The short answer
Protect people and stock first: move goods, catch the water, and keep clear of wet ceilings and electrics. Photograph the damage, then call a roofer to make the roof watertight before arranging the lasting repair. Where the water shows inside is rarely where it enters the roof, so a proper repair follows an inspection, not a guess. We prioritise leaks and storm damage.
Inside the unit: what to do before anyone reaches the roof
Start with the building, not the roof. Water and electricity share ceiling voids on most units, so if a light fitting is dripping or a ceiling tile is bulging, switch off the circuit and keep people out from under it. A sagging tile or board holds a surprising weight of water and can drop without warning. Only release trapped water from the floor, over a bucket.
Next, move what can be moved. Stock, paperwork, computers and machine controls go first; pallets of boxed goods can be covered with polythene if they cannot be shifted. Lay dust sheets or plastic under the drip line and put a container below it.
Note when the drip began and whether rain was falling at the time. That small detail tells a surveyor whether the leak follows rainfall, melt water or a plant fault. Leaks that continue on a dry day often point to a burst pipe, a condensation problem or a blocked outlet still draining, rather than a hole in the covering.
Photographs, notes and a call to your insurer
Before anything is cleared away, photograph the damage from several distances: the stain, the ceiling, the floor, the stock affected and any water on the surface of equipment. Take a wide shot that shows where in the unit it sits, then a close one. Video of a live drip with a timestamp is useful too.
If you own the building, or the lease makes the roof your responsibility, let your insurer know early. Policies often set conditions about notifying a loss and about taking reasonable steps to limit it, so a temporary repair that stops further damage is usually in your interest rather than against it. Tenants should write to the landlord or agent straight away, because the lease decides who arranges and pays for roof repairs. Our answer on who repairs the roof on a leased unit sets out the usual points to check, and the guide to commercial roof damage and insurance covers claims in more detail.
Keep every receipt, including for skips, dehumidifiers and cleaning. A claim is easier to support when the first photographs, the emergency repair and the permanent repair are all written down in order.
Where the water enters against where it shows
On a large flat or low-pitched roof, water can travel a long way between the point where it gets in and the place where it appears. It runs along the top of a deck, down the side of a steel purlin, along the underside of a sheet or between layers of insulation before it finds a joint or fixing to drip from. A wet patch is a clue, not an address.
On a profiled metal roof, the usual suspects are lap joints, sealant that has aged, fixings that have worked loose, and corroded cut edges. Our guide to cut-edge corrosion on metal roofs explains one of the slower causes. On single-ply and felt roofs we look at laps, upstands, pipe penetrations, rooflight kerbs and the outlets that carry water away. A box gutter or parapet gutter that has silted up can overflow into the building and look exactly like a hole in the roof, so we always check the drainage first, and our answer on commercial box gutters covers that failure.
A professional inspection works from the outside in: the surface, the details, the outlets, then the inside of the building. Chasing a leak by sealing the nearest bright patch often just moves the problem.
Making it watertight: emergency work against the proper fix
A temporary repair has one purpose, which is to stop the water getting in until the permanent fix can be planned. It might be a patch over a split in a membrane, tape and sealant over a lap, a temporary cover over a failed rooflight, or sheeting weighted down on a section that cannot be repaired in the rain. Access, weather and the type of roof decide what can be done on the day, and we treat leaks as a priority.
The permanent repair might be different. A split in a membrane may need a new section welded in. A whole run of sheets with failed fixings may need re-fixing with new washers and sealant. A box gutter that has failed might need a lining. If a roof has leaked in many places, the honest conclusion is sometimes that repeated patching costs more than a planned replacement, which is a decision for the owner once the facts are on the table. Our guide on choosing a roof system for a commercial building helps if the answer turns out to be a new covering.
Ask for the emergency cover and the lasting repair to be described in writing. You then know what the emergency cost and what the lasting fix will cost, which also makes an insurance claim cleaner.
Safe access, cost and keeping the trading going
Roof access is where commercial leaks become dangerous. Many industrial and retail units have fragile rooflights or old sheets that cannot carry a person, and a wet roof is slippery. Nobody should go up to look for a leak without edge protection, crawling boards or a proper platform. Unsure? Stay off it. Our guide to fragile roofs and safe access explains why.
For cost, MyBuilder's 2026 guide gives £150 to £1,500 on average for roof repairs and £500 to £1,500 for storm damage, but those are domestic-scale national figures and a commercial roof with access equipment can sit above them. Checkatrade's 2026 guide prices repairs by area, at £100 to £140 per m² for bitumen flat roofing and £120 to £170 per m² for EPDM, excluding access and VAT. After the survey, the written quote covers access, labour and materials.
You do not necessarily have to close. Where the leak is in one bay we can often work around the shop or the shift pattern; how that is planned is covered in our answer on repairing a roof while you carry on trading.
When the repair is done, it carries our 10-year workmanship guarantee, in writing, separate from any manufacturer warranty. The wider picture is on our commercial roofing page.