A leaking roof over a warehouse or factory floor is more than an inconvenience. Water onto stock, machinery or electrical equipment can stop production, spoil goods and create a slip hazard, and the cost mounts quickly. The pressure is to patch whatever drips and hope for the best, and that approach often fails because the point where water appears inside is not where it enters outside.
Industrial roofs are large and simple in outline, usually a single pitch or a shallow double pitch of profiled sheets, with a line of roof lights and a gutter along each eaves or between bays. That simplicity hides the details that matter: the laps between sheets, the fixings through them, the junctions with roof lights and walls, and the gutters and outlets that carry the water away.
This guide runs through the four common sources of leaks, explains the particular issues with asbestos-cement sheets, shows how a leak is traced and given a temporary stop, and sets out how the choice between repair and overcladding is made. A survey with photographs gives an owner the evidence to decide, and the options are explained in plain words.
Four places an industrial roof leaks
Almost every leak in a profiled-sheet roof can be traced to one of four places. Knowing them helps you describe the problem to a roofer, understand the answer and judge whether a quoted repair addresses the real cause. A line of drips along one purlin points one way, and a wet patch under a roof light points another.
Laps between sheets
Sheets overlap along their sides and ends, and the overlap is sealed with a strip of sealant or a close fit. When sealant dries out, sheets move or the overlap is too short for the pitch, water gets in. Low pitches and long sheets are the most exposed.
Fixings
Each sheet is fixed through the crown or valley with screws or bolts that carry a washer. Washers perish, fixings loosen or corrode, and holes elongate as the roof flexes in wind. A line of drips along a purlin often points to fixings.
Roof lights
The translucent panels let in daylight and are a frequent source of leaks. Their sealing strips age, their fixings loosen, they crack, and the lap with the neighbouring metal sheet is easy to get wrong.
Gutters and outlets
A valley or box gutter between bays carries a heavy volume of water. If it blocks, corrodes, splits at a joint or is laid with too little fall, it overflows into the building. Our answer on commercial gutter lining covers one repair route.
Other causes
Flashings at walls and ridges, penetrations for pipes and vents, damaged sheets from impact and cut-edge corrosion can all let water in. Each of these is checked in a proper survey, and the photographs show you which ones apply to your roof.
Metal sheets: laps, fixings and cut edges
Profiled steel and aluminium sheets are common on industrial units. Their failures follow a pattern.
Lap seals
At side and end laps, a sealant strip sits between the sheets. Over time it hardens, shrinks or is squeezed out. Where the pitch is shallow, water can be driven up the lap by wind or held there by capillary action. Re-sealing laps is a standard repair, though it needs clean, dry surfaces and the right product.
Fastener failure
A washer with a bonded seal crushes, hardens and splits in sunlight. The fixing may back out as the roof expands and contracts. Replacing a fixing with a larger diameter or a different type, with a new washer, cures many drips.
Cut-edge corrosion
Where a steel sheet was cut on site, the exposed edge can corrode if the coating has been damaged. The corrosion spreads under the coating and weakens the sheet. Our answer on cut-edge corrosion explains what to look for.
Condensation, not a leak
Not every drip is rain. In a cold building, warm moist air condenses on the underside of a single-skin metal sheet and drips like a leak. It happens in cool weather, often with no rain at all. A survey separates the two, because a roof cannot be repaired for condensation.
Impact and wear
Dents, holes from dropped tools or plant installation, and damage from foot traffic let water in at specific points.
Age
Coatings fail and the metal below corrodes. When the sheets are corroded across the roof, repairs stop being worthwhile.
Fibre-cement and asbestos-cement roofs
Many older industrial roofs are covered with corrugated fibre-cement sheets, and those installed before 2000 may contain asbestos. They leak differently from metal ones, and they carry extra risk.
How they leak
The sheets crack with age and thermal movement, and a hairline crack lets water through. Their fixings, usually hook bolts or screws with washers, corrode. The laps open as the sheets shrink or move. Moss holds water on the surface.
The safety issue
HSE guidance A14 puts asbestos-cement roofs in the fragile class in every case. Nobody should walk on one without proper protection. Our guide to fragile roof safety sets out the arrangements.
Treat as asbestos until tested
For a building from before 2000, assume asbestos is present until a survey shows otherwise. In the HSE's classification, asbestos-cement lifted out whole is non-licensed work, while heavy breakage moves the job into the notifiable non-licensed category. Waste is double-wrapped, labelled and moved on a consignment note.
Why patching is limited
Fixing a hairline crack in a brittle sheet by patching can disturb the material, and drilling or fixing through it needs care, because each hole is a new point of damage and a new route for fibres to escape. A temporary patch from above is a job for someone with the right access and plan.
Options
The realistic choices are limited repair, overcladding with a new sheet system, or removal and replacement. The survey and the condition of the sheets decide which is sensible.
Roof lights, gutters and flashings in detail
These three details account for a great deal of the leaks reported on industrial roofs, and they deserve a closer look.
Roof lights
A roof light is a section of translucent sheet set in the profile. Water enters at the laps with the neighbouring metal sheets, through cracked panels, around perished seals and at fixings. Plastic panels yellow and crack over time. Replacing a roof light involves matching the profile and sealing every lap. Our short answer on replacing industrial roof lights goes through it. Remember that roof lights can be fragile.
Gutters
Box and valley gutters are large and often lined. Joints, corners and outlets are weak points. Debris builds up, water ponds and the lining fails. BS 6229:2025 keeps a minimum finished fall of 1:80 for internal gutters on flexible-membrane roofs, which shows how much falls matter, and a metal gutter should be laid with a fall too. A gutter with standing water is a gutter that is going to fail.
Flashings and trims
Where the roof meets a wall, a ridge or a gable, flashings seal the gap. They fail at laps, at fixings and where sealant has aged. A line of staining down a wall below a flashing points to a leak at that level.
Downpipes
Blocked or undersized downpipes cause overflow. Check them as part of any leak investigation, by running water down them and watching how fast it clears, since a pipe that looks open from the top can be blocked at a bend.
Tracing the leak and stopping it temporarily
Finding where water enters a large roof is detective work. The wet patch on the floor is rarely beneath the hole.
Start with the evidence
Note where and when the leak happens: only in heavy rain, in wind-driven rain, after snow melts or in cold, still weather. Rain-only points to a hole in the roof. Cold-weather drips with no rain suggest condensation. Photographs of stains and a plan of the floor help.
Inspect from inside
Look at the underside of the roof for staining, rust streaks, wet insulation and daylight at laps. Water runs along sheets and purlins before it drips, so follow it uphill.
Inspect from outside
On the roof, with proper access and edge protection, check laps, fixings, roof lights, gutters, flashings and penetrations near the path the water followed. Where the roof is fragile, work from a platform or crawling boards.
Water testing
Where the source is unclear, controlled water testing, working from the lowest area up, can find it. It needs care to avoid flooding the building, and stock or equipment below should be covered or moved first. Each test area is isolated, so that a result can be tied to a single part of the roof.
A temporary stop
While a permanent fix is arranged, a temporary measure can keep the building dry: a sealant over a lap, a patch over a hole, a temporary cover over a roof light or a cleared outlet. Keeping further water out is the priority, and each step is recorded with photographs. The written price for the permanent repair follows once the survey is complete.
Repair, overclad or replace
After the survey, the question is how far to go. The right choice depends on the condition of the whole roof, not just the leak.
Repair when it is local
If the sheets are sound and the leaks come from a few identifiable places, such as one roof light, a run of fixings or a gutter joint, a repair is good value. Re-sealing laps, replacing washers and fixings, renewing roof lights and re-lining a gutter are all standard.
Overclad when the roof is tired
Overcladding fixes a new sheet system over the existing roof. It suits a roof where the sheets are widely worn but the structure is sound, and it avoids removing old material. It is often the answer for asbestos-cement sheets that are in poor condition, since the old sheets stay undisturbed beneath. Our guide on whether to overclad or replace an asbestos-cement roof sets out the trade-offs.
Replace when it has failed
If the structure is weak, the sheets are failing and the roof cannot take a second layer, replacement is the sound route. It is the most disruptive and costly, but it resets the roof.
What the survey tells you
It shows the proportion of the roof that needs attention, the condition of the structure and the likely life of any repair. A repair on a roof near the end of its life buys little.
Our price
You receive one written total for the route you choose, with the photographs behind it. Our commercial roofing page lists the related work.
