Roof leak detection: finding where the water actually gets in

Water travels before it shows

Once water is past the roof surface it follows whatever is easiest — along the top of the decking, down a rafter, across a ceiling joist — until it finds somewhere to drop. By then it can be several feet from where it came in, and on the other side of a room from the actual failure.

This is why chasing the stain is the classic mistake. Sealing the ceiling above the stain does nothing except delay the diagnosis.

Where we start looking instead

At the junctions uphill of the stain. Flashing at chimneys and walls, valleys, vent boots, skylight curbs, and anywhere a previous repair has been made. Those account for the large majority of leaks on roofs that still have life in them.

Then the field of the roof for lifted, cracked or missing shingles, and the eaves for signs of ice damming — which in Maryland is a genuine cause rather than a theoretical one, because meltwater refreezing at a cold eave pushes water back up under the shingles.

From inside, which is often more useful

The attic tells you more than the roof surface does. Daylight through the decking, staining on the underside of the sheathing, wet or compressed insulation, and rust marks on nail points all narrow it down quickly. Staining that runs along a rafter points you uphill to the entry point.

Bathroom and kitchen fans venting into the attic rather than out through the roof are a common find on mid-century houses, and they produce damp that looks exactly like a leak without any water coming through the roof at all. Worth ruling out, because the fix is completely different.

When a drone is the right tool

On a steep roof, a slate roof or anything that would be damaged or dangerous to walk, a drone gets close-up imagery of the junctions without anyone risking a fall or cracking slate. It is not a substitute for going up when going up is safe — it is the right answer when it is not.

What matters either way is that you see the photographs. If someone diagnoses a leak and cannot show you the cause, the diagnosis is a guess.