Metal roof leaks at laps
Why do metal roofs leak at the laps?
The short answer
Metal roofs leak at the laps because that is where two sheets meet and the seal between them has to do all the work. The usual causes are sealant that has dried out, fixings that have worked loose or corroded, sheets that move with the temperature, and a pitch too shallow to shed water quickly. Tracing the true entry point comes before any repair.
What goes wrong where profiled steel sheets overlap on a unit, barn or warehouse, how to trace the water, and which repairs last.
How a lap is meant to work
A profiled metal roof is a series of long sheets laid side by side and end to end. Where one sheet rides over its neighbour along the length, that is the side lap. Where a sheet in the next course overlaps the one below, that is the end lap. Water is shed by the overlap itself, and by a strip of sealant or a gasket pressed between the sheets to close the gap that capillary action would otherwise draw water through.
Fasteners hold the laps together. Screws through the sheets, and stitching screws along the laps, pull the metal tight onto the sealant. The system depends on three things staying in place for many years: the sealant, the fixings and the shape of the sheets.
When any one of them fails, rain finds its way through a gap that may be a fraction of a millimetre wide. That is why a roof that looks sound from the ground can still drip onto stock below.
The usual causes
Sealant ages. Under sun, cold and flexing it hardens, cracks and pulls away from one of the sheets. Mastic fitted at the time of building may also have been thin, displaced as the screws were driven, or missing from the end laps.
Fixings loosen. Steel expands and contracts daily, and over the years screws work out of their holes and the sealing washers harden or split. A screw head that has lifted even slightly gives water a path along the shank. Corroded fixings fail in the same way, which our guide to cut-edge corrosion relates to the sheets themselves.
Movement and shallow pitch complete the list. A long run of sheet flexes with the temperature and opens the lap, and a roof laid too flat holds water against the joint instead of shedding it. Damage from foot traffic can dent a rib and break the seal beneath it. Add the sea weather on this coast, with driving rain pushing water up laps that would stay dry in still air.
Tracing the real entry point
Water on a metal roof travels. It runs down the underside of a sheet, along a purlin and drips from a point that may be metres from where it came in. Marking the drip on the floor and looking up is not enough.
A proper trace works in stages. Inside, look for rust runs, stained insulation and daylight through the laps, and note which way the stains run. Outside, follow the line uphill along the sheet to the nearest lap, fixing or penetration. Where it remains unclear, a hose test applied one section at a time while someone watches from inside will narrow it down, and our guide to commercial roof leak detection describes the methods.
Sometimes the water is not rain. Condensation forms on cold sheets and drips in a way that looks exactly like a leak, which is covered in our answer on warehouse roof condensation. Telling the two apart avoids paying for a repair that cannot work.
Repairs that hold
The right repair follows the cause. Where sealant has failed, the lap is opened, cleaned and re-sealed, with a butyl tape or a suitable sealant matched to the sheet. Where fixings have loosened, they are replaced with new screws and sealing washers, and stitching screws are added along the lap to pull the sheets back together.
Where sheets are dented, cracked or corroded around the lap, a patch is a short-term answer at best. Replacing the affected sheets is the lasting fix, and our guide to replacing steel roof sheets shows what that involves. Where the whole roof is tired but sound, a liquid coating over properly prepared laps can extend its life.
Beware surface-only fixes. A bead of sealant squeezed over the top of a failed lap may hold for a few weeks and then peel, because it is stuck to dirty or weathered metal. Preparation takes longer than the sealing, and it is what makes the repair last.
Safety and when to call it
Metal sheets are slippery when wet, and old roofs can hide fragile rooflights and fibre cement panels. Never walk a roof you have not been shown is safe, and use crawling boards or proper access. If the sheets are asbestos cement, stop and find out first what the sheets are made of, because the work rules are different.
Call a roofer sooner than you think. A leak at a lap is cheap to cure while the metal is sound, and expensive once water has soaked the insulation, rusted the purlins or damaged stock. A survey tells you whether a series of lap repairs will do the job or whether the roof is nearing the point where recovering makes more sense.
Our commercial roofing team surveys metal roofs on units, barns and warehouses, photographs each defect and recommends the least work that will stop the water for good. The price is written down after the survey.
Related questions
Metal roof leak questions
Can I seal a lap leak myself from a ladder?
A ladder gives you the edge only, not the laps up the slope, and walking the sheets is risky. Dirty metal also takes sealant badly. A short-term bead from inside the roof space is possible, but a proper repair needs safe access.
Will extra stitching screws stop a leaking lap?
They can, where the cause is a lap that has opened. They cannot cure failed sealant, a split sheet or a leak caused by condensation, so find the cause before adding fixings.