Why does my roof only leak when it rains hard?

A roof that only leaks in a storm is almost always a flashing or a seal problem rather than a hole. In an ordinary rain, water runs down the roof and over every lap and joint the way it is meant to. In a storm, wind drives it sideways and uphill, under the edge of a shingle that has lost its seal, over the top of a piece of flashing that is lapped the wrong way, or into the gap where counter-flashing has pulled out of a chimney.

The usual suspects for a wind-driven leak are the windward edge of the roof, where shingles lift first; the flashing around chimneys, skylights and the walls where a second storey meets a lower roof; ridge vents, which are designed to shed rain falling down and not rain blown up; and any place a satellite dish or similar has been screwed through the shingles.

The fix is usually small, but the diagnosis has to be right, which means a contractor on the roof looking at the laps and seals rather than patching where the stain is. Tell him which direction the wind was coming from when it leaked. That alone often points straight at it.