Steel vs. Aluminum vs. Copper Roofing: What Changes When the Metal Changes

Two quotes say Galvalume steel. A third offers aluminum for a little more. And you have admired the copper over a neighbor's bay window and wondered what that would cost on a whole roof. The company's commercial metal page notes that most modern metal roofs are corrugated galvanized steel, with aluminum, copper, tin and stainless steel also used, which is a fair summary of what is out there.

The paint color will be the same on any of them. What changes is how the roof resists corrosion, how much it weighs, how it ages, what it can safely touch, and what it costs. This guide compares the three metals you are most likely to be offered for a home or small building in Anderson, Oconee or Pickens County.

The Metal Roofing Alliance (MRA) calls steel the most common metal roofing material thanks to its high strength, low weight and long-term durability, and says it is typically the most affordable. The catch is that steel rusts. The MRA notes that all steel roofing is treated with a metallic coating layer to prevent rust, and the IBHS low-slope metal guide says the same: steel roof panels require a metallic coating for corrosion protection.

Two coatings dominate. Galvanized steel carries a zinc coating under ASTM A653, with G90 the grade the MRA lists for residential work. Aluminum-zinc alloy coated steel carries a mostly aluminum coating under ASTM A792, with AZ50 for painted panels and AZ55 for bare; the MRA notes this product is sold under several brand names, including Galvalume. Paint goes over either one. The guide on Galvalume and PVDF coatings explains the layers.

The weak points are wherever the coating is gone: cut edges, deep scratches, and holes drilled after the fact. The National Park Service puts it plainly in its roofing brief: iron rusts unless it is well painted or plated, the protection lasts only as long as the coating remains intact, and once the plating is worn or damaged the exposed iron will rust. That is why installers use shears rather than abrasive saws and touch up cut edges, as the company's post on installing accessories on a metal roof describes.

Aluminum does not rust the way steel does. The MRA says aluminum roofing is lighter than steel and offers increased corrosion resistance, and calls it an ideal choice for homes built in marine and coastal environments. Its options guide adds that for coastal applications, non-ferrous metals such as aluminum and copper may provide a better solution, and recommends consulting the manufacturer about warranty terms near salt water.

Aluminum is specified by thickness rather than gauge. The 2021 International Residential Code requires roll-formed aluminum roof panels to meet ASTM B209 at a minimum of 0.024 inch (Table R905.10.3(1)), and the IBHS steep-slope guide lists common aluminum roofing at 0.023 to 0.040 inch. The MRA says anodized aluminum is not a coating, so it will not chip, flake, chalk or peel. The guide on what metal roof gauge means explains why steel and aluminum numbers do not compare directly.

Anderson is an inland Upstate community, in the National Weather Service Greenville-Spartanburg office's area along with Oconee and Pickens counties, and far from salt air. The main coastal reason for aluminum does not apply here. Aluminum still gets chosen inland for lighter weight on older framing, for roofs near pools or industrial exhaust, or by owners who want a metal that cannot rust at a scratch. Ask the manufacturer for the product's impact rating rather than assuming by metal; the MRA notes all roof types can take cosmetic hail damage.

Copper has been roofing American buildings since the 1700s. The National Park Service notes that copper with standing seams covered notable early roofs, and that flat-seamed copper was used on many domes and cupolas. It needs no coating. The IBHS copper guide says copper will show a progression in color as it ages, from bright copper to brown to green, and that a full patina will often require about 25 years of exposure on a sloped surface.

Copper is sold by weight, not gauge. ASTM B370 specifies copper roofing sheet in ounce-weight thicknesses, and the IRC requires at least 16 ounces per square foot for cold-rolled copper roofing sheets. IBHS notes that most fasteners in a properly installed copper roof are concealed, and that the fasteners should be copper, brass or 300-series stainless steel to avoid galvanic corrosion.

Copper is generally the most expensive of the three, and the labor is specialized: standing seam, batten seam and soldered flat seam are all hand-formed. That is why you see it on steeples, bay windows, porch roofs and historic buildings more than on whole houses. The company's post on the pros and cons of copper roofing and its historical roofing restoration page, which lists copper and ornamental roof metals among the materials it works with, are the places to go next.

Metals react with each other. The National Park Service describes galvanic corrosion as what occurs when dissimilar metals, such as copper and iron, are used in direct contact, and notes it can happen even when the metals are physically separated, because rainwater acts as an electrolyte. Its examples are a copper roof with iron cresting, or steel nails in copper sheets. The practical rules:

• Match the fasteners to the metal. NPS says fasteners should ideally be a metal sympathetic to the roofing; IBHS says fasteners should be galvanically compatible with the materials they touch. Copper or stainless on copper, coated or stainless screws on steel, as the manufacturer directs.
• Watch what drains onto what. Rainwater that runs off one metal onto another can carry the reaction with it. Copper gutters, flashing or a copper accent roof above a steel or aluminum roof deserve a conversation with the installer before they are ordered.
• Keep roofing metal away from wet masonry and some woods. NPS lists alkalis from lime mortar or portland cement washing down onto a roof, and tannic acids from cedar or oak sheathing, among the causes of pitting and streaking on sheet metal.
• Plan every penetration. Dissimilar accessories such as lead, copper or graphite parts can corrode a steel roof, as the company's accessories post notes. Use the pipe boots, clamps and sealants the panel manufacturer approves.
• Separate when you must mix. NPS notes that corrosion can sometimes be prevented by inserting a plastic insulator between dissimilar materials. The guide on painted vs. bare Galvalume covers contact with concrete and treated lumber.

For most homes, shops and commercial buildings in Anderson, coated steel is the sensible default: the widest choice of profiles and colors, the lowest relative cost, and no coastal salt to fight. Spend the money on the coating grade, the paint system, the gauge and the flashing details rather than on a different metal.

Aluminum earns its place when weight matters, when a rust-at-a-scratch failure would be unacceptable, or when the building is near a pool, a lake house dock or a corrosive exhaust. Copper earns its place on historic buildings, church steeples and architectural accents, where its look and its long record are the point. NPS notes that lead-coated copper, terne-coated steel and aluminum-zinc coated steel can replace tin, terne or lead on historic roofs when original materials are impractical.

Next step: have the metal, the coating designation, the thickness and the fastener metal written on every quote, and ask how any copper or other dissimilar metal on the building will be kept apart from the roof. Anderson Metal Roofing and Shingles gives free quotes and virtual consultations by Zoom or FaceTime, and verifies all measurements on site, which can adjust the price. Start at the residential metal page, or the historical roofing restoration page if the building is old.