Aluminum or Steel Roof on the North Carolina Coast: Which Holds Up Better?

Salt air treats them differently because steel needs a coating to survive outdoors and aluminum does not. Bare steel is mostly iron, and iron plus oxygen plus moisture makes rust. Roofing steel is protected with a metallic coating, either zinc (galvanized) or a zinc-aluminum alloy, and usually paint on top. That metallic layer is sacrificial: it corrodes slowly in place of the steel underneath.

Salt is an electrolyte, so it speeds that process up. On a beach house the coating is consumed faster, and the weak spots show first: panel cut edges, scratches from installation or falling limbs, drilled holes around exposed fasteners, and the drip edges where salty water sits longest. Once the steel is exposed, rust stains and creeps under the paint.

Aluminum works another way. When aluminum meets air it forms a thin, hard oxide film almost instantly, and that film re-forms if the surface is scratched. Aluminum does not rust, because there is no iron in it to rust. That is why aluminum is the usual choice for boat hulls, marine hardware and coastal trim. It is corrosion-resistant in salt air, not corrosion-proof: pitting can still occur in standing salt water or where aluminum touches the wrong metal, which is covered below.

FEMA's Home Builder's Guide to Coastal Construction says metal corrosion is most pronounced on homes within about 3,000 feet of the ocean, and it recommends stainless steel or hot-dipped galvanized hardware in that zone. On the North Carolina coast that band covers most oceanfront and second-row homes from Corolla to Sunset Beach.

Distance is only a rough guide. Salt travels farther on strong onshore winds, during tropical storms and on homes that face open water across a sound or inlet. A house in Beaufort, Swansboro or on the Intracoastal Waterway near Wilmington can see more salt than a house half a mile inland behind a dune line and trees.

Steel panel warranties often address this directly. Many limit or exclude coverage within set distances of surf, salt marshes or brackish water, and some exclude marine exposure altogether. Before you compare prices, ask each contractor for the actual panel and finish warranty documents and read the environmental exclusions. If your house falls inside an exclusion zone, the warranty on that steel roof may not cover the failure you are most likely to have.

A good paint system slows corrosion on steel but does not change what is underneath. High-performance PVDF paints resist fading and chalking well, and they protect the flat face of the panel. The trouble spots are the places paint cannot fully cover: sheared edges at eaves and valleys, field-cut pieces around chimneys and dormers, screw holes, and scratches from foot traffic during installation.

Standing seam steel with concealed clips has fewer exposed fasteners than screw-down panels, which helps. Exposed-fastener steel panels, common on barns and budget jobs, put hundreds of washers and holes in the weather, and each one is a place for salt to start working.

Aluminum panels carry paint too. Interlock aluminum roofing uses the Alunar 70% PVDF coating, built to the AAMA 2605 standard, and the color warranty matters for appearance. But if the paint on an aluminum panel is scratched, the metal under it still does not rust. On the coast, that difference shows up years down the road at every cut edge and scratch.

Wind performance depends more on the roof system and how it is fastened than on the metal. A properly engineered steel or aluminum roof can both resist high winds; a badly fastened one of either metal can fail. What you want to see is a published wind rating for the specific system, plus installation that follows the manufacturer's fastening schedule.

The North Carolina Residential Code requires roof coverings to resist the component and cladding wind loads for the house, and the Office of the State Fire Marshal has noted that aluminum roof panels must meet the ASTM B209 material standard. Coastal design wind speeds in parts of eastern North Carolina are higher than in the Piedmont, and the requirement for a given house is set by its location, exposure and height, with the engineer or local permit office deciding what applies.

Interlock aluminum shingles are rated for winds up to 120 mph when installed to manufacturer specification, with panels that interlock on four sides over concealed fasteners. Aluminum is also light, roughly 0.4 to 0.7 lb per square foot for Interlock panels, which keeps load off older framing. For more on storm performance, see hurricane-resistant roofing and roof wind ratings explained.

An aluminum roof still needs the right details at the edges, fasteners and joints with other metals. The common coastal mistakes are:

• Copper draining onto aluminum. The Metal Construction Association warns that copper runoff has a severe corrosive effect on aluminum, steel and zinc. Keep copper gutters, flashings and older copper roofs from draining onto aluminum.
• Bare or galvanized steel touching aluminum. Dissimilar metals in contact with salt water as the electrolyte corrode faster. Use compatible fasteners and separate unlike metals.
• Cheap fasteners. FEMA recommends stainless steel hardware within about 3,000 feet of salt water. Ask what the fasteners and clips are made of.
• Salt that never rinses off. Areas under deep eaves or porch roofs that rain never reaches can hold salt deposits. An occasional gentle rinse with a garden hose helps.

None of this is unusual work, but it separates a coastal installer from one who mostly roofs inland subdivisions. Our salt air guide covers maintenance in more detail.

Salt damage on a steel roof usually shows up first as orange or brown streaks running down from fastener heads, rust blooms along the bottom edge of panels, and paint that bubbles or flakes near cut edges and trim. On ridge caps and around vents, where pieces are field-cut, the coating breaks down early. On older galvanized roofs the surface turns dull gray and then rust-spotted as the zinc is used up.

On aluminum, the signs are milder: a chalky white film on unpainted trim, or small pits where salt water has sat in contact with another metal. Neither weakens the panel the way rust weakens steel.

If you are buying a coastal house, look at the roof edges, the gutters and any exposed screws with binoculars or from a ladder. Rust at the edges of a steel roof that is only ten or fifteen years old is a sign of how the rest of it will age. Our guide to signs you need a new roof covers the other warning signs, from leaks to sagging decking.

For a house within sight or smell of salt water, aluminum is usually the safer long-term choice, and steel makes the most sense farther inland where corrosion pressure is lower and budget is the main concern. Use these questions to decide:

• How far is the house from the ocean, a sound or a tidal marsh, and which way does it face?
• Does the steel panel warranty exclude marine or coastal exposure at your address?
• What are the fasteners, clips and flashings made of?
• Will any copper, bare steel or treated lumber be in contact with the roof metal?
• How long do you plan to own the house?

Metal Roof NC is an authorized Interlock aluminum roofing dealer serving the coast from Wilmington to New Bern, Morehead City, Jacksonville and Beaufort. The Interlock system is aluminum shingles in Slate, Cedar Shingle, Shake and Mediterranean Tile profiles, plus aluminum standing seam. Compare it on paper with any steel bid, warranty documents included. Our pages on aluminum roofing and the Wilmington area have more detail.