The stain is not
under the hole

Attic first

Almost every leak we have ever found was found from underneath. In the attic you can read the history of the roof: dark tea-colored staining down the side of a rafter shows the path, rusted nail points show where water has been sitting as vapor rather than running, and a wet insulation batt is a map with an arrow on it. We work from the stain backwards and uphill, because water only ever goes one way.

Then we look for daylight. A pinhole at a nail, light round a boot flange, a gap at the top of a wall intersection. Daylight is not always the leak, but it is always worth knowing about.

The four usual suspects, in order

Pipe boots. First place we look, every time. The sun destroys the rubber collar in eight to twelve years and it splits round the pipe. It is the commonest leak in this trade and the cheapest to fix, which is a combination that ought to make people suspicious of anybody who goes straight to quoting a full roof.

Wall intersections. Where a roof plane runs into a wall, water has to be stepped out over the shingles with individual flashing pieces, one per course. The lazy version is one long piece of bent metal caulked to the wall, and it fails at every course line. The version that fails most expensively is the missing kick-out at the bottom of that wall — without it, every drop running down the wall-roof junction goes behind the siding instead of into the gutter, and rots the sheathing for years before anything appears indoors.

Valleys. Cut too tight, cut wandering, or no membrane underneath. A valley carries the water off two planes at once and it is the highest-volume part of the roof.

Chimneys and skylights. Counterflashing that was caulked to the brick instead of cut into it, a missing cricket on the uphill side of a wide chimney, a skylight installed without its manufacturer's flashing kit and made watertight with sealant.

The water test

If the attic does not give up the answer, we hose it, and the discipline matters more than the water does. One person inside with a light, one on the roof. Start at the very bottom of the suspect area and work uphill, one small zone at a time, fifteen minutes each, and do not move up until the zone below has been given time to show. Wetting the whole roof at once and waiting proves nothing except that the roof leaks, which you already knew.

When it is not the roof

Roughly one visit in six ends with us telling somebody their roof is fine. In this climate, the imposters are a sweating air-conditioning line or an uninsulated duct in a hot attic — which drips exactly like a leak and leaks worst on dry days — a bathroom fan terminated into the attic instead of through the roof, a window head or a stucco crack when the rain is wind-driven, and a chimney crown letting water into the flue.

The tell is timing. A leak that appears only when rain comes from one direction is nearly always a wall. A leak that appears on a hot afternoon with no rain at all is condensation, and it is a mechanical problem, not a roofing one. We will say so, write it on the report, and not charge you for a roof you do not need.