Metal Roofing Glossary
A plain-English reference for the terms that come up most when specifying, buying, or installing metal roofing panels. No jargon — just clear definitions from people who've been on the job site.
Galvalume
Galvalume is a steel substrate coated with an alloy of aluminum and zinc. The aluminum content (typically 55%) provides superior corrosion resistance compared to standard galvanized steel, particularly in humid and coastal environments. AZ50 Galvalume (the most common designation) has a coating weight of 0.50 oz/sq ft. All four Hometown Metal Roofing profiles are available in Galvalume, and it is the default substrate for color-coated panels.
Galvanized Steel
Galvanized steel is coated with a zinc layer to prevent rust. G-100 galvanized (1.00 oz/sq ft coating weight) is available for Tuff Rib Ag panels and is commonly specified for agricultural applications where direct contact with fertilizers or ammonia is expected. Galvanized steel is generally less corrosion-resistant than Galvalume in typical roofing applications, but performs well in certain ag environments.
Grade 80 Steel
Steel grade refers to the yield strength of the base metal. Grade 80 steel has a minimum yield strength of 80,000 PSI — making it harder and stiffer than standard mild steel. Hometown Metal Roofing's Tuff Rib Ag and Commercial PBR panels are made from Grade 80 Galvalume. The higher hardness makes these panels more resistant to denting and deformation in service, but requires appropriate purlin spacing and substrate to prevent oil canning or waviness.
Grade 50 Steel
Grade 50 steel has a minimum yield strength of 50,000 PSI — softer and more ductile than Grade 80. Both standing seam profiles (Snap Lock and Nail Flange) are made from Grade 50 Galvalume. The softer grade is necessary because standing seam panel bends are sharper; Grade 80 steel would crack or delaminate at those bend radii. The lower yield strength is appropriate because standing seam panels are typically backed by a continuous substrate (plywood), which provides structural support the panel itself doesn't need to supply.
Exposed Fastener Panel
An exposed fastener metal roofing panel is one where the screws that attach the panel to the structure pass through the top face of the panel and are visible from the exterior. Tuff Rib Ag and Commercial PBR are both exposed fastener profiles. Correct screw placement is critical: on Tuff Rib Ag, screws must be driven into the flat plane beside the rib — never in the rib itself. Modern EPDM-washer screws provide a watertight seal expected to last approximately 40 years.
Oil Canning
Oil canning is the visible waviness or rippling that appears in the flat areas of a metal panel, caused by internal stresses from thermal expansion and contraction as temperatures change. It is a cosmetic characteristic, not a structural defect. Snap Lock Standing Seam panels include striations — shallow ridges running the length of the panel — that control where expansion occurs and reduce visible oil canning. Standing seam's hidden clip system also allows the panel to move freely without buckling.
Purlin Spacing (OC — On Center)
Purlins are the horizontal structural members that metal roofing panels attach to. Purlin spacing (measured on center, from the center of one purlin to the center of the next) affects which panel profile is appropriate for a given structure. Tuff Rib Ag supports roof purlins up to 24" OC; Commercial PBR supports up to 48" OC on roofs and 72" OC on walls. Both standing seam profiles require either plywood sheathing or purlins at 16" OC. Exceeding the manufacturer's maximum purlin spacing can cause panel deflection or failure.
WeatherXL Paint
WeatherXL is the paint system used on Hometown Metal Roofing's Tuff Rib Ag and Commercial PBR panels. It is a high-performance fluoropolymer coating designed for long-term color retention, fade resistance, and protection against weathering. WeatherXL panels are covered by a manufacturer paint warranty in addition to the Red Dot Galvalume substrate warranty.
Kynar Paint
Kynar (PVDF — polyvinylidene fluoride) is the paint system used on Hometown Metal Roofing's Snap Lock Standing Seam and Nail Flange Standing Seam panels. Kynar is more flexible than standard polyester or SMP paint systems, which is necessary for standing seam profiles: the sharper bends involved in roll-forming standing seam panels require a paint that can flex without cracking or delaminating. Kynar panels carry a separate manufacturer paint warranty.
Roll Forming
Roll forming is the manufacturing process used to produce metal roofing panels. A flat coil of steel is fed through a series of roller dies that progressively bend the metal into the desired panel profile. Hometown Metal Roofing roll-forms all panels in-shop at its Westfield, MA facility, enabling custom lengths and fast turnaround. Unlike some standing seam contractors who use mobile job-site roll formers (one machine per project, higher cost per panel), Hometown roll-forms in-shop and can supply approximately 10 jobs per day.
Custom Trim
Metal roofing trim pieces (also called flashing or accessories) are the formed metal components that finish the edges, transitions, and penetrations of a metal roof. Examples include ridge caps, eave drip edge, rake trim, valley flashing, and wall flashing. Hometown Metal Roofing fabricates 22 trim profiles to exact customer dimensions — unlike stock trim available through lumber yards, which is sold in standard lengths and may not match the panel profile precisely.
AZ50 Galvalume
AZ50 is the most common Galvalume coating designation, indicating a total coating weight of 0.50 oz per square foot on both sides of the steel sheet. The "AZ" prefix stands for aluminum-zinc alloy. AZ50 Galvalume is the default substrate for all color-coated metal roofing panels at Hometown Metal Roofing. It provides excellent corrosion resistance and is compatible with most paint systems.
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