The short answer
A leaking warehouse roof is usually a small failure in one of five places: a fixing that has lost its seal, a lap between sheets that has opened, a rooflight edge, a blocked or sagging gutter, or a cut edge that has rusted through. Water often travels along the steel before it drips, so the stain inside is rarely under the hole outside.
Where water gets into a profiled metal or sheeted industrial roof, how to read the clues from the floor, and what a repair involves.
Fixings, laps and why the drip is not under the hole
On a profiled sheet roof, every sheet is held by screws or bolts with a sealing washer under the head. Over time the washer perishes, the screw backs out as the sheet flexes in the heat, or the hole elongates in a gale. Each one then lets in a trickle. Laps are the other weak point: the overlap between two sheets, sealed with a strip of mastic or tape. If that seal dries out or the sheets are fixed too far apart, wind-blown rain gets between them.
Water seldom drops straight through. It runs along the underside of the sheet, follows a purlin and falls somewhere else, sometimes several metres away. That is why tracing a leak from the stain on the floor is unreliable, and why a roofer walks the roof, or uses a hoist or platform, and checks along the line of the slope above the stain. Our page on replacing metal roof sheets covers the point at which patching stops being sensible. Our commercial roofing page covers the rest of the work on industrial and retail buildings, from re-roofs to routine inspections. A leak that returns after a patch is usually telling you the fault was elsewhere on the same sheet or run, so ask the roofer to explain why they chose the spot they repaired.
Rooflights and their surrounds
Translucent rooflights are a common source of leaks because they are made of a different material from the sheets around them and move at a different rate. The sealant at their edges cracks, the fixings corrode, and old plastic rooflights turn brittle and split. Some are also fragile, meaning they cannot carry a person, and a fall through one is among the worst accidents in roof work. Nobody should walk on or across them without crawling boards and edge protection.
When a rooflight leaks, a roofer checks the side laps, the end laps, the fixings and the condition of the panel itself. Sometimes resealing is enough; if the panel has gone brittle or yellowed and cracked, replacing it is the lasting answer. Our page on industrial roof light replacement explains the process. Whatever the repair, the safe route to reach the rooflight has to be set up first, so ask how access will be arranged before you agree. Edge protection, crawling boards or a platform from below all cost time, and a firm that skips them is cutting a corner that matters. Mark the rooflights on a site plan for anyone else who goes up there, such as an aerial fitter or a gutter cleaner.
Gutters, outlets and ponding
Large industrial buildings often drain into a valley gutter between two roof slopes, or a box gutter hidden behind a parapet. When these are blocked with leaves, moss, bird droppings or debris, the water rises until it finds a way over a lap or under a flashing. Gutter linings also split at joints and corners, and sagging sections hold water that should be draining away.
The sign is staining below a gutter line, or water on the floor after heavy rain but not after light showers. A routine clear-out solves a blockage, but a gutter that holds standing water has a fall problem, and one with open joints needs relining or replacing. Our page on commercial roof drainage problems covers both, including how box gutters are lined. Ask whoever cleans the gutters to report what they see, because a regular note of the condition shows when the next job is due.
Cut edges, corrosion and age
Coated steel sheets are protected by a layer of paint over a metal coating, but wherever a sheet is cut, the bare steel is exposed. Rust begins at that cut edge and creeps under the coating, and at laps and at the base of sheets it can eat through. You will see it as orange staining, blistered paint or small holes along the lower edge. Our guide to cut edge corrosion explains how far it has to go before sheets need replacing.
Older fibre-cement roofs bring a separate concern. A building from before 2000 should be treated as one that may contain asbestos, and asbestos cement roofing is always fragile. Do not scrape, drill or clean such a roof yourself, and keep staff off it. Describe the building and where the water shows, and we will look at the roof, share photographs of what we find and put the repair into one written total. If the leak is damaging stock or machinery, say so, because we make a leaking roof safe first and then deal with the lasting repair.
More on this in Commercial Roofing in Crawley & Gatwick
Two leaking roof questions
Can I stop a warehouse leak with tape or sealant?
A temporary patch can slow a drip for a few days, but sealant over a failed fixing or corroded edge soon cracks again, and working on a roof without proper access is dangerous. Move stock if you can, catch the water, note where it appears and have the roof inspected so the cause is repaired rather than covered.
Does the leak always mean replacing the roof?
No. A few failed fixings, an open lap or a split rooflight can be repaired. Replacement becomes the better buy when rust has spread across many sheets, when new leaks appear as fast as old ones are fixed, or when the sheets can no longer hold fixings. A survey with photographs shows which case you have.