Why Does My Roof Only Leak in Heavy Rain?

A roof can be bone dry in a light shower and pour water in a downpour. Heavy rain overwhelms the parts of your roof that move water away — gutters, valleys and downpipes — and finds every loose screw, lifted sheet and gap that a gentle rain never reaches. So the leak isn't a new hole; it's water getting past a weak point that only shows up under load.

This is the most common cause I see. Gutters full of leaves and debris fill up in heavy rain, overflow, and run straight into the eaves and ceiling. If your leak is near the edge of the roof, under the eaves, or down an outside wall, start at the gutters.

Clearing the gutters and flushing the downpipes fixes a surprising number of mystery roof leaks. If the gutters are sagging or the downpipes don't flow, that needs a proper repair, not just a clean.

Colorbond and tin roofs are held down with screws that have rubber seals. Over time the sun bakes those seals, the screws back out, and the sheet lifts slightly. In light rain the water runs past. In heavy rain, especially with wind, it's driven up under the sheet and into your ceiling.

Rusted screws and flashings do the same thing. If you can see rust streaks down a roof sheet, or screws sitting proud of the sheet, those are your suspects. Replacing the screws and re-sealing the sheet stops it.

Flashings are the metal folds where the roof meets a wall, a skylight or a chimney, and ridge capping runs along the top peak. The bedding and pointing that seal them breaks down in the Cairns sun and storms. Heavy rain pools at those joins and finds a way in.

Re-bedding and re-pointing ridge capping, and re-sealing lifted flashings, is standard wet-season prep. It's fiddly work but it stops leaks that only show up when it's bucketing down.

Cairns storms bring wind, and wind-driven rain travels sideways and uphill. A roof that never leaks in calm rain can leak when the wind blows the water up under flashings and sheets on the windward side. Note which direction the wind was blowing and which room got wet — that narrows it down fast.

To find the source, don't just stare at the ceiling stain. Water travels along beams and inside wall cavities, so the entry point is often metres away. Check the roof, flashings, valleys and gutters from outside, and trace the water path back inside. If you're not confident on a roof, that's my job — call me rather than risk a fall.

Not every ceiling stain is a roof leak. A split air-conditioning condensate line, a leaking pipe in the roof cavity, or a cracked shower tray upstairs can all show up as a wet patch on the ceiling below. If the mark grows even when it's not raining, suspect plumbing. Any repair involving pipes or roof plumbing is a licensed plumber's job, so I'll tell you to call one in.

Condensation is another one — warm, humid Cairns air hitting a cool ceiling or metal frame can form drips that look exactly like a leak. Good ventilation and insulation usually sort it. The clue is it shows up in humid weather regardless of rain.

If water's coming in during a storm, move anything valuable out of the path, put a bucket down, and lay towels to protect carpet and floors. If you can see the ceiling bulging, don't puncture it — it can let a lot more water through. Call me and I'll come out, safely tarp the area to stop the damage getting worse, and do the permanent repair once it's dry.

Don't go climbing on a wet roof in a storm — that's how people get hurt. Wait for the rain to pass, or let a professional handle it. Any repair involving roof plumbing or stormwater pipes is a licensed plumber's job, so I'll refer those parts on.