What Is the Metal?
(Understanding Steel & Aluminum for Metal Roofing)
Introduction
When homeowners or business owners first explore metal roofing, they often ask: “What kind of metal do you use? Will it last? Will it rust?” These are excellent questions — the metal itself (its gauge, coating, finish) plays a critical role in durability, appearance, and performance. In this post, we’ll dig deep into the “what is the metal?” question: gauge, finishes, warranties, rust resistance, and comparing steel vs aluminum.
1. Gauge: How Thick Is the Metal?
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In metal roofing, gauge is the measure of thickness: the lower the gauge, the heavier/thicker the metal.
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For residential and commercial non-structural roofing, 24 gauge steel is commonly considered the sweet spot: thick enough to resist oil-canning and weathering, yet light enough to install without requiring heavy structural changes.
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Local material distributors sometimes stock 26 gauge steel, which is thinner and easier to handle. That is often used in projects where installers don’t have heavy equipment. But thinner metals tend to flex more, show waviness, and have less longevity in demanding conditions.
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For reference, a car door panel is about 14 gauge — so that gives perspective on thickness.
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In more demanding commercial or industrial roof systems (especially over open framing), installers may use 22 gauge, which has the same thickness as structural steel roof deck. But that is rarely used in standard residential roofing because of cost and handling constraints.
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Because 24 gauge is the practical upper limit for non-structural roofing, it’s offered in the broadest range of colors and styles. Many panel manufacturers decline to sell 26 gauge steel without stiffening ribs or striations (used to resist waviness).
Takeaway: 24 gauge steel is the “sweet spot” for most roofs — durable, manageable, and offered in many finishes.
2. The Finish: How We Protect the Steel
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Steel roofing panels go through multiple protective layers to resist corrosion and preserve appearance. Here’s how it works:
Galvalume base coat
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The steel is coated with a layer of aluminum–zinc alloy (Galvalume) to provide corrosion resistance.
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This layer is essential; it helps protect the raw steel from the elements.
Primer coat (~0.20 mil)
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Over the Galvalume, a primer is applied to promote adhesion of the topcoat and add further corrosion resistance.
Topcoat (Kynar / Hylar / Fluropon)
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The visible finish is a PVDF (“polyvinylidene fluoride”) resin-based coating.
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Popular trade names are Kynar 500® (Arkema) or Hylar 5000® (Solvay). These are high-performance finishes known for color retention, gloss retention, and weather resistance.
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Some manufacturers use lower-cost alternatives (50% PVDF formulations). However, these lower ratios don’t perform nearly as well over time.
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On the flip side, using more than ~70% PVDF is technically very difficult to do well in manufacturing — coatings may not adhere properly or cure correctly.
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Sheffield Metals (a known metal supplier) uses a true Kynar/Hylar finish.
Because of the expense and complexity of producing high-end PVDF coatings, many metal suppliers refuse to sell 26 gauge steel with a true (unstiffened) finish — the thinner metal flexes too much for the coating to reliably endure without cracking or failing.
3. Paint Durability & Warranty
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Sheffield Metals International offers a 40-year finish warranty on their coated steel, which speaks to the long-term expectations for performance.
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In real-world conditions, the finish often lasts longer than 40 years if properly installed and maintained.
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That said, warranties typically assume the finish is not fully compromised (i.e., no full-thickness gouges, chemical exposure, etc.).
4. Rust — Will It Happen?
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Steel panels won’t rust if the full protective system (Galvalume + primer + topcoat) remains intact. A scratch through all those layers (down to bare steel) is rare; even then, it’s exceedingly unlikely under normal conditions.
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During installation, minor surface scratches are almost inevitable. However, these do not necessarily lead to corrosion. Manufacturers expect this and include touch-up paint pens (color-matched, PVDF) for installers to repair small blemishes. These repairs do not void typical finish warranties.
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That said, in marine environments (within ~500 ft of offshore salt spray), corrosion is more aggressive. In those cases, aluminum or stainless steel components are often a better choice.
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Over many decades — especially if the roof is neglected, uninspected, or exposed to abrasive debris — signs of finish breakdown or rust may appear after the warranty period. Regular inspection helps catch problems early.
5. Why Not Use Aluminum?
At first glance, aluminum seems ideal: it doesn’t rust by nature. But there are trade-offs:
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Strength & rigidity: Aluminum is softer and weaker than steel of the same thickness. Steel offers better shear strength, less tendency toward waviness (oil-canning), and better structural stability under loads (wind, snow).
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Thermal expansion: Aluminum expands and contracts more with temperature changes. Over time, that higher movement can stress panels, fasteners, and finishes, potentially affecting durability.
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Cost & finish challenges: Heavier aluminum (to match steel strength) gets expensive. Also, because aluminum is more reactive and flexible, applying high-performance coatings can be more challenging.
In marine zones, or specialized cases, aluminum still makes sense. But for most roofs in non-extreme environments, 24 gauge coated steel offers a balanced, high-performance solution.
6. Summary & Buyer Tips
Gauge
What to Look For
24 gauge is ideal; 26 gauge only in limited cases
Why It Matters
Balance of durability, handling, and cost
Coating System
What to Look For
Galvalume + proper primer + true PVDF (Kynar/Hylar)
Why It Matters
Critical to long-term resistance to fading, corrosion
Warranty
What to Look For
30–40+ year finish warranty
Why It Matters
Indicates manufacturer confidence
Touch-Up Capability
What to Look For
Color-matched PVDF pens
Why It Matters
Essential for repairing installation damage
Environmental Fit
What to Look For
24 gauge is ideal; 26 gauge only in limited cases
Why It Matters
Balance of durability, handling, and cost
A well-specified, properly installed 24 gauge Kynar-finished steel roof can last 50+ years — an excellent investment in your building’s longevity, energy performance, and curb appeal.
When selecting a metal roof, ask your supplier or installer:
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What gauge do they use?
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What is their coating specification (percent PVDF, brand)?
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What is the finish warranty?
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Do they include touch-up materials?
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Do they have experience with coastal or challenging environments (if relevant)?