The short answer
An industrial unit roof usually leaks at a joint rather than across the sheet: failed sealant at laps, corroded fixings, a split in the valley gutter, a cracked rooflight or a worn detail around plant. Water often runs along a purlin before it drips, so the stain on the floor can sit several metres from the defect. A roof survey traces the entry point before anything is sealed.
Why the stain is rarely under the fault
Water entering an industrial roof does not drip straight down. It meets the underside of a sheet and runs along the slope, tracks along a purlin, the horizontal steel member that carries the roof sheets, and drops where the steel changes direction or a bolt hole gives it a way out. The drip you see over the packing bench may start at a lap near the ridge.
That is why daubing mastic over the wet patch so seldom works. The first job is to work out where the water gets in, and then to follow its path from there. On a large unit with a double-skin build-up, with insulation between an inner liner and an outer sheet, the water can travel a long way inside the insulation before it appears. Wet insulation stops doing its job as insulation, so a slow leak deserves quick attention, and the energy bill is often the first place it shows.
The usual leak points on a metal-clad roof
On a profiled metal roof, the sheets themselves seldom fail first. The weak points are almost always where one thing meets another.
- Side and end laps. The sealant between overlapping sheets dries out and shrinks over the years, and a lap that was once tight opens up to a capillary gap.
- Fixings. Screws pass through the roof sheet into the purlin and rely on a washer to seal the hole. Washers perish, screws back out as the steel expands and contracts, and the hole becomes a leak.
- Cut edges. Where a sheet has been cut on site, the coating is gone and the steel begins to rust. See our page on cut-edge corrosion.
- Ridge and flashings. Ridge cappings and the flashings at walls and eaves rely on fixings and sealant that are exposed to full weather.
- Cracked sheets. A sheet cracked by a dropped tool or by someone stepping between purlins will weep along the crack.
Valley gutters, rooflights and plant
Many industrial buildings are made of two or more bays side by side, and where the bays meet is a valley gutter. A valley gutter is the lined channel between two roof slopes. It carries the rainwater of both, and when its lining splits, its outlet blocks or its fall settles, the water goes into the building. The industrial valley gutter page has the detail.
Rooflights are the second place. Older rooflights are often plastic or glass-reinforced sheets that go brittle in sunlight, crack at their fixings and lose their seal to the surrounding metal. A rooflight is also the weakest part of the roof to walk across, which raises a safety question as well as a leak question.
The third is anything that has been added. Extraction fans, flues, cooling units, cable trays and solar mounting rails all pierce or sit on the roof, and each penetration needs a sealed collar or flashing. A hole cut for a service pipe years ago and filled with mastic is a classic source of a leak that keeps coming back, because mastic moves and cracks as the metal heats and cools.
Flat and low-pitch sections
Many industrial sites have a flat or low-pitched section over offices, loading bays or an extension. Here the covering is usually felt, a single-ply sheet or an applied coating, and the leaks follow a different pattern. Look for ponding, where water lies on the surface after rain. Look for blisters and splits in felt, open seams in a membrane and gaps at upstands, the upward turn of the covering against a wall.
Outlets are worth checking first on these roofs. An outlet fitted slightly proud of the surface, or blocked with silt, leaves a shallow pond that works on the covering for years. If a flat section sits next to a pitched metal roof, the junction between the two is a frequent entry point, because two systems with different movement are being asked to share one flashing.
When the covering is tired in several places at once, patching stops being sensible, and our guide to repairing or replacing a commercial roof shows how to weigh it.
Tracing the leak and getting it fixed
A proper leak investigation starts on the roof and ends in the building. We look at the stain, the nearest purlin, the line of the slope above it and every lap, fixing, rooflight and penetration along that line. Photographs of the defect are taken for you to look at, and they are useful later if an insurer or a landlord asks what failed.
If water is coming in now, the priority is to stop further harm: divert the water, cover the opening, protect stock and electrics. A lasting repair comes after the cause is established, and it is priced only once we have seen the whole run of the leak. Patching a symptom without finding the cause is how the same leak comes back in the next storm.
It is also worth reading what your insurer expects, because water damage claims often turn on whether the roof was maintained. What commercial property insurance expects of a roof is explained separately. For a leak you want looked at, we survey free, give a fixed price in writing and prioritise leaks. The wider picture of our work is on the commercial roofing page.
