The short answer
Industrial roofs leak at the laps because that is where one sheet meets the next and the weather works hardest. Sealant dries and splits, fixings loosen as the steel moves, and water is driven or drawn sideways under the overlap. Rooflights and cut edges fail for related reasons. Tracing the real entry point comes before any sealing.
A stain on the floor of a unit rarely sits under the hole. Water runs along purlins and sheet ribs before it drops, so the repair starts with the trace.
What the laps are and why they let go
A profiled metal roof is a set of long sheets laid side by side and end to end. Where two sheets meet along their length is the side lap. Where one sheet sits above another up the slope is the end lap. A strip of sealant, usually a soft mastic tape, is laid between the sheets so the joint stays watertight while the fixings pull it tight.
Why does a sound lap start to leak?
Steel expands in summer sun and shrinks on frosty nights, and over the years that movement works at the joint. The sealant hardens, loses its grip and splits. Dirt and moss settle in the lap and hold water against it. On a low pitch, rain that would run off a steep roof lingers long enough to creep upslope under the end lap, especially in a gale.
A lap that was marginal on the day it was built is the first to fail, and a lap in a corner that never dries out is next. When we look at a leaking unit we check these two kinds of lap before anything else, because they account for the leaks that appear after a wet, windy week. Sheets that were fixed in a hurry, with laps closed up tight on a dry day and never revisited, are the usual suspects, and so are sheets patched more than once with whatever sealant was to hand.
Fixings, washers and the holes they make
Every sheet is held down with screws that pass through the metal into the purlins, the horizontal beams that span the rafters or portal frames. Each screw has a washer that seals the hole it makes. The weak point is usually the washer, not the screw.
Are loose fixings a cause or a symptom?
Both. Wind lifts and drops a sheet thousands of times, and each cycle opens the hole a little more. The rubber of the washer perishes in sunlight, the screw backs out slightly and leaves a gap, and rust follows, which makes the hole larger still. A roofer looks for fixings that spin loose, ones that sit proud and ones that missed the purlin altogether.
Re-fixing with the right screws and fresh washers often cures a leak, but a screw into a rusted or rotten purlin has nothing to bite on. That is why we look at the structure under the sheet from inside the unit, where the building allows it, before deciding what the repair needs to be. A patch on a roof whose frame is failing only moves the problem along the building.
Rooflights and cut edges
Many leaks that look like lap faults begin at a rooflight, the translucent panel that lets daylight in. A rooflight sits in the same lap system as the metal sheets, so its own side and end laps, and its fixings, need the same attention. Plastic panels also yellow and crack with age, and a hairline crack lets water through without leaving any visible hole.
What is cut-edge corrosion?
Wherever a sheet is cut on site, around a vent, a flue or at the ridge, the protective coating is broken and the bare edge can start to rust. The rust spreads under the paint and thins the sheet at the very point where it is lapped. Our page on cut-edge corrosion explains how it develops and what slows it.
Gutters matter too. A blocked outlet or a clogged valley backs water up under the laps, and the leak appears far from the blockage. Our notes on box gutter lining cover that side of the roof, and the wider work sits under commercial roofing.
Tracing a leak before sealing it
Water on a unit floor does not tell you where it came in. It follows the ribs of the sheets, the flanges of the purlins and the line of a cable tray before it lets go. A bucket under the drip marks where water leaves the roof structure, not where it entered.
How do we find the real entry point?
We start inside, with the stains, the drip lines and the direction the roof falls. Then we go onto the roof, using crawling boards across the sheets or a mobile platform where that suits better, and study the laps, fixings, rooflights and edges above the stains. If you can tell us when it leaks, in wind-driven rain or after a long wet spell, that narrows the search a great deal.
Every fault is photographed, so what we saw is what you see, and the repair comes as a single written price. If the sheets are old fibre cement, read about fibre-cement roof sheets before anyone steps on them. To start, send photos of the stains inside and of the roof from the yard.
Leaks on unit roofs
Will a tube of sealant fix a leaking lap?
It can slow a leak for a season or two, but sealant over a dirty or moving lap often peels away. The lasting repair cleans the joint, replaces the lap tape or the sheet where needed, and renews the fixings. A sealed lap over a rusted sheet only hides a failing roof.
Does the leak always sit directly above the stain?
Rarely. Water travels along the sheet ribs and the steelwork below before it drops, sometimes several metres from where it came in. We trace it from inside and outside together, because a patch in the wrong place leaves the leak running and costs you the repair twice.
The full picture: Commercial roofing in Taunton ยท More short answers