Building Envelope
Coatings for Steel Roof and Wall Panels
Coatings for steel roof and wall panels are the layers applied to steel coil before it is roll-formed. A metallic coating of zinc or aluminum-zinc alloy protects the steel from corrosion, and an optional factory paint system adds color, extra durability and resistance to fading and chalking.
Key facts
- Metallic coating mass is specified as a coating class in each standard.
- Aluminum-zinc alloy coated (AZ) steel is widely used for roofs because of its barrier protection.
- Zinc coated (Z) steel provides sacrificial protection at cut edges and scratches.
- Paint systems range from polyester through silicone-modified polyester to PVDF.
- Coastal, industrial and chemical environments need a higher coating specification.
How metallic coatings protect steel
Zinc coatings protect steel in two ways: as a barrier and by corroding sacrificially in place of the steel where the coating is damaged. Aluminum-zinc alloy coatings combine the barrier durability of aluminum with some sacrificial action from zinc, and generally give longer life in many atmospheric environments.
Coating thickness is expressed as a coating class or mass in each standard. Heavier coatings last longer. Unpainted AZ steel is common for roofs and liners, while painted steel is used where color or added protection is required.
How standards in each region describe coated sheet
In North America zinc coated sheet is specified to ASTM A653 and aluminum-zinc alloy coated sheet to ASTM A792, each with designations for coating mass. Canada generally uses the same ASTM standards. Prepainted coil is often described by reference to coil coating standards such as ASTM A755.
In Australia and New Zealand, AS 1397 defines Z and AZ coating classes and steel grades, and AS/NZS 2728 classifies prefinished metal products by environment. In Europe, EN 10346 covers continuously hot-dip coated steel, and EN 10169 covers organic coated flat products, including categories for corrosion and UV resistance.
Choosing a paint system
Polyester paints are economical and suit mild environments. Silicone-modified polyester improves gloss and color retention. Polyvinylidene fluoride, or PVDF, systems resist fading and chalking very well and suit demanding climates and architectural facades. Heavier or specialist systems, including thicker organic films, are available for aggressive exposures.
The reverse side of the sheet also needs consideration. A backer coat protects the underside, and roofs over humid or chemical processes may need a heavier reverse-side coating.
Matching coatings to the environment
Environment matters as much as the coating. Distance from surf, prevailing wind, industrial pollution, fertilizer or animal housing, and internal humidity all affect life. Sheltered areas that are not washed by rain, such as under eaves and overhangs, can corrode faster because salts accumulate.
EN ISO 12944 provides atmospheric corrosivity categories that are widely referenced for steelwork, and similar classifications exist in AS/NZS 2312.1 and AS/NZS 2728. These help the specifier choose a coating suited to the site.
Compatibility and care
Coated steel must not be in contact with copper, lead or wet timber treated with certain preservatives, and water should not run from incompatible metals onto it. Swarf from cutting and drilling must be removed the same day because it rusts and stains the surface.
Cut edges, scratches and fastener heads should be treated as recommended by the coating supplier. Coating selection is confirmed by the project engineer and specifier for the site conditions.
Coating points to specify
- Site environment, including distance from the coast and industrial sources.
- Internal environment, such as humidity, chemicals or animal housing.
- Metallic coating type and class to the relevant standard.
- Paint system, color and reverse side coating.
- Expected service life and maintenance plan.
- Compatibility with gutters, fasteners and adjacent materials.
Frequently Asked Questions
Is aluminum-zinc coated steel better than galvanized steel?
In many atmospheric environments aluminum-zinc alloy coated steel lasts longer than zinc coated steel of similar mass. Zinc coatings can perform better in some alkaline conditions, such as contact with wet concrete or animal waste, so selection depends on the environment.
What is PVDF paint?
PVDF is a fluoropolymer paint system with excellent resistance to fading and chalking. It is often chosen for facades and buildings in strong sunlight where appearance must be maintained.
Why does steel under an eave corrode faster?
Surfaces not washed by rain collect salts and pollutants that hold moisture. These areas need regular cleaning or a more durable coating.
Engineering content is general guidance. Every World Steel building is designed by qualified engineers to the code and loads of the project.
