Roof or Ceiling Leaking Only in Heavy Rain? Why It Happens and How to Find It
For most of the year everything is dry. Then a summer storm rolls through, rain hammers the roof for twenty minutes, and a brown stain appears on the ceiling or water starts dripping from a light fitting. The next day the sun is out, someone climbs onto the roof, and they can't see anything wrong.
If that sounds familiar, you're not alone. A roof or ceiling that only leaks in heavy rain is one of the most common calls we get across Melbourne, Sydney, Brisbane and the Gold Coast, and one of the most misunderstood. This guide explains why it happens, why the stain is rarely under the actual leak, what to do while it's raining, and how the source can be found even on a dry day.
Why a roof only leaks when it rains hard
Light rain runs off a roof the way it was designed to. Heavy or wind-driven rain pushes water into places it normally never reaches. These are the causes we find most often:
Overflowing gutters and box gutters. When the rain falls faster than a gutter, outlet or downpipe can carry it away, the water level rises and spills back over the edge into the roof space or wall cavity. Blocked outlets, missing sumps and undersized or missing overflows make it worse. Box gutters, the internal gutters common on townhouses, apartments and parapet walls, are a frequent culprit.
Wind-driven rain. Strong wind pushes water sideways and even uphill, under roof tiles, through sheet laps and behind flashings that keep out normal rain without any trouble.
Failed flashings and sealant. Flashings seal the joins where the roof meets a wall, parapet, chimney or skylight. Over time silicone perishes and flashings lift or crack, leaving gaps that only flood when a lot of water is running past them.
Valleys and laps that get overwhelmed. Roof valleys collect water from two slopes. In a downpour they can run over, especially if leaves, cracked pointing or rust are slowing the flow.
Penetrations. Vents, flues, solar panel mounts, aerials and pipes all need a seal through the roof. A small gap around one can stay dry for months and let water in during a storm.
It isn't always the roof. Water coming through a ceiling after rain can also come from a window, a balcony or terrace above, cracked render or a wall. That's why we test every likely path, not just the roof.
Why the stain is rarely under the leak
Water that gets in at roof level rarely drops straight down. It runs along the sarking, the underside of the sheets or tiles, rafters, battens and the top of walls, sometimes for several metres, before it finds a low point and soaks through the plaster. So the stain on your ceiling tells you where the water ended up, not where it got in. Patching the roof directly above the stain is one of the most common reasons a roof leak "comes back".
A real example: a townhouse that only leaked in cloudbursts
We were called to a late-1980s double-storey brick townhouse in Melbourne's north, in the middle of a row, with a metal roof. In normal rain it stayed dry. In heavy downpours, water trickled down the exposed brick walls inside both upstairs rooms, at roughly the same point on each side of the house. The owner had noticed that neighbours in the same row reported similar problems and wondered whether blocked drains were to blame.
Our technician inspected the roof and roof space and traced the water to the junctions where the roof meets the dividing walls between the townhouses. The apron and parapet flashings at those junctions had failed, and the old silicone sealant had deteriorated, letting rainwater into the roof cavity whenever enough water was running across those joins. Because the dividing walls are shared, the water was getting into the units on either side as well.
The owner received a written report with photos showing the failed flashing junctions and what needed rectifying: removing the degraded sealant, repairing the flashing junctions and reinstating proper weatherproofing. We referred him to a trusted roof plumber to quote the repair, with the report in hand so the work could go straight to the right spot.
What to do while it's raining
Put a bucket under the drip and move furniture, rugs and electronics out of the way.
If water is near a light fitting, a ceiling fan or the ceiling is sagging, switch that circuit off at the switchboard if it's safe to do so, and keep clear.
Take photos and short videos of where the water appears, and note the time, how heavy the rain is and which way the wind is blowing. This is gold for finding the leak later.
Don't climb onto a wet roof. Roofs are slippery and dangerous in the rain, and the leak can be found safely once the weather clears.
Call us on 1300 346 353, 24/7, if water is coming in and you need help now.
Can a roof leak be found when it isn't raining?
Usually, yes. A leak that only shows in heavy rain can be found on a dry day by recreating the conditions that cause it, one part of the roof at a time:
Roof space inspection to look for water staining, wet timber and daylight around penetrations.
Thermal imaging and moisture mapping to show where the ceiling, insulation and walls are holding water.
Controlled water testing, running water over one section at a time, such as a flashing, a valley or a skylight, from the bottom of the roof upwards, until the leak is reproduced.
Flood testing of box gutters and outlets to see whether they overflow into the building.
Drone inspection of steep, fragile or high roofs that aren't safe to walk on.
Testing one area at a time is what separates finding the leak from guessing at it. It also gives you evidence of the cause for a roofer, a body corporate or an insurer.
Who should you call: a roofer or a leak detection specialist?
If you can clearly see the problem, such as a missing tile or a gutter full of leaves, a roofer or roof plumber can fix it. If the roof looks fine, the leak only happens in certain storms, or someone has already repaired it and it's still leaking, get the source found and proven first. We mark the entry point and give you a report, then your roofer fixes the right spot the first time. We explain more in Leak detection specialist or plumber: who should you call first?
Roof leaks and insurance
Insurers generally treat sudden storm damage differently from gradual wear and tear, such as perished sealant or rusted flashings, so the cause matters. Check your policy and talk to your insurer. Our report records where the water came in, what caused it and the damage it has done, with photos, which is the information insurers and assessors normally ask for.
What it costs
Roof and ceiling leak detection starts from $398 inc. GST for the first hour on site in metro Melbourne, Sydney, Brisbane and the Gold Coast, with a written report included. Found or you don't pay.* *If we can't find the leak, you don't pay for the inspection. Ask for details when you book or read our guarantee terms. See prices for all areas, or our roof leak detection pages for Melbourne, Sydney, Brisbane and the Gold Coast.
Seeing water come in around windows in a storm as well? Read leaking windows during heavy rain.
Frequently asked questions
Why does my roof only leak when it rains hard?
Heavy or wind-driven rain pushes water into places light rain never reaches. The most common causes are overflowing gutters and box gutters, failed flashings and sealant at walls, parapets and skylights, water driven under tiles or sheet laps, and gaps around roof penetrations.
Can you find a roof leak when it isn't raining?
Usually, yes. We recreate the rain with controlled water testing on one section of the roof at a time, while thermal imaging and moisture readings show where the water travels inside.
Is a ceiling leak after rain always coming from the roof?
No. Water through a ceiling after rain can also come from a window, a balcony or terrace above, a box gutter, or cracked render or brickwork. Testing each likely path is the only way to be sure.
Will insurance cover a roof leak after a storm?
It depends on your policy and the cause. Insurers generally treat sudden storm damage differently from gradual wear such as perished sealant or rusted flashings. Our report documents the cause and the damage, which is what insurers normally ask for.
Should I call a roofer or a leak detection specialist?
If the problem is obvious, a roofer can fix it. If the roof looks fine, the leak only happens in certain storms, or a repair hasn't worked, have the source found first so the roofer repairs the right spot.
