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Why does my roof only leak in heavy rain?

The short answer

A leak that appears only in driving rain is usually water being pushed uphill or sideways past a lap, flashing or valley, rather than a hole. That is why it can be invisible on a still, wet day.

TradieAnswers editorial · written in-house and sourced to the authorities listed below

Checked against 4 published sources, listed in full at the end of this page. How we source this

Roofs shed water by gravity, and they are built assuming water runs downhill. Wind breaks that assumption. It drives water horizontally, lifts it up under laps by pressure difference, and forces it into gaps that gravity alone would never reach.

The usual sources

  • Insufficient roof pitch for the sheet or tile profile, so laps that work at higher pitches do not at this one.
  • Valleys that are too narrow, silted with debris, or where the tiles have been cut too close to the valley centre.
  • Flashings at walls, chimneys and penetrations that have lifted, corroded or were never turned up far enough.
  • Missing or torn sarking, which is the secondary barrier that catches exactly this kind of water.
  • Ridge capping with failed pointing, which lets wind push water under the cap.

Why they are hard to find

The stain inside is rarely below the entry point. Water travels along battens, rafters and the underside of sarking before it drops, sometimes metres away. That is why a competent investigation looks at the roof structure from inside the roof space during or straight after rain, rather than only at the ceiling stain.

Who does this work

You may need a roof plumber

Because the diagnosis needs someone on the roof safely, and roof drainage is a licensed plumbing class in several states.

Depending on the cause it can also involve builder.

This work is licensed, though the threshold and the licence class differ between states. Check a licence in your state

Before you call anyone

  1. Photograph where the water appears inside, and note whether it only happens in driving rain, in any rain, or regardless of weather.
  2. Look at the gutters from the ground during rain. Water going over the front edge is a different problem from water coming through the ceiling.
  3. Mark the wet area and check whether it grows, so you can tell the roofer whether it is one point or a spreading path.
  4. Stay off the roof. Wet roof sheets and tiles are the single most common cause of serious household falls in this kind of work.

Sources

Every source below was fetched and its response recorded on the date shown. Where a government site blocks automated clients, that is noted rather than reported as a successful check.

  1. National Construction Code · Australian Building Codes BoardChecked 2026-08-10 · responded 200 OK
  2. NCC Volume Two · Australian Building Codes BoardChecked 2026-08-10 · responded 200 OK
  3. Standards Australia · Standards AustraliaChecked 2026-08-10 · responded 200 OK
  4. Guidance material and handbooks · Australian Building Codes BoardChecked 2026-08-11 · responded 200 OK

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