Salt air and metal roofs in Florida: steel or aluminum, and the fasteners that decide it

The Florida Building Code sets the minimum metal for a roof panel. It does not say how close to salt water that metal may go. The manufacturer's warranty draws that line, and each manufacturer draws it in a different place.

What the code sets

The Florida Residential code marks the IRC's metal-panel corrosion table, R905.10.3(2), as "Reserved." Instead it says: "Metal-sheet roof coverings installed over structural decking shall comply with Table R905.4.4." A quote citing the IRC table for a Florida house cites a table Florida reserved.

• Aluminum: ASTM B209, 0.024 inch minimum thickness for roll-formed panels.
• Aluminum-zinc coated steel (Galvalume): ASTM A792, 0.013 inch minimum thickness, AZ50 coating.
• Galvanized steel: ASTM A653, 0.013 inch minimum thickness, G-90 zinc coating.

The Building volume, in Table 1507.4.3(2), lists the same AZ50 and G-90 coatings and requires painted steel to sit on a listed metallic coating. For screws, FBC-R R904.5.2 sets a measurable test: "not more than 5-percent red rust after 1000 hours exposure" in ASTM B117 salt spray. Under R904.5.3, clips must meet a corrosion resistance of 0.90 ounce per square foot or an equal corrosion resistance, or be Type 304 stainless.

We searched the roofing chapters of both the Residential and Building codes for "dissimilar" and "galvanic" and found no such section, and no coastal-distance rule for metal panels. The general provision, R904.2, is a compatibility clause: roof assemblies "shall be of materials that are compatible with each other and with the building." The 2023 edition is current. A 9th Edition is in rulemaking, and the Florida Building Commission's August 2026 workplan sets its effective date at six months after publication.

The warranty draws the coastal line

PrimeBid installs two metal lines: Union Corrugating Galvalume steel (5V and standing seam) and PAC-CLAD aluminum standing seam. Here is how their current warranty forms treat salt water.

• PAC-CLAD 35-year PVDF warranty, Galvalume substrate: installations "less than 2,800 feet from a seacoast, 1,200 feet from a large bay, or 600 feet from a canal or brackish water, are excluded." Outside those distances, the steel substrate term is 25 years, with no reduced coastal tier.
• PAC-CLAD 35-year PVDF warranty, aluminum substrate: the same three distances apply, but the house stays covered. Inside them, the substrate perforation term drops from 30 years to 25, the owner must rinse twice a year and keep records, and clear edge protection goes on all cut edges at installation.
• Union Corrugating bare Galvalume and Galvalume Plus 40-year perforation warranty, issued by NCI Group, Inc.: excludes "areas within two (2) linear miles of saltwater marine atmospheres."
• Union Signature 300 paint warranty: no coastal exclusion, but within 1,500 feet of a salt-water environment the owner must do a fresh tap water rinse twice a year. The film term is 45 years against cracking, checking, or peeling, and 35 years for chalk and fade.
• Union Signature 200 paint warranty: within 1,000 meters (3,280 feet) of salt water, a fresh water rinse once a year.
• Union general Product Warranty: excludes deterioration from "marine (salt water) atmospheres," with no distance stated.

A common rule of thumb says to use aluminum within about a mile of salt water. One manufacturer TechTip, from ATAS, says panels have traditionally not been recommended closer than one mile from the coast, and that this applies to metallic coated steel, not aluminum. No FEMA document or Florida code provision we found uses one mile. A mile from the beach would cover PAC's 2,800 foot seacoast distance, but it misses the 1,200 foot bay and 600 foot canal triggers, and Union's two-mile Galvalume exclusion is stricter still. The warranty distance for the specific panel decides it.

Why steel and aluminum differ

Galvalume is steel coated with a 55 percent aluminum-zinc alloy. The GalvInfo Center, part of the International Zinc Association, explains that "the aluminum-rich dendrites perform like a barrier coating, while the zinc-rich areas provide the galvanic protection," which limits rust staining at sheared edges.

That advantage shrinks closest to the surf. GalvInfo's Kure Beach, North Carolina data compare AZ50 Galvalume to G90 galvanized: AZ50 corroded about 3.8 times more slowly at 25 meters from the ocean (severe marine), against 8.2 times more slowly at 250 meters (moderate marine).

Aluminum behaves differently. The Metal Construction Association (MCA) notes that aluminum alloys "quickly form oxide layers that are non-conductive," and that chlorides in seawater are strong electrolytes. That does not make aluminum immune: galvanic corrosion needs two dissimilar metals plus moisture, and FEMA lists "stainless steel and aluminum" as an example of a dissimilar pair. ATAS, a manufacturer, states its prepainted aluminum passed 3,000 hours of ASTM B117 salt fog, against 1,000 hours for metallic coated steel. That is the manufacturer's own test claim, without published test details.

Where the salt sits

FEMA P-499 says metal corrosion "is most pronounced on coastal homes (within 3,000 feet of the ocean)." FEMA Technical Bulletin 8 adds that airborne salt declines rapidly in the first 300 to 3,000 feet inland, yet "studies have shown accelerated corrosion rates as far inland as 5 to 10 miles."

Rain helps. Per the same bulletin, sheltered or covered areas "do not benefit from occasional rinsing from rain and therefore accumulate more salt." PAC-CLAD's warranties exclude finish failure where the coating is sheltered from rainfall, naming "underside eaves and soffits."

Dissimilar metals on one roof

• Copper: MCA says copper "has a very severe corrosive effect on steel, aluminum, and zinc." It does not need to touch the panel. A copper roof or pipe draining onto a Galvalume surface is enough.
• AC condensate: Union's maintenance manual says condensate from AC units contains dissolved copper ions that cause galvanic corrosion.
• Treated lumber: MCA says preservative-treated lumber "should not be used on metal roofs."
• Runoff direction: Steelscape, a ZINCALUME (Al-Zn) producer, says its products should not catch water from roofs of more noble metals such as aluminum or copper. An aluminum roof should not drain into Galvalume or galvanized gutters or valleys.
• Fasteners: MCA's classic example of failure is "a galvanized (zinc-coated) nail in an aluminum roof."

FEMA P-499 gives the general rule: avoid joining dissimilar metals, especially those with high galvanic potential, such as copper and steel. PAC-CLAD's warranty forms and Union's Galvalume, Signature 200, and Signature 300 warranties exclude damage from copper or lead runoff.

Fasteners: where the sources disagree

The code's fastener rule is a fallback. FBC-R R905.10.4 begins "In the absence of manufacturer's installation instructions," then calls for galvanized fasteners on steel roofs, accepts stainless for metal roofs, and accepts aluminum-zinc coated fasteners for aluminum-zinc coated roofs. FBC-B 1507.4.4 says "Stainless-steel fasteners are acceptable for all types of metal roofs," and allows aluminum fasteners only on aluminum roofs attached to aluminum supports. The code names Type 316 stainless for coastal areas only for wood shingle and shake nails.

FEMA is more specific. P-499 Fact Sheet 7.6 says: "For roofs located within 3,000 feet of the ocean line, 300 series stainless steel clips and fasteners are recommended." Technical Bulletin 8, written for structural connectors, says Type 316 is more corrosion resistant than 304 and that G60 and G90 galvanized connectors are not recommended in coastal areas. MCA calls 316L "generally a minimum requirement" in marine settings, and its fastener table rates 300-series stainless "Yes" and hot-dip galvanized nails and zinc-plated screws "No" for aluminum cladding, defining coastal marine as within 1,500 feet of salt water.

Steelscape disagrees for coated steel: "Most stainless-steel hardware, such as clips and fasteners, are not intended to be used with ZINCALUME coated steel profiles in corrosive environments." Its bulletin shows paint bubbling at a stainless fastener 300 feet from the ocean. These sources conflict. The fastener and clip have to match the panel manufacturer's instructions for that substrate.

Read the warranty, not only the approval. PAC-CLAD's product page states: "Aluminum substrates, with stainless steel clips and fasteners, must be used to qualify for the coastal warranty." Its current aluminum warranty form does not mention stainless and says selection of fasteners and flashings "rests solely with the OWNER." The Florida approval for the same panel permits galvanized, Galvalume, or stainless clips. For a coastal lot, the warranty is the stricter document, so get the fastener requirement in writing from the manufacturer.

An approval is not a coastal warranty

FL24423-R5, held by Petersen Aluminum Corp., shows status Approved under Roofing, Metal Roofing. It covers Tite-Loc Plus standing seam in 0.032 inch aluminum and in 24 gauge steel, each over a wood deck, through Miami-Dade NOAs; the aluminum NOA was approved January 19, 2023 and expires April 18, 2028. Its NOAs list "Galvanized Steel, Galvalume Steel or Stainless steel stamped clips." A related Petersen approval, FL5562-R11, lists G90 coated steel clips for both its steel and aluminum products. Approval status does not tell you whether the warranty covers your lot. The warranty form does.

What Hurricane Ian showed

FEMA's Hurricane Ian Mitigation Assessment Team report (P-2342) documented "numerous instances of corrosion on critical building component connectors and fasteners," some of which failed. It made no corrosion finding specific to metal roof panels. Its roof findings were about wind. The corrosion lesson is about connectors and fasteners.

Rinse schedules are warranty terms

Inside their coastal distances, PAC-CLAD's aluminum warranty requires a rinse twice a year with records kept, Union's Signature 300 requires a fresh water rinse twice a year within 1,500 feet, and Signature 200 requires one a year within 1,000 meters. Union's maintenance manual says maintenance should be performed more often than twice a year on coastal buildings. Rinse the eaves, soffits, and covered edges rain does not reach. FEMA says that is where salt builds up.

Read the limits too. PAC-CLAD's aluminum form says PAC is not liable "for the cost of labor expended by others on any nonconforming material," and that the warranty "is non-transferable and non-assignable."

What to ask on a metal roof quote

1. The substrate: aluminum, Galvalume (AZ50), or galvanized, and the gauge or thickness.
2. The manufacturer's warranty form by name, and its distance test applied to your lot: seacoast, large bay, canal, and brackish water.
3. The fastener and clip material, per the panel manufacturer's instructions for that substrate, and what the coastal warranty requires.
4. How cut edges are treated at installation.
5. The metal of the gutters, flashings, and valleys, and whether any copper, treated lumber, or AC condensate reaches the panels.
6. The rinse schedule the warranty requires, and what records it asks you to keep.
7. The FL Product Approval number for the panel, revision and decimal included.

PrimeBid Energy LLC is a Florida Certified Roofing Contractor working across Florida, and it checks the panel warranty's distance test against the lot during design. On a Florida lot, the warranty picks the metal, and the canal out back can matter more than the beach.