you did in Step 4. Allow the patch to dry.


The fibreglass will add life to your patch and provide added protection to your home. If you have a larger repair you need to do, you can use a Type IV fibreglass felt which can be laid into the first layer of cement.


Try to stay away from liquid asphalt repair products as these may not last and can make it difficult for a professional to find the leak if it doesn't work.

Things You'll Need

  • Roofing cement
  • Broom
  • Putty knife or trowel
  • Fibreglass reinforcing fabric
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About the Author

R.L. Cultrona is a San Diego native and a graduate of San Diego State University. She holds a Bachelor of Arts in theater, television and film with a minor in communications and political science. She began writing online instructional articles in June 2009.