How are leaks found on large commercial roofs?

The building tells you where to begin. Stains on ceilings and liners, drips onto stock, damp patches on walls and rusting on steelwork are all clues, and each is worth recording with a photograph and a note of when it was seen and what the weather was doing.

The weather matters. A leak that only shows in wind-driven rain points to a lap or a vertical joint facing the weather. A leak that appears after long, steady rain points to a low spot or an outlet. A leak that comes and goes with no rain may be condensation, a different problem altogether, covered in our answer on warehouse roof condensation.

We ask who saw the leak, when, and how long it ran. Those answers narrow a huge roof down to a few likely strips. Our commercial roofing page shows the buildings we are usually asked to look at.

Water entering a roof does not drop straight down. It runs along the underside of a sheet, down a purlin, along a liner and through insulation before it finds a place to drip. On a pitched metal roof the entry point is normally higher up the slope than the stain. On a flat roof it can move sideways along the deck under the membrane for a surprising distance.

So the work is to trace each stain back along the likely paths: up the slope, along the purlin line and towards the nearest laps, fixings, penetrations and rooflights. The line from the stain to the suspect defect is usually a short list.

On a membrane roof, the water can run between the covering and a wet layer of insulation, which is why the entry may be well away from the stain. A moisture survey helps map that spread, and it saves opening up more of the roof than necessary.

With a list of suspects, the roof itself is inspected on foot or from a mobile platform, with fragile areas protected first. Our guide to fragile roof safety explains the care involved. On metal we look at lap sealant, fixings and washers, cut edges and rooflight laps. On membranes we look at seams, upstands, penetrations and the edges around outlets.

The usual suspects repeat themselves. Plant penetrations, flues, rooflights, parapet junctions and the places where one trade's work meets another's are the likeliest weak points. A defect that sits directly uphill of a stain is a strong candidate. One that does not is rarely the cause, however poor it looks.

Photographs of each find are kept, so you see what the surveyor saw and what was ruled out, and a second pair of eyes can check the reasoning if you want one.

When inspection does not settle it, the roof is tested. A hose is run over a small zone at a time, starting at the lowest point and working up the slope, while someone inside watches for the drip. The test stops the moment water appears, and the zone that was being wetted is the source. Working from the bottom up avoids wetting the whole roof and confusing the result.

Moisture meters can show where insulation or a deck is wet beneath an intact surface. Thermal imaging, used at the right time of day, can also pick out wet areas because they hold heat differently, though it supports the survey rather than replacing it.

The simplest method is often the best: one careful person, a hose, a torch and patience. A hose test is slow, but it is reliable and needs no special kit, which is why surveyors return to it when other methods disagree. Our page on commercial roof surveys sets out what a survey covers.

First comes a temporary seal, so the building stays dry while the proper repair is planned. We treat leaks as a priority and stop the water first, before planning the lasting repair. A durable repair follows, sized to the fault, with a photograph of the defect and of the finished work.

If the leak followed a storm, let your insurer know early, hold on to receipts for temporary work and photograph the damage before anything is cleared, which matches Which? advice. Our page on commercial roof insurance covers the order of calls.

If tracing a leak shows that the roof has many weak points, you will be told plainly, with the reasoning set out in our guide to repair or replace on a commercial roof. A written quote with a single price follows, and the smaller job gets the recommendation whenever it will properly hold.