How do I fix a leaky concrete flat roof?

Updated February 21, 2017

Concrete roofs wear down over time, developing cracks, chips and fissures that allow water down into your home. The best way to waterproof these roofs is to apply a waterproof roof coating. Urethane offers the best degree of waterproofing for concrete and is not too expensive, according to Wade L. Vorley, a Seattle, Washington, roof consultant for more than 20 years. Work on this project on a clear, dry weekend when no rain is forecast, and expect to use a lot of elbow grease.

Get up on your roof on a dry day to find the leak. Look for rings of dirt or small holes in the concrete to indicate spots where large puddles form and leak. Also examine the flashings or metal edges of the roof, common locations for leaks. Cracks or breaks in flashings indicate leaks.

Clean the roof thoroughly, using either a pressure washer or a stiff bristle brush and water. Remove all dirt, pollen, mould and other contaminants so the protective coating can adhere and effectively stop leaks.

Cover leaky areas with rubber seam tape, which adds a waterproofing element. Do this from flashings to cracks in the concrete.

Pour urethane primer into a paint tray. Fit a paint roller with a roller cover with 1-inch nap, which helps the finish adhere to concrete. Work the roller back and forth until you cover the roof. Allow it to dry according to the recommended wait time listed on the primer can.

Apply a base coat of urethane waterproof roof coating in the same manner as you applied the primer. Let the first coat dry the recommended wait time, perhaps eight to 12 hours. Follow up with a second coat to fully waterproof your old leaky roof.


To clean your roof by hand, use a 10 per cent bleach solution and scrub all areas with a stiff bristle brush dipped in this solution.

Things You'll Need

  • Pressure washer (optional)
  • Bucket
  • Bleach
  • Stiff bristle scrub brush
  • Rubber seam tape
  • Urethane primer
  • Paint tray
  • Paint roller
  • 2 roller covers with 1-inch nap
  • Urethane waterproof roof coating
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