Why does my flat roof keep leaking in the same spot?

If the spot was already repaired, the patch was probably put in the wrong place, or it was only sealant that cracked after a season. Sometimes the repair did work, and what you are seeing is wet insulation still draining.

Why the water comes out in the same place every time

Water that gets through the roofing does not fall straight down. It runs along the plywood deck or a joist until it reaches a low point or a gap. Then it drips through.

That path is the same every storm. The low spot does not move. Neither do the joists, or the gap around a light fixture. So the water comes out in the same place, even when it got in 10 or 20 feet away.

Ceiling tiles along a fluorescent light fixture with several brown water stains, some tiles showing rings inside rings, above an orange beam, a black duct and a wall-mounted air conditioner — A flat-roofed building in Beaverton, Oregon. The stains sit on the same few tiles along the light, storm after storm.

The rings show this. Each brown line is the edge of a wet patch that dried. Rings inside rings mean that tile got wet in storm after storm.

So the stain tells you where the water comes out. It does not tell you where it gets in.

One spot, or the whole ceiling?

A stain that keeps coming back in one place almost always traces to one opening. Find it and close it, and the leak stops.

Stains across most of the ceiling are a different problem. The roof is letting water in at many places, and fixing one opening at a time rarely keeps up. Repair, restoration, or replacement covers how that decision gets made.

A bakery back room with mixers, work tables and shelves of supplies, under a suspended ceiling where most of the tiles carry brown water stains and two tiles are missing — Stains across nearly every tile in a bakery. This is not one leak coming back. It is a roof letting water in at many places.

Where the water usually gets in

When one spot keeps leaking, our roofers usually find the opening in one of these places:

The roof edge, where the roofing meets the metal
Around rooftop units and pipes
Around drains
Along seams, where two sheets of roofing overlap
Worn or peeled spots on older roofing

Some of these have their own articles: roof edges in wind-driven rain, rooftop units, and drains.

A finger pointing into a gap where silver-coated roofing has lifted off the metal edge of a wet flat roof, with the dark joint between two pieces of edge metal visible below — Roofing pulled away from the metal edge. Every rain that reaches the edge can run under it here.

A hand lifting a peeled flap of gray roof surface off a white-coated roof, showing a dark patch underneath and a crack running through the coating — Roofing pulled away from the metal edge. Every rain that reaches the edge can run under it here.

Why the last repair did not hold

When the same spot leaks again after a repair, the repair usually went wrong in one of these ways.

It was made above the stain. The roofer went up, found the spot over the stain, and patched it. The water was getting in somewhere else, up the slope or at an edge or unit nearby, so the leak came back on the next storm. Our roofers start inside at the stain, then work outward on the roof from the point above it, checking every edge, unit, drain and seam in the area.

It sat on top of the opening. Mastic, caulk or tape spread over a gap stops water for a while, but it does not join the roofing back together. When it cracks or peels, the same opening leaks again. Why roof seams open again after a repair shows what that looks like.

It went onto wet roofing. Nothing bonds well to wet roofing. In the Pacific Northwest, a repair squeezed into a dry afternoon between showers can go onto roofing, or a seam, that is still damp inside.

A good repair can still drip for a while

Insulation under a flat roof soaks up water like a sponge. Once the opening is closed, that water still has to drain out, and it can keep dripping at the same spot for days. That does not always mean the repair failed.

If the dripping keeps going, or starts again in the next rain, it did. And insulation that has been wet for a long time usually needs to be cut out and replaced, not left to dry.

What years of leaking in one spot do underneath

A spot that leaks in every storm never fully dries. The plywood deck under it softens, comes apart, and rots. The joists under it can follow.

Rotted decking sags. A sag holds water on the roof above it, so the weakest spot on the roof ends up with the most water sitting on it. Each storm makes it worse.

A section of flat roof cut open beside a rooftop generator, showing plywood decking rotted through to the joists, broken wood scattered around the hole and old insulation visible inside — The deck beside a rooftop unit, opened up. The plywood had rotted through to the joists after years of water getting in at the same place.

A ceiling cut open to show dark, stained framing above, with brown water streaks running down the wall below it along a white pipe — The deck beside a rooftop unit, opened up. The plywood had rotted through to the joists after years of water getting in at the same place.

None of that shows from the roof surface. A roof can look fine from above while the deck under one spot is rotting.

What a repair that holds looks like

A repair that stops a repeat leak for good does four things:

Finds the opening, starting at the stain inside and working outward on the roof.
Cuts out wet insulation and rotted decking, and replaces them.
Closes the opening with a material that bonds to that roof.
Runs the patch well past the opening, onto sound roofing on every side.

If nothing shows on the roof, a roofer can water-test one area at a time with a hose while someone watches the ceiling. On TPO or PVC, the patch is usually heat-welded to the roofing around it; how a TPO roof patch is heat-welded walks through it. On older or asphalt roofing, our roofers often use silicone.

A clean white rectangular patch welded onto a dirty white flat roof, with a seam down its middle and a round cover piece at each end of the seam, a roofer's boots at the bottom of the frame — A welded patch over one leaking spot. It runs well past the opening, and the round pieces seal the ends of the seam across its middle.

A wide area of fresh white silicone around a roof drain on an older dark flat roof beside a brick wall, with a parking lot below — A welded patch over one leaking spot. It runs well past the opening, and the round pieces seal the ends of the seam across its middle.

If the stains have spread past one spot, see flat roof leaks: what a Property Manager needs to know for when repairing stops making sense.

Common questions

The roofer patched right above the stain. Why is it still leaking?

The water was probably getting in somewhere else. It can travel 10 or 20 feet under the roofing before it comes through the ceiling. The repair needs to be where the water gets in, not above where it comes out.

Why does it only leak in heavy rain?

In light rain, the opening lets in so little water that the insulation soaks it up. Heavy or wind-driven rain pushes in more than the insulation can hold, and it comes through the ceiling. It is the same opening either way.

It stopped after the repair, then came back months later. Why?

Usually because sealant was spread over the opening instead of closing it. Mastic and caulk hold while they are fresh and fail after a season or two of sun and rain.

It is still dripping a day after the repair. Did the repair fail?

Not necessarily. Wet insulation can drip for days after the opening is closed. If it is still dripping after a week, or starts again with the next rain, the repair missed.

Does a leak that keeps coming back mean I need a new roof?

Not on its own. One spot that keeps leaking usually has one opening that was never fixed. A replacement is the question when stains show up all over the ceiling, not in one place.

What should I tell the roofer when I call?

Where the stain is, measured from the nearest walls. Whether it drips in every rain, only heavy rain, or only when the wind blows from one direction. How long after the rain starts it drips. What was done to fix it before, and when. Photos of the stain from past leaks help.