Flat roof repair: what to know

What the job involves, what changes the price, and what to ask a pro.

What flat roof repair is

Flat roof repair fixes a low-slope roof by sealing leaks, patching or replacing membrane sections, and clearing drains so water does not stand. People call when a ceiling stains after rain, when water sits on the roof for days, or when the membrane is blistering or tearing.

Signs you need it

  • Ponding water that stays on the roof a day or more after rain stops
  • Blisters, bubbles, cracks or tears in the membrane, especially along seams and around drains
  • Ceiling stains or dampness along a parapet, wall edge or near a roof drain
  • Slow or clogged roof drains and scuppers filled with leaves, grit or sediment
  • Lifting or open seams at the roof edge, around skylights, vents and wall junctions

What the pro does

  • Inspect the membrane for blisters, open seams, punctures and failed flashing, and mark each problem location
  • Clear the drains, scuppers and outlets so water flows freely, and check the drain bodies for cracks
  • Clean and dry the membrane before patching, since adhesives and seam tape fail on wet or dirty surfaces
  • Cut out and replace damaged membrane sections, or patch small punctures with a membrane patch and the matching sealant
  • Reseal penetrations, parapet flashing and seams, then test the repair with a controlled water test if the leak source is unclear

What changes the price

Every job is different, so we do not publish prices. These are the things that move the quote up or down.

  • Membrane type, such as TPO, EPDM, modified bitumen or built-up roofing, since each needs different materials and methods
  • Roof size and the number of seams, penetrations and drains that need attention
  • Extent of damaged membrane, and whether wet insulation under the damage has to be replaced
  • Roof access and height, plus any lift or staging needed for materials
  • Whether ponding comes from a sag in the deck that needs structural correction
  • Drain clearing, or replacement of drain bodies and scuppers
  • Whether the job is a patch, a full recoat or a re-cover

What a written quote should spell out

  • Membrane type, thickness and manufacturer, and the repair method for each area
  • Each seam, flashing and penetration named, with the material for each
  • Insulation replacement shown as a separate line, with how it is priced if found
  • Drain clearing and any drain body or scupper repair listed by location
  • The test method used to confirm the repair, such as a controlled water test
  • Warranty terms for the patch or the membrane system, and what conditions void them

Questions to ask before you hire

  • What membrane type is on my roof, and can you confirm the patch material is compatible with it?
  • How do you find the leak source on a flat roof, and will you test the repair with water?
  • Will the repair address the ponding, or is there a sag in the deck that needs correction?
  • How do you clean and dry the membrane before sealing it?
  • What happens if you find wet insulation under the membrane?
  • Can you show me a recent flat roof repair on a membrane like mine?
  • How will your crew stay safe near the roof edge, since there may be no guardrails?

Red flags

  • A roof that is only sprayed with sealant over blisters and cracks, without any repair
  • No plan for the drains, or a repair that leaves the ponding untreated
  • Patches applied over wet or dirty membrane without cleaning or priming
  • A full replacement recommended without showing the damage or explaining why a repair will not work
  • Refusal to name the membrane type or the adhesive and sealant to be used

What you can do yourself

You can safely clear leaves from gutters and downspouts you can reach from the ground, and watch the ceilings below for new stains. Flat roofs can be slippery and often have edges without guardrails, so do not walk the roof to inspect or patch it. Call a pro as soon as ponding lasts, the membrane blisters or tears, or a ceiling stains near a roof edge.

Licenses and permits

Flat roof repair is commonly done by licensed roofing contractors, and reroofing or structural work often needs a permit. Check your state and local licensing and building office requirements before work begins.

Flat roof repair questions

Can a flat roof be patched, or does it need replacing?

Many flat roofs can be patched, especially when the damage is limited to a few areas. A pro should test the membrane and insulation before recommending replacement.

Why does water pond on my flat roof?

Ponding usually comes from a low spot in the deck, a clogged drain, or a sag in the structure. A pro should check the slope and the drain height before sealing anything.

Should I coat the roof instead of repairing it?

Coating can help some membranes, but it does not fix a leak or a failed seam. Repair the damage first, then ask whether a coating suits your membrane.

How do I know the membrane is failing?

Look for blisters, cracks, open seams, exposed fabric and a chalky surface. Those signs mean the membrane is aging and a pro should inspect it.

About this guide

This national guide is written and checked by infinibell. Rules and conditions differ by place, so local pros are credited on guides for the cities they serve, not on national ones.

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