How to Repair a Flat Roof That Is Leaking

So you've got a little leakage problem in your flat-roofed house. Or a major one. Don't worry, you might be able to fix the leak yourself. To repair a flat roof that is leaking you need to first be able to identify where the leak is coming from.

This is the part where you put your keen observation skills to work, looking for imperfections and following water stains like a detective. Once you've identified the culprit, you can get to work actually repairing the section of flat roof that is to blame.

Identify the Source of the Leak

  1. Stand outside your house with a pair of binoculars and look closely at every square inch of your roof. Take your time and enjoy the weather, because this will take a little while if you are thorough. And thorough you should be. Here you should be looking for any and all irregularities: cracks, tears, rot, missing pieces of sheathing and the like.

  2. Get on the roof and mark any irregularities you found in Step 1. Mark them by placing an object that will not blow away. While you are up there, inspect everywhere that one type of material meets another. For example, check all flashings, air conditioning units, chimneys, vents, skylights, anyplace where brick meets sheathing and so on.

  3. Stay on the roof and do an overall inspection of the roof, pace by pace, one side of the roof at a time. There may be more than one imperfection to tend to. Be overly judgmental and make this single inspection count. No use having to do multiple inspections and repairs.

  4. Now go inside your attic and use a flashlight to follow any water trails to where they meet the ceiling. Poke a nail up through the offending area to locate it once you get back on the roof. Water trails are devious, winding creatures, so be thorough in this step.

Repair the Source of the Leak

  1. To repair a crack in the sheathing, first clean the offending area by scraping off any dirt or debris. Use a trowel to squeeze roofing cement into the crack. Let the cement completely cover and overlap the crack or imperfection in the sheathing. Deeply embed a strip of roof patching fabric into the cement. Add more cement, feather the cement for drainage. You are done with that crack.

  2. To repair a bubble in the sheathing, cut an \"X\" in the sheathing with a carbide blade or heavy-duty razor. Peel up the wings of sheathing, sweep the area free of dirt and debris, use a rag to sop up any moisture you find, and let the remaining water residue dry out in the sun on a warm day for a few hours. Then work some roofing cement into the offending area and cover it back over with the wings of sheathing. Flatten thoroughly, and continue on to Step 3.

  3. Apply more cement overlapping the \"X\", embed some patching fabric. Apply yet another layer of cement, more patching fabric, and a final layer of cement. Feather the cement into the sheathing for drainage.

  4. To repair cracked or separated cement between roof features (chimneys, vents, etc.) and the roof sheathing, first clean the offending area. Peel, scrape, or cut off any old cement that might get in the way. You may need to remove the fixture and clean underneath it. Apply a coat of cement, working it between the feature and the sheathing. Finish it off by feathering the cement into the roof sheathing for drainage.

  5. Tip

    When working on any non-metal roof, you should wear flat-soled shoes to prevent marring or damaging the surface.\n\nUse latex gloves to minimize skin exposure to roofing cement.\n\nYou can wash off roofing cement from your hands by by rubbing them with penetrating oil (such as WD40).


    Always use caution when walking on rooftops. Never go onto a roof while under the influence of alcohol or other intoxicant.