Galvalume coil (aluminum-zinc coated) is the dominant roofing and cladding material for agricultural buildings. This guide compares AZ50 and AZ55 coating designations and explains how to pick the right coating for barns, poultry houses, dairy sheds and grain storage.
Galvalume coil, formally 55% aluminum-zinc coated steel per ASTM A792, offers roughly 2–4 times the atmospheric corrosion resistance of standard galvanized in most rural and agricultural environments. The coating sacrifices itself slowly, but the aluminum content forms a stable oxide that slows the corrosion rate, so AZ-coated roofing routinely outlasts G60 galvanized on the same barn. The two common coating weights are AZ50 (0.50 oz/ft² total both sides, about 150 g/m²) and AZ55 (0.55 oz/ft² total both sides, about 165 g/m²).
AZ50 vs AZ55: The Practical Difference
The numeric difference between AZ50 and AZ55 looks small on paper but reflects regional convention. AZ50 is the typical North American designation per ASTM A792, while AZ55 is the traditional Australian and Asian specification per AS 1397 and JIS G3321. In service life, the two are essentially equivalent; the choice is driven by which standard your roll-forming line is set up to receive, and by the local market expectation. For export orders, confirm whether the buyer's spec calls out AZ50 or AZ55 so that the mill test certificate matches the RFQ.
Why Galvalume Outperforms Galvanized on the Farm
Agricultural buildings combine several corrosive stresses: high humidity from animal respiration and bedding, ammonia vapor from manure and poultry litter, fertilizer dust, and intermittent wetting cycles. Galvalume's aluminum-zinc alloy coating is much more resistant than pure zinc to ammonia attack, which is why AZ-coated barn roofing has become the standard for poultry, swine and dairy operations. The coating also resists sulfur compounds from feed additives and provides better long-term color retention for pre-painted AZ products.
Thickness, Paint System and Fastener Detail
For agricultural roofing, the most common galvalume thicknesses are 0.35–0.55 mm (26–22 gauge), with 0.42 mm and 0.47 mm being the workhorse sizes for poultry and dairy barns. Bare galvalume has a bright spangled surface that weathers to a soft gray; for color, specify a polyester or SMP paint system designed for AZ substrate (not all paint systems are compatible with galvalume). Fasteners should be galvanized or AZ-coated to match the panel lifetime, and should include EPDM sealing washers to prevent leaks at the penetrations. Avoid direct contact with copper, treated timber, or wet concrete, which cause premature AZ coating failure.
What to Specify on the Agricultural Roofing RFQ
State the standard (ASTM A792, AS 1397 or JIS G3321), the coating designation (AZ50 or AZ55), the substrate thickness, the coil width, and whether a paint system is required (and which: polyester, SMP, or PVDF). For poultry and swine buildings with high ammonia exposure, AZ55 (or heavier) is preferred. For dry hay barns and machinery storage, AZ50 is sufficient. Confirm that cut edges and scratches are repaired with AZ-compatible touch-up paint to prevent edge creep.
Related Resources from Baobin Steel
- Steel coil catalog – galvanized and galvalume options
- Mild steel overview – grades and standards
- Request a quote – send your barn spec for galvalume coil pricing