The short answer
You find a warehouse roof leak by working backwards from the drip. Note where water lands and where stains run, follow the line up the underside of the sheets and purlins, then inspect the roof above that line for open laps, failed fixings, cracked rooflights, blocked gutters and loose flashings. Water often runs sideways along a rib or purlin before it falls, so the hole is usually higher up the slope than the puddle. Rule out condensation before assuming a leak.
Start inside the building
Begin from the floor and look up, ideally during or just after rain. Mark every drip point and any stain on the ceiling, racking or stock. Then look along the roof's underside. On a sheeted warehouse the water usually runs down the inside face of a profile or along a purlin, and falls where there is a break, such as a bolt, a joint or a sag. Follow the wet trail uphill, towards the ridge. Photograph it with a date and a marker, since the pattern in a dry week can be misleading.
Check stock and equipment first. Move or cover anything that matters, and keep clear of electrics and of any area where the roof lights or sheets look sagging. Only trained people with the right access gear should go up on the roof itself, which brings us to the next point.
Rule out condensation
Keep a simple log as you go: the date, the weather, the wind direction, where the water landed and how long after the rain started. Three or four entries often show a pattern that points straight at one side of the roof or one run of gutter, and the log is useful to whoever repairs it.
Not every drip is a leak. In a cold, damp warehouse, warm moist air meets the underside of uninsulated metal sheets and condenses into drops that look exactly like a leak. Clues are drips across a wide area, drips on cold mornings that stop by midday, and drips with no matching wet patch on the outside. Condensation is a ventilation and insulation problem, so it needs a different fix. A true leak follows rain, concentrates in lines and tracks back to a feature on the roof.
Where leaks start
On a sheeted roof, the common sources are the side and end laps where one sheet overlaps the next, sealant that has dried and cracked, and fixings that have loosened or whose washers have perished. Cut edge corrosion, where rust eats along the cut edge of a coated steel sheet, also opens up leaks. Our page on cut edge corrosion on metal roofs explains how to spot it. Rooflights are the next suspect: old rooflights crack, and the sealing round their edges fails. Gutters and valleys can overflow in heavy rain, and a blocked outlet will push water back under the sheets. Our guide to commercial gutter lining covers the box and valley gutters where that often happens.
On a flat or membrane roof, look at seams, upstands, parapet copings, outlet connections and penetrations such as flues, vents and pipework. The same rule applies: the water enters at a feature, not in the middle of an open sheet of membrane.
Tracing it on the roof
Roof work on a warehouse is high-risk. Many sheets and rooflights are fragile and give no warning. Safe access means a proper plan, edge protection, crawling boards or a mobile platform, and no one working alone. Read our page on fragile roofs and rooflights before anyone goes up.
Once up there, the survey follows the interior lines. Check the laps, fixings, flashings and rooflight edges along that route. If the source is not obvious, a water test works in stages: a hose is run on one small section at a time, starting low on the slope and working up, while someone inside watches for the first drip. Wetting the whole roof at once tells you nothing. Thermal imaging or a drone survey can find wet insulation under a membrane, and is a reasonable step for a large flat roof where a simple look has not found the cause.
Making it safe and getting it fixed
A temporary patch, such as sealant, a lap strip or a rooflight cover, can stop the water until a proper repair is arranged, but a patch is not the cure. Once the source is found, the repair might be re-sealing laps, replacing fixings, fitting a new rooflight, or lining a gutter. If the pattern shows wide wear, a larger plan is more honest. Regular checks help prevent the next leak, so see our page on planned maintenance for commercial roofs.
We prioritise leaking roof calls and will make the roof safe first. Our free survey includes photographs of the source and a fixed written quote for the repair. You can read more about our work on commercial roofing in Basingstoke.
Every roof is different. A free survey with photos settles it for yours.
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