Why your roof only leaks in heavy rain or wind

A roof that only leaks in heavy or wind-driven rain usually does not have a hole in it. It is being overwhelmed. Water is being pushed under tile laps by wind, drawn between them by capillary action, backing up behind a blocked valley, or overflowing a gutter that cannot cope. The roof performs adequately in ordinary rain and fails once a threshold is crossed.

That distinction matters because it changes what you are looking for. A hole leaks every time it rains. A threshold failure leaks four times a year, which is why these are the hardest leaks to find and the most likely to be misdiagnosed.

Most of the roofing advice available online was written for houses with asphalt shingles, plywood decking, attics and ice dams. Australian roofs are mostly concrete or terracotta tiles and steel sheet, with battens, sarking and no attic to speak of. The failure modes are genuinely different.

Why intermittent leaks are so hard to trace

Three things conspire.

It only happens under specific conditions — a particular rain intensity, often from a particular wind direction. By the time anyone is on the roof with a ladder, the weather has passed.

Water travels before it appears. It enters at one point, runs along a batten, a rafter or the top of the sarking, and drops through the ceiling somewhere else entirely. The stain on your ceiling is frequently metres from the entry point and almost always lower down the slope.

Nothing looks wrong from the ground. Threshold failures are usually small gaps, short laps and blocked drainage rather than visible damage.

The Australian causes, in rough order of likelihood

Wind-driven rain under tile laps. Tiled roofs are not waterproof in the way a membrane is. They shed water, and they rely on gravity and overlap to do it. Strong wind pushes water sideways and upwards, past the overlap, at pressures the roof was never intended to resist. This is the single most common answer to why a roof leaks in a storm and nowhere else.

Capillary action between tiles. Water climbs between two closely fitted surfaces without any wind at all. In sustained heavy rain, enough gets in this way to matter.

Missing, torn or badly lapped sarking. Sarking is the second line of defence — the layer that catches whatever gets past the tiles and carries it back out into the gutter. An older roof may have none, and a newer one may have it torn during an antenna or solar installation. Where sarking is absent or damaged, ordinary tile leakage that should be harmless becomes a leak into the house.

Blocked or corroded valleys. The valley carries the largest concentration of water on the whole roof. Leaves and debris pack in, water backs up over the edge and under the tiles either side. Older galvanised valleys also rust through at exactly the point where they are permanently wet.

Gutters and downpipes at capacity. A gutter that overflows in heavy rain sends water back under the eave and into the wall or ceiling. This presents as a leak but is a drainage problem — see the end-of-winter roof and gutter check.

Box gutters. The highest-consequence detail on any Australian roof, because a box gutter sits within the building rather than at its edge. When one overflows, it overflows inside. Box gutters need adequate capacity, a working outlet, and a genuine overflow provision. Many older ones have none.

Anti-ponding boards. At the base of a tiled roof, a board or tray supports the sarking so water runs into the gutter rather than pooling behind the fascia. Where it is missing or has sagged, water ponds at the eave and backs up under the bottom course.

Flashings at penetrations. Chimneys, flues, vent pipes, skylights and the junction where a roof meets a wall. Flashings fail at the upstand — too short, or dressed over rather than into the surface — and only leak when water arrives with enough force to get above them.

Ridge and hip pointing. Traditional mortar bedding cracks and loses adhesion with age. Wind-driven rain enters through the cracks.

If your roof is metal

Sheet roofing fails differently. The usual suspects are fastener washers that have perished, leaving each screw a small open hole; side laps in the wrong orientation relative to prevailing wind; capillary action between sheets where the anti-capillary groove has been flattened by overtightening; and pitch below the product minimum, which is a design problem rather than a maintenance one and shows up only in sustained heavy rain.

Finding it

1. Record the conditions. Note the wind direction, rough rain intensity and how long it took to appear. If it only leaks in a southerly, that eliminates most of the roof immediately.
2. Get into the roof space during or straight after the rain. This is by far the most productive thing you can do, and the only time the evidence is visible. Take a torch and look for active water, wet timber, and tracks running along battens.
3. Work uphill. Water always enters above where it appears. Follow the wet trail up the slope to its highest point.
4. Mark what you find with tape or chalk while it is wet. It will be dry and invisible tomorrow.
5. Check the obvious drainage first from the ground — valleys, gutters, downpipes, and whether anything is overflowing.
6. Consider a controlled hose test on a dry day, working one small area at a time from the bottom up, with someone in the roof space watching. Flooding the whole roof at once tells you nothing.

What not to do

Do not apply sealant blindly. Sealing the area around a stain, without knowing where water is entering, is the most common response and it rarely works. It also tends to block the drainage paths the roof relies on, which can make things worse.

Do not walk on a wet tiled roof. Tiles are slippery, they break under point loads, and roof falls are among the more serious domestic injuries. Working at height is regulated for good reason — if the inspection requires being on the roof rather than in the roof space, that is a job for someone equipped for it.

Do not assume the stain marks the spot. Cutting into a ceiling directly above a stain frequently finds nothing at all.

Common questions

Why does it only leak from one wind direction?

Because wind-driven rain is being forced past a lap or flashing that only faces that way. It is a very useful clue and narrows the search enormously.

My roof is only a few years old. Should it leak at all?

No, and a threshold leak on a newer roof points more toward a detailing or workmanship issue than wear — flashing upstands, sarking laps, or valley installation. If the house is still within the statutory warranty period in your state, that is worth establishing before paying for repairs yourself.

Could it be condensation rather than a leak?

It could, and the two are regularly confused. Condensation appears in cold weather regardless of rain, tends to be widespread rather than localised, and often shows as dampness across a broad area rather than drips from a point. A leak follows the weather; condensation follows the temperature.

Is it worth replacing the whole roof?

Rarely, on the strength of an intermittent leak alone. Most threshold failures are repairs — valley clearing, re-pointing, flashing rework, sarking repair. Full replacement is a decision about age and overall condition, not about one leak.

The plumber could not find it either. What now?

Ask for a hose test with someone in the roof space, section by section, rather than a visual inspection alone. Intermittent leaks are genuinely difficult and a visual check on a dry day frequently misses them.

Where to go next

A leak that only appears under load is a capacity problem, and capacity problems are usually about drainage rather than damage.

Read next: the end-of-winter roof and gutter check, or how a new roof ties into an existing one if the leak started after an extension was built.

The most valuable half hour you can spend on this problem is the one during the next downpour, in the roof space, with a torch.