Plastisol Delamination: What It Is and What to Do About It

Plastisol coatings sit on a large share of the UK’s industrial and agricultural roofs, and a great many of them were installed decades ago. When the bond between that coloured PVC layer and the steel beneath it fails, you get plastisol delamination: the surface lifts, cracks and peels back in flakes or whole ribbons. It is not a cosmetic issue you can leave, because once the steel is exposed the roof begins to corrode.

What plastisol is and why it lets go

Plastisol is a thick, grained PVC coating applied to galvanised steel cladding during manufacture. It gives the sheet its colour and leathery texture, and its real job is to keep water and UV off the zinc and steel underneath. Like any PVC product it relies on plasticisers to stay flexible. Over years of sun, heat and rain those plasticisers slowly migrate out and the coating hardens. As it stiffens it loses its grip on the metal and its ability to move with the sheet as the roof heats and cools.

Why plastisol delamination happens

Three forces usually combine. UV breaks down the surface, which is why south-facing slopes and exposed ridges tend to go first. Thermal cycling flexes the sheet every day, working at any weak bond. And moisture finds its way under the coating at cut edges, fixings and laps, lifting it from behind. None of this happens to a fixed timetable. Two roofs of the same age can be in very different condition depending on orientation, pitch, pollution and how they were maintained, which is why condition, not age, decides what needs doing.

What it looks like at each stage

Early on you see fading and a fine chalky residue that comes off on your hand. Next comes crazing, a network of small surface cracks, followed by the coating curling and lifting at edges and around fixings. In advanced cases whole strips peel away and expose bare or rusting steel. The tell-tale sign that it has moved from surface wear to true delamination is that you can lift or peel the coating by hand and find it no longer bonded to the metal.

Why it spreads once it starts

A lifted edge is an open door. Wind gets under it and pulls, rain sits behind it, and every warm day expands the sheet and works the gap wider. Water trapped under the coating also reaches the steel, so corrosion creeps outward from each failed patch. Left alone, isolated flaking joins up into large delaminated areas, and the repair you could have made with a clean and recoat becomes a replacement job.

What a survey looks at

Before anyone can say whether a coating will take an overcoat or whether the sheets are finished, the roof has to be walked and tested. A survey checks:

• Whether the existing coating is still bonded, tested by hand and blade at several points across the roof
• The condition of cut edges, laps and fixing points, where failure usually starts
• Corrosion on the steel itself, including any perforation or soft spots
• Cleanliness and adhesion, whether a wash and primer will give a new coating something to key to
• The state of gutters, flashings and rooflights that any coating scheme has to work around

The realistic options

What can be done depends entirely on what the survey finds. Where the steel is sound and enough of the coating is still bonded, cleaning and recoating is often viable. Where the coating is widely delaminated or the steel is corroded through, no coating will stick to it and replacement is the honest answer.

Can a delaminating plastisol roof be coated over? Sometimes. If the survey shows the remaining coating is well bonded and the steel is sound, a proper clean, edge treatment and recoat can extend the roof’s life. If it is lifting, it has to be removed or the sheets replaced first.

Is delamination the same as rust? No, but they travel together. Delamination is the coating losing its grip; rust is what follows once bare steel meets water. Catching the first limits the second.

Does it always mean the whole roof needs replacing? Not necessarily. Many roofs have sound and failed areas side by side, so recoating the good areas alongside replacement of the worst sheets is common.

If your cladding is showing these signs, a survey is the only way to know which route fits your roof; see our roof coatings service or request a free quote to get one booked.

Published by National Coating Specialists • survey-led commercial, industrial & agricultural coatings across the UK.