The short answer
Yes, a rusty metal roof can be coated when the sheets are still sound. The result depends on preparation: loose rust is removed, the cut edges and fixings are treated first, and the laps between sheets are made watertight before any coat goes on. A coating does not bridge holes or replace sheets that have rusted through, so a survey decides whether coating is worth doing.
A coating can add years to a tired metal roof, but only on a sheet with metal left in it. Paint over the wrong roof and you pay twice.
Is the sheet still sound?
Surface rust is staining on the outer skin of the sheet. Rust that has eaten right through is a different problem. The first can be treated and sealed. The second is a hole, and a coating bridges nothing wider than a hairline.
How do you tell the difference?
From inside the building, look up at the underside on a bright day. Pinpricks of daylight show perforation. Flaking rust that comes away in plates, or sheets that flex and sag between the purlins, point the same way. A screwdriver handle tapped on the underside of a suspect patch rings sharp on sound metal and sounds dull on rotten metal.
Treat a rusted roof as unable to take your weight until a roofer says otherwise. Sheets weakened by corrosion are fragile too, so edge protection and spreading boards belong in the plan, and the plan is made before anyone steps out onto the sheets. Check what the sheet is as well. Painted asbestos cement can pass for metal from the ground, and a building from before 2000 needs a survey before any work starts.
Preparation decides how long it lasts
On a coated roof, most failures trace back to the day it was prepared and not to the coating itself. Dirt, moss, grease and loose rust all stop a coating from gripping, and a coating that does not grip peels in the first hard frost or hot summer.
What does proper preparation involve?
The roof is cleaned of moss and dirt, then loose and flaking rust is taken back to sound metal by wire brush or mechanical abrading. Bare metal is treated with a rust converter or primed straight away, because steel left bare overnight flashes back to rust.
A pressure wash alone is not enough on corroded sheets, and a heavy jet can drive water into the laps. Cleaning is followed by drying, because a coating over damp metal blisters. Mild weather with a dry forecast matters more than the calendar, and the survey sets out a sensible window so the work is not rushed. Every sheet gets this treatment, not only the worst ones, and the primer and topcoat have to come from the same system so that they bond to each other.
Cut edges, fixings and laps first
Metal sheeting usually fails first where it has been cut, because the cut exposes bare steel. Our page on cut-edge corrosion covers that in detail. On a coating job those edges, along the eaves, around rooflights and at the ridge, get attention before the main surface.
Why do the fixings come first?
Screws and bolts with rusty heads or worn washers let water in under the sheet and start the next rust patch. They are replaced or tightened and sealed, as the survey finds, and the laps where sheets overlap are cleaned out and sealed with a suitable mastic or tape. Skipping these and coating the open face is how a coated roof starts leaking within a season.
Gutters at the eaves matter as well. A rusted gutter drips water onto the lower sheets, and once the roof is coated the gutter is often the next thing to fail. The liquid roof coatings for commercial buildings page describes the systems used. The coat, its thickness and any fabric reinforcement come from the survey, not from guesswork.
When coating is a waste of money
Coating is the wrong answer when the roof has gone too far, and saying so early saves you money.
What are the signs it is too late?
Holes through the sheets in several places, fixings that pull straight through the metal, purlins (the beams that carry the sheets) that are corroded or rotten, a roof that sags and holds water, and laps that have opened along their full length. A coating cannot hold a roof together, and it hides the weak points until the next storm tests them.
In that case the choices are overcladding, which means fitting new sheets over the old, or a full strip and replacement. Our page on roof overcladding sets out those routes, and insulated panels are one of the usual choices for the new layer.
No single price fits every building, because roof area, the state of the sheets, access and the coating system all set the cost. The survey comes first, our report photographs show the roof as it stands today, and a single written price follows from them. Within our commercial roofing service, a photo of the rusted areas is the way to begin.
More on coating metal roofs
How long will a coating last on a rusty roof?
That depends on the sheet, the preparation and the system used, so no single number fits. A well-prepared roof on sound sheets lasts far longer than a quick coat over loose rust. Ask for the manufacturer's data sheet for the product and treat any life quoted as a typical range, not a promise.
Can I do it myself with a tin of roof paint?
Small outbuildings can be treated by an owner, but working on a roof carries a risk of falling, and a corroded sheet may not carry you. For a unit, shed or farm building, the access set-up is usually the real cost. Ask a roofer to survey first and tell you whether paint is worth buying.
The full picture: Commercial roofing in Taunton ยท More short answers