Start from the drip and work up the slope
Water that gets through a metal roof drops where a purlin dips or a bolt hangs down, so the drip tells you where to start looking, not where the hole is.
- Mark the drip on the floor and the spot above it on the underside of the roof, and photograph it in daylight. Wet stains and rust trails on the purlins show the path the water took.
- On a dry day, go up with walk boards, in a harness clipped to a lifeline, and look upslope of that spot along the same line of sheet. Check the nearest lap, fastener, flashing or penetration first.
- If nothing shows, the next step is a hose test, which is a job for a trained roofing crew. They start at the lowest point, at the gutter, and work upslope one section at a time, with someone inside watching the drip, giving each section ten minutes before moving up. Water poured at the ridge first soaks everything below it and tells you nothing.
- Write down what was found and where, and keep it with the roof's records.
Nobody should go on a roof alone, in the rain, without a harness, or off the purlin lines. Skylight sheets will not carry a person's weight, and falls through them are a common cause of fatal accidents in factories. Our nine rules for factory staff and vendors, in the guide on how often to inspect a factory roof, cover who may go up and how.
Where factory roofs usually leak
These are roughly in the order we find them on audits.
Fasteners. Each screw carries an EPDM washer under its cap. Over-tightened, the washer squeezes out and splits in the sun. Under-tightened, it never seals. Washers also perish with age, and a screw driven into the valley of the sheet instead of the crest sits in running water. A rusted screw head is often one whose washer has also gone. The minimum roof pitch guide explains how they should be fixed.
Gutters. A gutter set with its front edge lower than the back spills outside when it overflows. One set the other way, or one that is blocked, backs water up under the eave sheet. On a box or valley gutter between two roofs, the overflow goes over the back of the gutter and straight into the building, and it looks exactly like a roof leak. Check the outlets, the number of downpipes and the gutter joints before blaming the sheet.
Laps. Side laps laid against the wind, end laps that are too short or on too flat a roof, and laps with no butyl tape inside them. The pitch guide covers these in detail.
Skylights. FRP and polycarbonate sheets age faster than steel. They go brittle, crack around their fixings, and leak at the laps when their profile does not match the metal sheet next to them. We make our FRP sheet in our 2 Rib or 3 Rib profile, or to any profile you send us, so that it laps in with the metal sheet on either side.
Pipes, ducts and fans added after handover. Exhaust fans, air-conditioning ducts, cable trays and solar mountings are often put through a roof by someone other than the roofer and sealed with a ring of silicone. A penetration needs its own flashing, formed to the profile of the sheet. "No unapproved penetrations" is the first of our nine rules for factory staff and vendors.
Ridge, gable and wall flashings. A flashing with the wrong profile, gaps at its ends or no foam fillers lets wind-driven rain in at the top and sides of the building, often only when the wind is from one direction.
Rust holes. These appear where water sits: at end laps on flat roofs, along valleys under silt and leaves, beside a gutter that holds water, and around specks of swarf left from cutting. One or two can be patched. When they appear across the roof, the coating has gone and repairs stop making sense.
When it is not a leak at all
Water dripping from the roof on a cold morning, when it has not rained, is usually condensation. Warm, damp air inside the shed meets the cold underside of the sheet overnight, the moisture collects, and it drips. It is common in winter in sheds with steam, cooking, dyeing, washing or a lot of people working, and in any building with no insulation under the sheet.
Sealant will not fix it. The cure is insulation under the sheet, more ventilation through ridge or turbo ventilators, or both, and it is worth ruling out before paying for roof repairs.
A temporary fix in the monsoon
If a joint leaks in the middle of the rains and the proper repair has to wait, a butyl or aluminium foil tape over the joint will hold for a while. It only sticks to a surface that has been cleaned and dried first. Note where it went, so that the joint is opened and repaired properly once the roof is dry.
Fixes that make a leak worse
- Silicone or bitumen smeared along the outside of a lap. It cracks in the sun, traps water inside the lap, and has to be stripped off before the real repair.
- A coat of tar or a liquid membrane over a roof whose joints are failing. It hides the joints that are letting water in and makes the sheet impossible to inspect.
- Extra screws through the valleys of the sheet to hold it down.
- Sealant applied to a wet sheet. It does not stick, and it hides the leak path from whoever comes to find it.
- Walking on the sheets between purlins, which dents the crests and opens the laps.
Repair or reroof
Most leaks on roofs under about 12 years old are at the joints, and a repair fixes them: new fasteners, resealed laps and penetrations, new flashings, cleared and relined gutters, and a few new sheets or skylights. When the sheet itself has rusted through in many places, or the coating has gone across whole slopes, repairs only move the leak along, and the roof needs new sheet over all or part of it.
That decision should come from a survey of the whole roof, zone by zone, not from where the drips are. Our roof audit scores each zone so that you repair the parts that need repair and reroof only where the sheet has failed, and its price is set out on that page. The reroofing page explains how we do the work bay by bay with production running.
Before the next monsoon
Most leaks show up in the first heavy rain after months of dry weather, when the gutters are full of dust and leaves. Clear the gutters and outlets, check the fasteners and skylights, and look for anything new cut into the roof since the last check. The monsoon checklist in the minimum roof pitch guide and our guide on how often to inspect a factory roof set out what to look at and when.
Sources and standards referred to
- MNF Projects roof audit reports and Roof Health Passport
- MNF Roof Rule Book for factory staff and vendors