Fasteners, Dissimilar Metals, and Underlayment Three Blocks from the Gulf
A 2,400-square-foot hip roof sits three blocks from the beach in Cameron Parish. Slope is 5:12. Tear-off exposes 1x6 skip sheathing spaced about 5/8 inch apart, so the crew overlays 15/32-inch ACQ-treated plywood. The foreman then (a) fastens both the new sheathing and the shingles with 1-1/4-inch electro-galvanized coil nails left over from an inland job, (b) reuses the existing 16-ounce copper chimney flashing but butts new aluminum drip edge tight against it in three places, and (c) installs the same 30-pound organic felt used on the last job. The shingle supplier's published coastal requirements call for hot-dip galvanized or stainless fasteners and a synthetic or self-adhered underlayment. Four years later the owner reports rust streaks below the chimney, loose shingles along the eaves and rakes, and a denied warranty claim.
1. Identify each material or fastener incompatibility in the foreman's scope and explain the physical mechanism that produces the failure.
Read the worked answer
Three separate mechanisms are at work; they are not one problem.
1. Electro-galvanized nails in ACQ-treated plywood (decking fasteners). ACQ is alkaline and copper-bearing, and copper is cathodic to zinc. Electro-galvanizing is a very thin coating (typically well under 0.001 in) applied by plating, so it is consumed quickly and the steel shank corrodes, losing shank friction and head bearing. The code does not leave this to judgment: fasteners for preservative-treated wood must be hot-dip galvanized per ASTM A153, stainless steel, silicon bronze, or copper (IRC R317.3). Rust streaks starting at the sheathing are the tell.
2. Aluminum drip edge in direct contact with copper flashing. In a marine electrolyte, copper and aluminum form a galvanic couple with a potential difference on the order of half a volt. The aluminum becomes the anode and perforates locally; copper salts also stain the shingles below. Isolation (bituminous tape or membrane separator) or matching metals is required whenever the two must meet.
3. Organic felt in a coastal, wind-driven-rain environment. 30-pound felt is a temporary membrane: it wrinkles when it takes on moisture, has no adhesion, and provides no secondary water barrier. The supplier's coastal requirement is a warranty condition, so using felt converts a possible product claim into an installation claim against you.
There is also a fastening defect independent of corrosion: roofing nails must penetrate 3/4 inch into or through the sheathing with a 12-gauge shank and 3/8-inch head, and coastal zones require the manufacturer's enhanced pattern. Light coil nails in the perimeter are why the shingles are loose at the eaves and rakes, not just rusted.
2. Write the corrected specification for decking fasteners, roofing fasteners, flashing, and underlayment, and state the authority behind each item.
Read the worked answer
Specify by standard, not by brand, and attach the product data to the submittal.
- Sheathing fasteners: hot-dip galvanized per ASTM A153, or stainless steel, sized per the sheathing manufacturer's schedule. Authority: IRC R317.3 (fasteners for preservative-treated wood) plus the panel manufacturer's installation instructions, which is what an inspector will actually read.
- Roofing fasteners: 12-gauge, 0.120-inch shank, 3/8-inch head, hot-dip galvanized per ASTM A153 or stainless; length to penetrate 3/4 inch into or through the deck. Nailing pattern per the shingle manufacturer's high-wind table (typically six nails per shingle plus enhanced perimeter nailing at eaves and rakes). Authority: IRC R905.2.8.5 plus the printed installation instructions that govern the warranty.
- Flashing: keep copper at the chimney, and use copper or lead-coated copper drip edge where the two meet; if aluminum must be used, require a bituminous or elastomeric separator tape and a stainless or copper cleat. Authority: manufacturer compatibility tables and accepted flashing practice; there is no code section that will save a galvanic couple.
- Underlayment: self-adhered SBS membrane at eaves, valleys, and all penetrations, with a synthetic underlayment in the field, lapped per the membrane manufacturer. Authority: IRC R905.1.1 and the supplier's coastal requirement, which is a condition of the material warranty.
The key move is that the submittal - not a verbal instruction - is where the coastal requirement becomes contractual.
3. The warranty claim is denied. Who is economically responsible, and what should have been done at submittal to avoid this outcome?
Read the worked answer
The denial is almost certainly correct as to the manufacturer, because the installation did not follow the published instructions the warranty incorporates. That does not make the contractor responsible for everything.
Allocate by what each party controlled:
- Contractor: fastener selection, fastener embedment and pattern, metal-to-metal contact at the chimney, and underlayment choice - all within the crew's scope and all specified on the supplier's literature that was available before the order was placed. Expect to absorb rework of corrosion at the drip edge, replacement of perimeter shingles, and the flashing separator.
- Owner: any condition the contractor documented at bid but did not price (existing corrosion, prior repairs). If no pre-bid photo log exists, this defense is gone, which is the real lesson.
The submittal fix is procedural, not technical: submit the fastener and underlayment specification with the shingle product data, get written acceptance from the supplier's technical desk and the AHJ before material is delivered, retain the coil-nail cartons with coating labels on site, and photograph the flashing separations before closing them in. Four years later, that file is the only thing that separates a $2,000 repair from a $30,000 reroof argument.