Flat Roof Repair and Maintenance

Without a significant slope, a flat roof is more vulnerable to water damage. A waterproof membrane prevents leaks but needs periodic maintenance.

A flat roof is the easiest type to build and offers perks you don’t get with a sloping roof.

For one, it’s a potential garden area. It also gives you a place to put your lawn chairs, hang out in the sun and gather with friends. And it’s easy to walk on, simplifying maintenance and repair.

The biggest drawback is the potential for leaks. Most flat roofs aren’t actually flat; they slope slightly to one side (a 1/4-in. per foot is considered minimal) to allow drainage. After a heavy rain, puddles are inevitable. And the longer water stands, the more likely it is to find a way through the roof covering.

Still, a flat roof shouldn’t leak for 25 to 30 years, the length of a typical roof warranty. But warranties don’t mean much without proper maintenance.

Flat Roof Maintenance

The whole point of roof maintenance is to prevent leaks. You can avoid leaks if you do the following regularly:

Keep the drains clear

You’ll find the drain openings in the barrier on the downward-slope side of the roof. When they’re blocked by leaves or other debris, water pools on the roof.

Clear the drains by pulling gunk out of the openings in the raised sides and putting it in a bucket. Do this at least once every month or two, more often in the fall when leaves fly around. While you’re at it, clear out the downspouts with a garden hose.

Dirt and silt can stick in the drain openings and clog the downspouts. That’s less likely to happen if the roof is clean. Regular sweeping is another bi-monthly task. Collect the sweepings in the bucket and bring it all down from the roof when you’re finished.

Prune overhanging branches

Branches that touch the roof can scratch and damage the covering. Those that aren’t actually touching the roof can break off and fall during a storm, also causing damage. Branches are also sources of leaves, seeds and small twigs that can block the drains.

Inspect the roofing

Most flat roofing systems have seams. While you’re cleaning debris, check the seams for signs of lifting or separating. Even small cracks can cause leaks and should be repaired promptly.

Check also for blisters that occur when air is trapped underneath the membrane. They need to be “popped” to let the air out, then patched with roofing cement or overlaid with a patch of the same roofing material.

If you have a spray polyurethane foam (SPF) roof, which is usually white, check for areas of discoloration. That means the material has deteriorated. These areas need to be cut out and filled with caulk.

Inspect the flashing

Flashing protects plumbing stacks, roof vents and other protrusions to prevent leaks. Look for cracks or gaps that could allow water to seep in. Repair and cracks with roofing caulk.

Flat Roof Repair

There are several flat roofing systems in common use. When leaks occur, you can repair most with a patch of the same material as the roofing.

For materials like modified bitumen (similar to asphalt roll roofing), apply the patch with roofing tar. Other materials, like EPDM rubber, TPO (a type of plastic) and PVC (another plastic), need to be heat-sealed.

EPDM roofs tend to be the most DIY-friendly to repair using a heat gun and/or adhesives. But TPO and PVC roofs are trickier and best left to pros.

Pinpointing a leak can be challenging because water may seep under the membrane for a considerable distance before finding a path through a seam. Here’s a savvy way to go about it:

Go indoors, note where water is dripping or the roof decking or drywall is discolored, and measure the distances from that spot to the walls on either side. Use those measurements to pinpoint the same spot on top of the roof. Then proceed sideways or along the upward slope until you find the damaged seam or deteriorated section.

If that doesn’t identify the spot of the leak, you may need an oversized patch that covers all the likely spots where water could enter. Or, on the advice of a contractor, you could apply a liquid elastomeric membrane over the affected area or the entire roof with a paint roller.

Leaks are easier to spot on an SPF roof because the foam and its elastomeric coating are usually deteriorating and discolored around where water gets in. To repair, cut out all the discolored material in a V-shaped depression, then fill it with elastomeric caulk.

Flat Roof Replacement

When the roof covering exceeds its warranty period, it’s time to think about replacement. But if you notice any of the following conditions, replacement should happen sooner than later:

  • Damage to the membrane, seams or flashing in several places;
  • Multiple leaks;
  • Water habitually pools in certain areas;
  • There’s impact damage from a fallen tree branch or other debris.

Leave roof replacement to the pros. They’ll remove the old membrane, rotted plywood and fascia, damaged roof vents and plumbing stacks. They may also replace insulation. After cleaning up, they’ll lay a new membrane, re-flash all the vents and install new perimeter flashing and fascia.

Cost varies depending on roofing materials, but it usually ranges from $4 to $13 per square foot.

Chris Deziel
Chris Deziel has been active in the building trades for more than 30 years. He helped build a small city in the Oregon desert from the ground up and helped establish two landscaping companies. He has worked as a carpenter, plumber and furniture refinisher. Deziel has been writing DIY articles since 2010 and has worked as an online consultant, most recently with Home Depot's Pro Referral service. His work has been published on Landlordology, Apartments.com and Hunker. Deziel has also published science content and is an avid musician.