Bare Galvalume Sheet
Metal Roofing Sheets
What a bare galvalume sheet is
Bare galvalume is steel with an aluminium-zinc alloy coating and no paint over it. The steel carries the load and the alloy coating does the corrosion work. What you see is the mill finish, a bright spangled silver that dulls to a soft grey over the first year or two outdoors.
It goes by several names in the trade: BGL, zincalume, silver sheet, Silveron+ and satin silver. They all describe the same product.
Coating mass is the specification that matters
Galvalume is quoted as AZ70 or AZ150. The number is the grams of alloy per square metre counted across both faces of the sheet, so AZ150 carries roughly twice the coating of AZ70.
This is the number to argue about, not the steel thickness. Thickness decides how the sheet handles load and foot traffic; coating mass decides how long it lasts. A thicker sheet with a thin coating will fail at the surface long before the steel is anywhere near its structural limit.
For a normal inland industrial building AZ70 is usually adequate. Within a few kilometres of the coast, or near a plant putting acid or ammonia into the air, AZ150 is worth the difference, because the coating is what fails first.
Why choose bare over colour coated
Two reasons. The mill finish reflects more heat than most paint colours, which helps on an uninsulated shed. And with no paint layer there is nothing to chalk, fade or peel, so the sheet ages evenly instead of looking tired in patches.
The trade-off is appearance. There is no colour choice, and a silver roof is a silver roof. Where the building is visible or the client wants to match a corporate colour, colour coated is the answer.
Cut edges and site handling
The alloy coating gives some protection to a cut edge, but it is not unlimited, and a cut made with an abrasive disc is worse than one made with a shear. The disc burns the coating back for several millimetres either side and throws hot particles onto the sheet that embed and rust.
Bundles left in the rain are the other common problem. Water trapped between stacked sheets causes white storage staining on the coating, which is cosmetic at first but marks the sheet permanently.
What to send us for a quote
The covered area, the profile, the purlin spacing, the thickness, and the coating mass if the specification names one. If you are not sure between AZ70 and AZ150, tell us how far the site is from the coast and what the building is used for.
Frequently asked questions
What is the difference between AZ70 and AZ150?
The grams of aluminium-zinc alloy per square metre across both faces. AZ150 carries roughly twice the coating of AZ70 and lasts correspondingly longer in aggressive conditions. It is the usual specification for coastal and chemical sites.
Is galvalume the same as galvanised?
No. Galvanised is a zinc coating. Galvalume is an aluminium-zinc alloy, which gives better corrosion resistance for the same coating mass, particularly against cut-edge and weathering attack.
Will the silver finish stay bright?
No, and it is not meant to. It dulls to a soft grey over the first year or two. That is normal weathering of the alloy surface, not a fault.
Can I paint a bare galvalume sheet later?
It is possible but rarely worth it. A factory-applied paint system on a colour coated sheet is cured onto the coil and far more durable than anything applied on site.
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