You spot a roof leak in three places: the attic, the ceilings and walls under the roof, and the roof itself seen from the ground. Go into the attic with a flashlight during or just after rain and look for wet wood, damp insulation or light coming through. Then match what you find to the weather, because the timing tells you whether it is a leak, an ice dam or condensation.
That last part is the one most people skip, and it is the one that saves money.
Start in the attic, with the rain still falling
The attic is the only place you can see water on the way in rather than where it lands. The best time to look is during a steady rain or within an hour or two after it, while the wet areas are still wet.
Take a strong flashlight and stay on the joists. Look for:
- Dark streaks on the underside of the roof deck, running downhill from one spot. Follow the streak up to where it starts.
- Wet or matted insulation, or insulation that is packed down in one patch.
- Water trails on rafters or trusses, shiny or stained, that run toward a wall.
- Daylight around a vent pipe, a chimney, or at the ridge. Where light gets through, water can too.
- Drips from nail tips in one area only.
Look hardest where the roof is interrupted: around the chimney, around the plumbing vent, under a valley, and where a lower roof meets a wall. Those are the places the roof leak repair page describes as the usual culprits, and most owners who find a leak find it at one of them.
If the attic is finished, or a cathedral ceiling means there is no attic at all, skip to the rooms below and the view from the ground.
Inside the house
A roof leak rarely announces itself with a drip. The quieter signs come first:
- A brown or yellow ring on a ceiling, often with a darker edge.
- Paint that bubbles, cracks or peels on a ceiling or near the top of an outside wall.
- Drywall that feels soft, or tape seams that sag.
- A musty smell in an upstairs closet or a room under a low slope.
- Water marks around a skylight frame or a bathroom fan.
Write down the room, the spot, and the date you first saw it. Put a piece of tape at the edge of the stain and date it. If the stain grows past the tape after the next rain, you have a live leak and not an old mark.
From the ground, with binoculars
You do not need to climb up. Stand back far enough to see the whole slope and look for:
- Missing or lifted shingles, especially at the edges and the ridge after a wind.
- Shingles that are curled, cupped or cracked.
- Flashing that has pulled away from a chimney or a wall, or a dark gap along the step flashing where a roof meets siding.
- Heavy granules in the eavestrough or at the bottom of the downspouts.
- A split or sun-cracked stack boot around the plumbing vent.
- Stained fascia or soffit, or paint peeling under the eave, which can mean water getting in at the edge.
- A sag in the roof line, which means the deck has been wet for a long time.
Granules alone do not mean a leak, and a young roof sheds some. They are one of the signs covered in the guide on how long does a roof last, where the question is age rather than a single leak.
Read the timing. It is the best clue you have.
The same stain can have three different causes, and each one shows up in different weather.
| When it appears | Most likely cause |
|---|---|
| During or just after rain | A leak at flashing, a vent, a valley or a damaged shingle |
| Only in rain driven from one direction | Flashing or siding on that side of the house |
| During a thaw after a heavy snow | An ice dam backing water up under the shingles |
| After a cold spell, often on a dry day | Condensation melting off the underside of the deck |
The thaw leak
In the snowbelt, around Stratford, Listowel and inland from Goderich off Lake Huron, a leak that only appears in a thaw after snow has sat for weeks is usually an ice dam. Water pools behind a ridge of ice at the eave and backs up under the shingles. The fix is not the shingles, and the reasons are in the guide to an ice dam on the roof. On the Erie shore, around Chatham and Leamington, snow rarely stays long enough for this to happen, so a winter leak there points more often to wind or flashing.
The wind leak
A leak that turns up only in a storm with rain coming from the west or southwest is often wind pushing water sideways under a lifted tab or behind loose flashing. On flat farmland with nothing upwind, that is common after the big summer storms. If the storm also took shingles, photograph before anything is touched; the guide to an insurance claim for roof storm damage sets out the order.
Leak or condensation?
This is the mistake that costs owners the most. A cold snap builds frost on the underside of the roof deck and on the nail tips poking through it. When the weather turns, it all melts at once and drips onto the insulation and the ceiling. From inside the room it looks exactly like a leak. CBC reported roofers in Calgary fielding call after call about this “attic rain”, with one technician saying that people think it is the roof leaking.
The US Department of Energy’s building guidance explains why: in cold weather the condensation often forms as frost, and when warmer conditions arrive the moisture returns to a liquid state.
Signs it is condensation, not a leak:
- It appears after a cold spell, not after rain.
- The whole attic shows it: rows of rusty nail tips, dark or mouldy wood across wide areas, frost on the deck.
- There is no single wet trail you can follow up to one entry point.
A new roof will not fix condensation. The fix is keeping warm, damp house air out of the attic and moving air through it: sealing gaps around ceiling fixtures and the hatch, venting bathroom fans outside rather than into the attic, and clear soffit and ridge vents. Replacing shingles over a wet attic only means the stains come back the next cold spell.
What to note before you call
A roofer finds a leak faster with good notes. Before anyone comes out, have:
- The rooms and spots, with the dates you first saw each one.
- The weather each time: rain, wind direction, a thaw, or a dry day after cold.
- Photos from the attic, if you could get up safely.
- Photos of the roof from the ground.
- Anything done to the roof recently: a new vent, a satellite dish, an antenna taken down, a solar install.
If the signs point to one spot and the roof is otherwise sound, it is a repair. If you are finding signs in several places at once, ask for a roof inspection report instead, so the decision between repair and replacement is made on paper and not from the stain.
Where we come in
Spotting the signs is your job, and this guide covers it. Tracing water from the stain to the place it gets in, and fixing that place, is ours. When your notes point to a leak, book a trace and repair through the roof leak repair service, and bring the dates and photos with you.