Industrial roof leaks: common causes

A profiled sheet roof is a series of overlaps. Each side lap and end lap relies on sealant, the right amount of overlap and tight fixings to keep water out. Over the years the sealant dries and shrinks, the sheets move, and a gap opens.

End laps are the more exposed of the two, particularly on shallow pitches where water moves slowly and can be pushed back up the roof by wind. An end lap that was marginal when built becomes a leak once the sealant has aged.

A fault at a lap can be small. A line of fresh sealant, a stitching screw and a cleaned surface will often cure it. If sheets have deformed or corroded at the lap, though, patching only delays the problem, and the survey will say so. Our page on roof systems for commercial buildings shows how laps behave on different types of roof.

Every sheet is held down with screws or bolts, and each one passes through the waterproof layer. The sealing washer under the head does the work of keeping water out. As the washer ages, it hardens and splits. The fixing may rust, back out of the purlin through repeated movement, or be crushed by heavy tightening.

On a large roof there can be thousands, and a handful of failures can account for a leak that looks mysterious. The cure is to replace the failed fixings with ones of the right type, fix them to the right place in the profile and renew the sealing cap. Fixing pattern matters as well: sheets are meant to be fixed through the crown or the trough depending on the profile, and a roof fixed in the wrong place by a previous contractor will leak at every one of them, however new the screws are.

If many fixings are rusted, the problem is bigger. The sheets and sometimes the purlins are decaying, and it is time to consider more than a repair. Our guide to cut edge corrosion on metal roofs helps explain what is going on.

A great deal of rain ends up in gutters on an industrial building, and when they fail the water goes into the walls, the eaves or the roof void. Joints between lengths open as the gutter moves. The lining in box and parapet gutters cracks. Outlets block with silt, leaves, moss and packaging.

Overflow is a common sign. If you see water coming over the front of a gutter in heavy rain, or stains below the gutter line, the outlet is probably not coping. A blocked outlet on a flat roof leaves a pond that sits against every seam and upstand.

Clearing and re-sealing is often enough. Where a lining has gone, there are options, set out in our page on industrial gutter lining. Water running from a valley across the inside of a wall is a sign worth attention.

Every break in the roof is a chance for a leak. Ridge flashings, wall abutments, eaves closures, vents, flues, fans, cable and pipe penetrations, and rooflights all depend on sealant and well-shaped metal. Sealant fails; flashings loosen or are bent by wind; rooflight panels crack.

The leak in these cases appears close to the feature, so they are usually the easiest to find. Check the sealant bead and the fixings holding each flashing, and look for gaps between the flashing and the sheet. Wind-driven rain is the real test: a flashing that stays dry in a calm shower may leak when rain blows sideways, which is why some leaks appear only on one face of a building.

Penetrations added later, such as extractor fans, solar frames and aerial mounts, are a favourite source of trouble because they were cut and sealed after the roof was built. Our page on whether a commercial roof can take solar panels explains how fixings should be handled when equipment is added.

Not every drip is a leak. Warm, humid air inside a building meets the cold underside of a metal sheet and condenses. The water collects, runs to the low points and drips. It shows up on cold mornings especially, and in buildings where processes give off steam or moisture.

Signs that point to condensation include drips across a wide area at once, no obvious link with rain, and droplets forming on the underside of sheets. Signs of a leak include a single wet patch that follows rainfall and traces back to a lap, fixing or flashing.

The cures differ. Condensation calls for ventilation, insulation or a liner. A leak calls for repair. Mixing the two up wastes money, which is why the survey checks both. Our page on shop roof repairs deals with a similar mix of causes in retail buildings.

Protect stock and equipment first, using sheeting and buckets, and move anything electrical away from the drip. If the water is coming through near lights or wiring, switch off the supply at the board and call an electrician.

Do not go onto the roof to look. A wet profiled roof is slippery, and rooflights are fragile. Photograph the stain and record the timing: in the rain, after it, or only on cold mornings. That detail saves time.

We prioritise leaks and storm damage, and the first visit stops the water doing more harm. The survey that follows gives you pictures of each defect and a written quote. We would sooner re-seal a lap than re-sheet a roof, we clear up after ourselves, and the workmanship carries a 10-year written guarantee. The wider range is on our commercial roofing page.