Building problem
Corrosion of metal roofs: the rust starts underneath, where the water sits.
A coated steel sheet is protected on both faces until the coating is broken. Fixings break it, cut edges expose it, and condensation keeps it wet from below through every cold night. Rust streaks at the fixings are the first sign; a sheet you can push a screwdriver through is the last.
- Coated steel and galvanised sheets
- Condensation stopped at source
- Nationwide coverage
01What you see
Rust streaks at the fixings, brown lines along the laps.
Underside corrosion starts where the coating is broken and the steel stays wet longest: at the fixings, along the cut edges of the laps, and in the valleys of the profile above the purlins. It shows as streaks first, then as bloom, then as holes.
- At the fixings first. Every fixing hole is a break in the coating, and the washer that once sealed it has hardened. Water sits there every cold morning.
- Along the laps and cut edges. The coating stops where the sheet was cut. Side laps and end laps expose bare edges to the same wet.
- Where the water sits longest. The valleys of the profile and the flanges of the purlins hold water after the rest of the sheet has dried.
Not a leak from outside.Not cured by paint alone.The water arrives from the room every cold night. Stop it arriving.
02Why it happens
Wet steel, a broken coating, and no chance to dry.
Steel rusts when it is wet and its protective layer is broken. Condensation supplies the water every cold night; fixings and cut edges supply the breaks; and because the underside is shaded and still, it dries slowly. Cut-edge corrosion at the sheet ends and laps is the same mechanism seen from the top.
A cold night. Condensation forms across the underside, runs to the fixings and the lap edges where the coating is broken, and sits there. Rust streaks follow, then bloom, then thinning.
Schematic, not to scaleValues shown are example values, not a surveyed building.
03What it costs you
Fixings that lose their grip, sheets that thin, and rust-stained water below.
Corrosion is slow and then sudden. Fixings rust until they no longer hold or seal; sheets thin until they hole; and everything beneath takes rust-stained water in the meantime. The end of that road is a roof replacement.
The fixings
Grip and sealA fixing that rusts loses both its grip on the purlin and its seal against the weather. Sheets loosen in wind, and every rusted fixing is a new place for rain to enter from above.
The sheets
Laps and valleysSteel that thins along its laps and in its valleys eventually holes. Patching a holed sheet is a stopgap; enough of them and the roof is replaced, with everything that involves.
Below the roof
Stock, plant, floorsRust-coloured drips stain stock and packaging, and corrode plant and vehicles parked beneath. In a hangar or a workshop the things under the roof can be worth more than the roof.
04What usually gets tried
Paint on wet steel does not hold. New fixings make new breaks.
Each usual remedy addresses the coating or the fixing and leaves the water supply where it was. Paint applied over a sheet that wets every night fails at the same places it was meant to protect, and replacing fixings adds fresh breaks in the coating.
Repainting the underside
A coating needs clean, dry, prepared steel and a surface that stays dry afterwards. Under a bare sheet neither holds for long: condensation returns the next cold night and gets behind the paint at every edge.
Replacing the fixings
New fixings hold better for a while. Each one is a new hole in the coating, and the condensation that rusted the old ones is still arriving.
Overcladding or replacement
A new roof over or instead of the old one removes the corroded steel. It is also the largest spend the building will see, and on many roofs it is not yet necessary.
Spray foam versus overcladding
