People use Colorbond as a generic term for metal roofing the way they use Esky for a cool box. It is a brand, and the material underneath it is not the only option on Sydney roofs.
Telling them apart matters, because the right cleaning method is not the same for both.
What each one is
Zincalume is steel coated in an aluminium-zinc alloy. Unpainted. It is the bare metallic silver-grey sheet, bright when new and weathering to a duller grey over years. The alloy coating is what protects the steel, and it is sacrificial — it corrodes preferentially so the steel underneath does not.
Colorbond is that same Zincalume base with a baked-on paint system applied over it. The paint is the weather surface and the colour you see; the alloy layer is still there underneath doing the protective work.
So Colorbond is Zincalume plus a coating, which is why the cleaning differs: on one you are working on paint, on the other you are working directly on the sacrificial alloy.
How to tell which you have
Colour. Any colour other than bare metallic is Colorbond or another painted product. Bare silver-grey is Zincalume.
Careful here — Colorbond does make a light silver-grey, and a weathered Zincalume roof can look similar from the ground.
Look at a cut edge or a gutter end. On Colorbond you will see a thin paint layer over a metallic core. On Zincalume the edge is the same material as the face.
Look underneath. Colorbond is painted both sides, usually with a lighter backing colour. Zincalume is metallic both sides.
The spangle. Zincalume has a faint crystalline pattern in the surface — a slightly mottled, frosty look. Paint covers it.
Rub test. An old Colorbond roof chalks: run a hand over it and coloured powder transfers. Zincalume does not chalk, it just dulls.
Cleaning Colorbond
You are cleaning a paint film, and the risks are chalking and abrasion.
Mild detergent, soft brush, thorough rinse. Low pressure — enough to rinse, not enough to abrade. On a chalked roof, aggressive washing strips chalk unevenly and leaves permanent patchy fade, which is the thing people most often do to themselves. We cover it in cleaning Colorbond without chalking it and for roofs in how to clean a Colorbond roof.
Keep strong hypochlorite contact time short. It attacks the cut edges and the fasteners even where the paint film resists it.
No abrasive pads, no solvents, no acid.
Cleaning Zincalume
Here you are working directly on the alloy coating, and it is thin and sacrificial. Anything that removes it shortens the life of the roof.
No acid, ever. Acidic cleaners dissolve zinc rapidly. A brick-cleaning acid used on or splashed onto Zincalume will take the coating off in minutes and leave bare steel.
Strongly alkaline products are also a problem. Zinc is amphoteric, meaning it is attacked at both ends of the pH scale. Aim for neutral to mildly alkaline detergents.
Minimal mechanical abrasion. No wire brushes, no abrasive pads, no high pressure. There is no paint film buffering the coating.
Rinse thoroughly and let it dry. Water trapped between sheets or under debris causes wet storage stain — a dark grey or black bloom that is zinc oxide corrosion. It is largely cosmetic early on but it indicates the coating is being consumed.
And do not expect a Zincalume roof to come back to its original brightness. Weathering is permanent and even. A clean removes growth and debris; it does not restore the finish.
Where they fail differently
Colorbond fails from the surface in: fade, then chalk, then eventually coating breakdown exposing the alloy, then rust at those points. It fails first at cut edges, screw penetrations and scratches.
Zincalume consumes its alloy coating gradually and uniformly, then rusts once it is gone. It is more vulnerable to localised attack from contact with incompatible materials.
That contact issue is worth knowing. Zincalume should not be in contact with, or downstream of runoff from, copper, lead, or unseasoned or treated timbers such as CCA-treated pine and some hardwoods, whose tannins and preservatives are corrosive to it. A copper pipe or an old lead flashing draining onto a Zincalume roof will cut a line of corrosion straight through it.
The coastal factor
Both materials come in grades rated for marine environments, and both are compromised by salt accumulation.
The critical detail is that corrosion happens where salt sits and is not washed off by rain — sheltered areas, not exposed ones. Under eaves, behind gutters, on the underside of sheets, in the lee of a parapet. Those are the places to inspect and the places regular fresh-water rinsing actually helps.
On a coastal property, a periodic fresh water wash of unwashed areas is genuinely the highest-value maintenance you can do on a metal roof. See what coastal Sydney does to your home’s exterior.
Rainwater tanks
Both are acceptable catchment surfaces for drinking water, with Colorbond generally preferred. In either case the tank comes off before any cleaning work and goes back on after the first good rain — the detail is here.
If you are not sure which you have, we will tell you when we look at it — see Colorbond roof cleaning.
