A leak in a flat roof is rarely where it looks. Water gets in at one point, travels under the membrane or along the deck, and drips through somewhere else, often many feet away. That is why so many commercial leaks get patched three times before they stop. Here is where flat roofs actually leak, how a leak is traced, and what to do inside the building while you wait.
First: protect the inside
- Move what can be damaged: inventory, files, computers and anything on the floor under the drip.
- Kill power to wet fixtures: if water is near a light fixture, panel or outlet, shut that circuit off at the breaker.
- Catch and contain: buckets, plastic sheeting and wet-vacs. Photograph everything for insurance.
- Stay off the roof in bad conditions: low-slope roofs have open edges, skylights and slick surfaces. Wet, icy or dark is not the time to look.
- Keep people clear of sagging ceilings: a ceiling holding water can come down.
Roof leaking right now?
Dayton Commercial & Flat Roofing repairs, coats and replaces commercial and flat roofs across Dayton and the Miami Valley, with a written price before any work starts. Call (937) 669-7303 to request service.
The five places flat roofs leak most
1. Flashings
Wherever the roof meets something vertical, such as a parapet wall, a rooftop unit curb, a skylight or a wall of the building next door, there is a flashing. Flashings move with the building, take the most sun and weather, and get stepped on. On older Dayton buildings with brick parapets, the flashing and the masonry above it (coping, mortar joints) are the first place to look.
2. Seams
Every roll of membrane overlaps the next. Glued and taped seams on EPDM can open as the adhesive ages; welded TPO and PVC seams can fail if they were welded cold. Freeze-thaw cycling in the Miami Valley works open any seam that has started to let go.
3. Penetrations
Pipes, vents, conduit, gas lines and pitch pockets (the sealant-filled boxes around odd-shaped supports) all need their own seal. Pitch pockets in particular dry out and shrink away from the metal inside them.
4. Drains and scuppers
A drain clogged with leaves leaves water standing against every seam and flashing nearby. Drain bowls also crack, and the clamping ring that holds the membrane down can loosen. See ponding water on a flat roof.
5. Punctures and wear
HVAC service techs, dropped screws, hail, and foot traffic all puncture membranes. On old roofs, the membrane itself eventually wears through: EPDM gets thin and crazed, built-up roofs lose their gravel and the felts crack, and modified bitumen loses its granules and alligators.
How a leak gets traced
A roofer starts inside, marking where the water appears, and then works up the slope on the roof toward the drains it would naturally flow away from. The nearest details uphill of the drip, such as flashings, seams and penetrations, get inspected and probed. Single-ply seams are checked with a probe tool. On harder leaks, a section of roof can be water-tested in stages, starting downhill, to find the exact point where water gets in. Infrared scans, done in the evening when wet insulation holds heat longer, show where the water has spread under the membrane.
When a leak means more than a repair
A single leak at a detail is a repair. Several leaks at once, wet insulation spread over large areas, or a membrane that is brittle and failing everywhere is a different conversation. That may mean a coating if the insulation is still mostly dry, or replacement if it isn't. See are roof coatings worth it? and recover vs. replacement.
The bottom line
Protect the inside first, then get the leak traced to its real entry point rather than patched over the stain. Most commercial leaks are a few-hundred-to-a-few-thousand-dollar repair at a flashing, seam, penetration or drain. See our commercial roof repair and flat roof repair for homes.