What Is The Composition Of Metal Roofing?

The base metal is the structural heart of a metal roofing panel. It is the layer that gives the panel its rigidity, its load-bearing capacity, and its resistance to impact. Everything else in the system, the protective coating, the paint finish, the fasteners, is either bonded to this layer or designed to protect it. The choice of base metal is the single most consequential decision in the composition of a metal roof.

Steel is the most widely used base metal for residential and commercial metal roofing in the United States and in San Antonio specifically. Modern roofing steel is cold-rolled to precise thicknesses, designated by gauge. The higher the gauge number, the thinner the steel: 24-gauge steel is approximately 0.024 inches thick and is the standard for residential standing seam roofing, while 26-gauge at approximately 0.019 inches is used in some commercial applications and less demanding residential projects.

Aluminum is naturally rust-proof, roughly one-third the weight of steel, and the correct choice for properties near coastal or high-humidity environments. It is softer than steel, which makes it more vulnerable to denting under hail impact, a meaningful concern for San Antonio homeowners. Aluminum roofing panels are typically specified by thickness in inches rather than gauge: 0.040 inches is the standard residential minimum.

Steel (most common): Strong, hail-resistant, affordable, requires protective coating Aluminum: Rust-proof, lightweight, softer than steel, higher cost Copper: 100+ year lifespan, self-protecting patina, highest cost Zinc: Self-healing patina, 80 to 100 year lifespan, requires specialty installation

Bare steel rusts. That is not a deficiency unique to metal roofing; it is a fundamental property of iron-based alloys exposed to oxygen and moisture. The metallic coating applied directly to the surface of the steel base is what prevents this from happening. Without it, a steel roofing panel would begin to rust within months of installation. The type, thickness, and chemical composition of this coating is one of the most critical specifications in the entire metal roofing system.

Galvalume is the industry-standard metallic coating for residential and commercial steel roofing panels in the United States. It was developed by Bethlehem Steel in the 1970s and has become the dominant coating for metal roofing because it significantly outperforms the older galvanized (pure zinc) coating in long-term corrosion resistance. Galvalume is composed of approximately 55% aluminum, 43.4% zinc, and 1.6% silicon applied to the steel surface by a continuous hot-dip process.

Why Galvalume outperforms galvanized: The aluminum component of the Galvalume coating creates a dense, stable oxide layer on the surface that is highly resistant to progressive corrosion. The zinc component provides sacrificial cathodic protection at cut edges, scratches, and any point where the coating is broken. These two mechanisms work together in a way that pure zinc coating cannot replicate. In accelerated corrosion testing and in real-world performance data spanning four decades, Galvalume-coated steel panels consistently outlast equivalent galvanized panels by a significant margin.

Galvalume composition: 55% aluminum + 43.4% zinc + 1.6% silicon Protection mechanism: Barrier protection (aluminum) + sacrificial protection (zinc) AZ coating weight: AZ50 (minimum) to AZ55 (standard residential) to AZ60+ (premium) Galvanized (G90): Pure zinc coating significantly less corrosion-resistant than Galvalume

On colored metal roofing panels, the paint system sits on top of the metallic coating and is what you actually see from the ground. It serves two purposes: it provides the color and aesthetic appearance of the roof, and it adds another layer of protection against UV radiation, moisture, and environmental exposure. The quality of this paint system determines how the roof looks after 20 or 30 years under San Antonio's intense sun exposure, which is one of the highest UV environments in the continental United States.

There are two primary paint systems used on metal roofing panels: PVDF (polyvinylidene fluoride) and SMP (silicone-modified polyester). They look identical on day one. The difference appears over time, particularly in a market like San Antonio where annual UV exposure levels are extreme.

The underlayment is installed over the roof decking and beneath the metal panels. It serves as a secondary moisture barrier in the event that water finds its way past the panel surface, and it provides additional protection to the structural deck during installation and before panels are fully secured. Selecting the wrong underlayment for a metal roof is a common installation error that can shorten panel life, cause noise problems, and even void the panel manufacturer's warranty.

The key thing homeowners need to understand is that traditional asphalt felt underlayment (felt paper) is not the correct specification for a metal roof. The temperatures that build up on a metal roof deck in the San Antonio summer can exceed 200 degrees Fahrenheit in some configurations. Standard felt paper degrades, releases volatile compounds that can stick to the underside of the metal panel, and fails at those temperatures over time. Metal roofing requires an underlayment specifically rated for high-temperature applications.

The fastening system is where most metal roof failures originate, and it is also where the most variation exists between a quality installation and a cost-cut one. Getting the base metal and coating right means little if the system is held together with the wrong type of fasteners at incorrect spacing and torque. Conversely, a properly designed fastening system is what allows a metal roof to handle decades of thermal expansion and contraction without developing leaks at every attachment point.

Standing seam floating clips: In a standing seam system, the panel is not directly screwed through its face. Instead, a metal clip is fastened to the deck and the panel seam snaps over or slides through this clip. The clip design allows the panel to move horizontally as it expands and contracts with temperature changes without stressing the fastener field. This is the fundamental mechanical reason why standing seam roofs outlast exposed fastener systems: the panels are free to move, so the metal is never fighting its own fasteners.

EPDM-gasketed screws (exposed fastener systems): In screw-down or corrugated panel systems, the fastener penetrates directly through the face of the panel. Each screw has an EPDM rubber washer bonded under the head that compresses when the screw is driven, creating a watertight seal. These washers harden, crack, and compress permanently over 10 to 15 years in San Antonio's temperature extremes, creating potential leak points at every fastener location.

Floating clips: Allow thermal movement in standing seam systems, no face penetration EPDM washers: Seal exposed fastener penetrations, require periodic inspection Fastener material: Stainless steel or zinc-coated steel never galvanized with copper panels Torque control: Over-driven screws strip holes and void the EPDM seal

Sealants and butyl tape: At panel overlaps, ridges, valleys, and penetrations, mechanical fasteners alone are not sufficient. Butyl tape (a flexible, non-curing sealant tape) is installed at panel overlaps in standing seam systems to seal the seam before the panels are crimped or snapped together. At ridge caps, penetrations, and transition flashings, a compatible sealant is applied in addition to mechanical fastening. The correct sealant is specified by the panel manufacturer and is not interchangeable with general-purpose caulk.

Flashings are the sheet metal components installed at all transitions, edges, penetrations, and changes of direction on a roof. They are what seal the roof at chimneys, vent pipes, skylights, valleys where two roof slopes meet, and at the eave and ridge edges of the panel field. Flashing failures are responsible for a disproportionate share of metal roof leaks, particularly on roofs that are otherwise installed correctly. The panels themselves rarely fail. The transition points do.

In a properly composed metal roofing system, all flashings are fabricated from the same material as the panels or from a compatible material that will not cause galvanic corrosion. Copper flashings should never be installed on a steel or aluminum roof. Galvanized steel flashings should not be substituted for Galvalume-compatible trim components. The metal in every flashing and trim piece needs to have the same corrosion resistance as the panel it is protecting.