Why does my roof only leak in heavy rain?

It is the most common call we get after a storm, and it is the one people most often misdiagnose. The instinct is to look for a hole. If there were a hole, it would leak in light rain too.

What heavy rain does that light rain does not

• It fills the gutter faster than the downpipes can empty it, so water backs up over the rear edge of the gutter and onto the fascia and roof battens.
• It overwhelms valleys, the sloped channels where two roof planes meet, which carry more water per metre than anything else on a roof.
• It arrives at an angle. Wind-driven rain travels sideways and upwards under tiles, flashings and ridge capping that shed vertical water perfectly well.
• It finds gaps that surface tension closes in light rain, particularly around penetrations like vents, aerials and skylights.

If the leak tracks with rainfall intensity rather than simply with rain, start at the gutters, the valleys and the downpipes before anyone lifts a tile.

Why the stain is not under the leak

Water entering at a high point runs along sarking, battens and rafters until it finds somewhere to drop. The mark on the ceiling can sit metres from the entry point, which is why patching the obvious spot so often fails and the leak returns with the next downpour.

What to check before you call anyone

• Look at the gutters during rain if you safely can, from the ground. Water spilling over the back edge rather than the front is the classic sign of a blocked or undersized system.
• Check whether the downpipes are actually discharging. A downpipe that trickles while the gutter brims is blocked below the inlet.
• Note whether the leak appears only when wind comes from one direction. That points at a flashing or a ridge rather than drainage.
• Photograph the ceiling stain and the weather. Two data points across two storms narrow the search more than any single inspection.

Why this matters more in the Shire

A lot of Sutherland Shire housing is original post-war tile with gutters sized for the rainfall patterns of the day, and a good deal of it backs onto bush. Leaf load reduces an already marginal capacity, so the first heavy rain after a dry spell is when these faults show up.