Flat roof leaking: patch, coat or replace?

Patch: one failure on a sound roof

Most flat roofs leak at a seam, an edge, a drain or a pipe through the roof. Not in the middle. If one of those let go and the rest is sound, patch it.

The patch has to be the same family of material as the roof. Mixing materials on a flat roof is how a patch peels off in a year.

Coat: an aging roof with dry insulation

A coating adds life to a membrane that is worn but still holding. It only works over dry insulation.

Coat over wet insulation and you seal the water in. It keeps rotting underneath, and the next failure is bigger. Anyone quoting a coating should tell you how they know the insulation is dry.

Replace: wet insulation or failure everywhere

Replace when the insulation is soaked, when seams are opening across the whole roof, or when it has been patched so often the patches are the roof.

Replacement is also the time to fix the drainage. A roof that ponds after every rain will wear out its next membrane the same way.

Ponding water is the warning sign

Standing water that is still there days after rain wears the membrane faster in that spot and finds any weak seam. It points to a drainage or a deck problem, not only a membrane problem.

Tell us where it ponds and for how long. It changes which of the three options makes sense.

Side by side

• Patch: one failed seam, edge, drain or penetration. Rest of the roof sound.
• Coat: worn membrane, no active soaking, insulation confirmed dry.
• Replace: wet insulation, widespread seam failure, or a roof that is mostly patches.

Commercial building? Short-notice, after-hours and weekend work is available for commercial clients, quoted per job. See commercial roofing.