When does a factory-coated steel roof need its first repaint?

A factory-coated steel roof needs its first repaint when the factory finish has faded and started chalking, and before white rust or corrosion appears, not at a fixed age. On a sheltered inland roof that point can be a long way off; on a roof exposed to salt spray, or with areas the rain never washes, it arrives much sooner, and it arrives first in specific spots rather than across the whole roof.

Catching it at the faded-and-chalky stage is what keeps the job cheap. Once corrosion has started, you are paying for rust treatment as well as paint.

What are the signs the factory coating is ageing?

Resene's description of an older pre-finished steel roof is a good checklist: faded and chalked, and in severe cases white rust and corrosion. In rough order of how they appear:

1. Loss of gloss. A slight dulling in the first year or two is normal. NZ Steel describes a uniformly weathered surface with a slight reduction in gloss as typical after one to two years of service, and says it gives adequate adhesion for overpainting. This is not a reason to repaint.
2. Colour fade. The roof looks noticeably lighter or flatter than the flashings or a protected offcut of the same colour.
3. Chalking. Wipe a finger across the sunny side of the roof. A powdery residue on your fingertip means the paint surface is breaking down. This is the ideal time to plan a repaint.
4. White rust. White or grey powdery deposits, usually at laps, around fixings, under flashings or in areas that stay damp. The coating underneath the paint is now being consumed.
5. Red rust and flaking. The steel itself is corroding. Resene's technical team is blunt about what this means: once corrosion has formed on the surface it will always come back, and a paint system will only slow it down.

If the factory paint is peeling away in sheets rather than weathering, Resene's advice is to refer it to them, because delamination can indicate a failure of the pre-finished coating system rather than normal ageing. Do not paint over that until it has been looked at.

Why do coastal and sheltered roofs age differently?

Salt and dirt sit on the surface and attack the finish. Rain washes most of it off the open, exposed parts of a roof. The areas it cannot reach keep their salt, and that is where the damage starts.

NZ Steel calls these unwashed areas and lists them specifically: soffits, wall cladding under eaves, undersides of gutters, fascias, sheltered areas of garage doors, unwashed roof areas, and high-risk spots around flues, under television aerials and solar panels, and anywhere prone to mould, lichen, bird droppings or debris. Its maintenance guide requires these to be washed manually, and more often as the environment gets harsher, ranging from every six months in milder categories to every month in the most severe. Washing them is also a condition of the warranty.

In Wellington terms, that means a south coast or harbour-edge roof, or one on an exposed ridge taking salt-laden southerlies, will usually show chalk and white rust well ahead of a similar roof in a sheltered inland valley. It also means the first corrosion on almost any roof tends to appear under a solar array, behind a chimney flashing or beneath an overhanging upper storey, while the open slopes still look fine.

Stage	What you see	What the repaint involves
Weathered	slight loss of gloss, colour even	no repaint needed; keep washing unwashed areas
Faded and chalky	flat colour, powder on your finger	thorough wash to remove chalk, then the paint system; the cheapest time to act
White rust in patches	white deposits at laps, fixings and sheltered areas	wash, remove white rust, spot prime, then full system
Red rust and flaking	brown rust, lifting paint	mechanical rust removal, specialist primers, more labour; the rust will keep trying to return

Why is earlier cheaper?

A chalky roof in otherwise good condition is mostly a washing and painting job. Every stage after that adds preparation: removing white rust, grinding back red rust, spot priming, feathering edges. Preparation is labour, and on a roof labour is slow because of access and safety. The paint itself is a small part of the bill either way.

There is also a limit. Paint can protect a roof whose metallic coating is still largely intact. It cannot rebuild steel that has rusted through. Leave it long enough and the conversation moves from repainting to replacing sheets, which is what our article on when a roof is too rusty to paint covers.

What should you check before painting a newer roof?

The warranty. NZ Steel states that its warranty does not cover failure caused by post-paint systems, or failure of the paint systems themselves, and that failure or damage resulting from overpainting will not be covered by any of its warranties. If your roof is still within its warranty period and has a specific problem, talk to the manufacturer before a painter touches it.

Three other points from NZ Steel's overpainting guide are worth knowing:

• Site-applied paint dries in the air, while the factory finish is oven-cured, so they weather differently. Patch-painting one area to match will likely look different over time.
• Touch-up paint on scratches is not recommended. Minor scratches should be left alone, because the metallic coating underneath protects them.
• In more corrosive salt-marine locations, a paint system with enhanced corrosion resistance may be needed, and a paint supplier should specify the system for your roof and environment.

If you collect drinking water from the roof, the paint system also has to be one the supplier confirms is suitable for that.

Can you do anything to delay the first repaint?

Yes, and it costs very little. Wash the unwashed areas on the schedule your manufacturer sets for your environment, keep gutters clear, remove leaves and debris from valleys and behind solar panels, and treat lichen early. None of that needs a painter.

When the chalk test starts leaving powder on your finger, that is the time to get a price, not when the rust shows. Our metal roof painting page explains how we prepare and coat pre-finished steel roofs. If you would like an honest read on whether yours needs painting yet, contact us for a free quote.