Guide
Finding a flat roof leak
On a flat roof water travels. It enters at one point, runs along the deck or insulation, and drips somewhere else. Finding the source takes patience and a method.
Why the stain is rarely under the hole
A sloped roof sends water downhill in a straight line. A flat roof has layers: membrane, insulation, deck. Water entering through a split in the membrane can run sideways between layers, follow a joist, a pipe or a seam in the deck, and drop through the ceiling several metres away. The stain tells you which room is affected, not where the water came in.
This is why guessing and sealing the roof above the stain often fails. The patch goes on the wrong place and the leak returns at the next rain.
A method that works
Write down when it leaks
After every rain? Only wind-driven rain? Only during snow melt? Only when the heating is on? Each pattern points to a different source.
Map the stains inside
Draw the room and mark each stain, drip or damp wall. Several stains in a line often follow a seam or a drain line.
Look at the roof above and around
The cause is usually within a few metres but can be farther along the slope. Look uphill of the stain.
Check the usual suspects first
Flashings at walls, curbs and vents; drains and scuppers; seams; skylights; any cuts or punctures; anything attached to the roof, such as aerials or railings.
Test with water, if you can
With a helper inside, wet small areas one at a time, starting low and moving up, waiting several minutes at each. Stop when the drip starts. Do not do this in freezing weather or on a roof you cannot reach safely.
Use a moisture survey for stubborn leaks
A roofer can scan the roof with a meter or infrared camera and mark wet insulation. That map often shows where the water is entering.
Where flat roof leaks most often start
Wall and curb flashings
The joint between roof and wall, a chimney, an AC curb or a hatch. See flashing leaks.
Drains and scuppers
Loose clamp rings, cracked sumps, a blocked outlet that lets water rise over the edge. See clogged drains.
Seams
A lifting lap or a poorly welded seam. See open seams.
Penetrations
Vent stacks, conduit and cable entries. See skylight and penetration repair.
Blisters and cracks
Cracks let water in directly. See blisters and cracks and alligatoring.
Condensation
Warm moist indoor air hitting a cold deck can drip without any hole in the roof. A leak that appears only in cold weather with no rain is worth checking for this.
What to do while you wait for a repair
Protect the room: move furniture, put plastic under the drip, and make a small hole in a bulging ceiling so that water drains into a bucket instead of collecting and bringing down the ceiling. Turn off lights or circuits where water is near fixtures. Take photos with dates. If it is winter, read flat roof in winter.
Avoid covering the roof with large amounts of roof cement, plastic and tape. It rarely solves the problem and can trap water, and it makes the real source harder to find.
How a roofer finds it
A roofer will ask about the pattern, check the interior, walk the roof, and probe suspect spots. For hard cases the next step is a moisture survey, which separates dry areas from wet ones. A written report with photos gives you something to hand to an insurer or a board.
A leak repair starts from $350 per repair before HST, and a house roof inspection with a photo report starts from $250 per roof. If the leak is from a part that is failing everywhere, a repair is a short step before a replacement; read repair or replace a flat roof.
Reading the pattern
A leak after every rain points to an opening in the membrane or a flashing. A leak only after wind-driven rain points to an edge, a wall joint or a vent. A leak during a thaw but not in summer rain points to ice at a drain or snowmelt behind a flashing. A drip that appears on cold, dry mornings can be condensation.
Several leaks across a large area with no clear line suggest wet insulation, where water has spread through the layers. In that case chasing one hole will not help, and the next step is a moisture survey and a conversation about repair or replace.
Questions about finding leaks
Can I find a leak with a hose?
Sometimes, but it needs two people, patience and dry weather. Start at the bottom of the suspected area and work up in small sections.
My ceiling stain dried. Is the leak fixed?
Not necessarily. A stain dries between rains. Watch for new marks, or have the roof checked.
How long does a leak survey take?
A simple roof can be walked and checked in under an hour. A moisture scan of a large roof takes longer.
Leak, damage or an old flat roof?
Send a few details and photos. We reply by email with next steps and a written quote.