How Does Salt Air Affect a Metal Roof in Coastal North Carolina?

Salt air deposits a fine film of sodium chloride on every outdoor surface, and when that film gets damp from dew, fog or humidity, it becomes an electrolyte. Electrolytes let corrosion reactions run faster. That is why a mailbox, grill or porch light near the beach looks worn years before the same item inland.

On a roof, salt affects three things. It speeds up corrosion of metal parts, including the panels, fasteners, flashing and gutters. It breaks down some sealants and coatings faster, especially when combined with strong sun. And it accelerates galvanic corrosion, the reaction that happens when two different metals touch while wet. The Metal Construction Association's bulletin on dissimilar metals names chlorides from seawater as strong electrolytes for exactly this reason.

Salt does not affect every roof the same way. Asphalt shingles do not corrode, but their metal flashings, drip edges and nails do. Steel roofing loses its zinc or zinc-aluminum coating faster. Aluminum roofing forms a protective oxide skin and does not rust, which is why it is widely used for coastal and marine applications. Our comparison of aluminum and steel on the coast goes deeper on the metals themselves.

The parts that fail first are almost never the middle of the roof panels. On coastal North Carolina homes, look at these spots:

• Fasteners. Exposed screws and nails are small, thin and often a different metal from the roof. FEMA's coastal guidance recommends stainless steel or hot-dipped galvanized hardware within about 3,000 feet of the ocean.
• Flashing and drip edge. These are frequently cut on site, which exposes raw edges, and they sit where water and salt collect.
• Gutters and downspouts. Gutters hold wet debris and salty water for days at a time. Mixed metals in gutter systems corrode quickly.
• Vent caps and pipe boots. Painted steel vent caps and rubber boots age fast in salt and sun.
• Sheltered areas. The underside of porch roofs, deep eaves and the lee side of dormers collect salt that rain never washes away.

When you look at a coastal roof, these are the places to look first. A small rust streak from a fastener is a cheap fix. Left alone for years, it can stain siding, loosen trim and let water in.

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, so the heaviest exposure is on the barrier islands and the first blocks behind the beach. That includes oceanfront and second-row homes on the Outer Banks, Bogue Banks, Topsail Island, Wrightsville Beach and the Brunswick County beaches.

Salt exposure does not stop at a fixed line. Homes along the sounds and tidal creeks in towns like Beaufort, Swansboro, Southport and Manteo see salt from open water on the other side. Strong onshore winds carry it farther. Hurricanes and tropical storms are the extreme case: NOAA's North Carolina climate summary describes Florence in 2018 dropping 20 to 36 inches of rain in eastern North Carolina, and storms like that bring wind-driven spray and flooding to homes that normally see little salt.

In Raleigh, Charlotte or the Triad, salt air is not a meaningful roofing factor. The concerns there are heat, humidity, hail and the occasional ice storm. If your home is east of I-95, ask about exposure on site rather than relying on a map.

Salt air affects asphalt shingle roofs through their metal parts rather than the shingles themselves. The asphalt and granules do not corrode, but every shingle roof depends on steel nails, galvanized drip edge, step flashing along walls, valley metal and vent flashings. Near the ocean those parts can rust well before the shingles wear out, and a rusted-through step flashing behind siding leaks just as surely as a missing shingle.

Coastal wind is the other pressure on shingles. Tabs that lift in strong gusts break their seal strips, and once the seal is broken the next storm can peel them back. Warm, humid coastal summers also help algae grow on shingles, which leaves the dark streaks common on North Carolina roofs; see roof algae and mold in NC.

If you are replacing a shingle roof near the coast with new shingles, ask for corrosion-resistant nails and flashing, and ask what wind classification the shingles carry. If you are weighing metal instead, our comparison of metal and architectural shingles covers cost, wind and upkeep side by side.

A metal roof near the beach needs simple, regular attention rather than special treatment. Rain rinses most of a sloped roof naturally. What it cannot reach is where salt builds up.

A garden hose at normal pressure is enough to rinse salt from protected areas a few times a year, from the ground or a ladder at the eaves. Never use a pressure washer on a metal roof; high pressure can force water under trim and damage coatings. Keep valleys and gutters clear of pine needles, leaves and sand, which hold salt water against the metal. After a tropical storm, rinse the roof and gutters once the weather clears and check for damage.

Have a roofer look closely at flashing and sealant around chimneys, skylights and walls every few years. Sealants are the parts of a roof most likely to need renewal over time, and coastal sun and salt shorten their life. Our guide to pine needles and debris on metal roofs covers cleaning safely, and roof damage after a hurricane covers what to look for after a storm.

Ask questions that show whether the contractor has thought about your specific exposure, not just roofing in general:

• What is the panel metal, and what is the finish system? For aluminum, ask for the coating specification; for steel, ask what the metallic coating is and how thick.
• What are the fasteners, clips and flashing made of, and are they compatible with the panels?
• Does the panel or finish warranty exclude coastal or marine exposure, and is my address inside that zone?
• How will copper, existing galvanized gutters or bare steel be separated from the new roof?
• What maintenance does the manufacturer's warranty require, if any?

Get the answers in writing with the estimate. A contractor who can answer these clearly has usually worked on the coast before. One who waves them off may be pricing a roof meant for an inland subdivision. Our pages on how to vet a roofer and choosing a metal roofing contractor cover the rest of the checklist, including licensing and contracts.

The Interlock system is built from heavy-gauge aluminum, which does not rust and is corrosion-resistant in salt air. About 98% of Interlock roofs are aluminum; the rest are solid copper. The aluminum panels are finished with the Alunar 70% PVDF coating, made to the AAMA 2605 standard, and the coating carries a 30-year warranty from Interlock Roofing. Panels lock together on all four sides with the fasteners concealed under the next course, so most fasteners are not exposed to spray.

Material and finish are covered by the Guardian Lifetime Limited Material Warranty from Interlock Roofing for the original purchaser while they own the home, transferable once, with the second owner covered to 50 years from substantial completion. Workmanship is not part of that warranty; Metal Roof NC provides the Metal Roof NC 5-Year Labor Warranty for workmanship. Details are on our warranties page.

Metal Roof NC, an authorized Interlock aluminum roofing dealer, serves the North Carolina coast from Wilmington to New Bern, Morehead City, Jacksonville and Beaufort. A visit to the house is the way to judge your actual exposure.