Secondary water resistance and roof-deck attachment

Two details hidden under your shingles decide how a roof behaves in a hurricane. Roof-deck attachment is how firmly the plywood or OSB sheets are nailed to the rafters or trusses; secondary water resistance (SWR) is whether the seams between those sheets are sealed, so rain cannot pour into the attic if the shingles blow off. Both can be improved most cheaply when the roof is replaced, both appear on Florida's wind mitigation form, and both are central to the IBHS FORTIFIED Roof standard.

This guide covers what each does, what Florida and FORTIFIED actually require, and how to prove you have them. For the inspection form itself, see wind mitigation inspections and discounts.

What each one does

Deck attachment. Wind creates uplift on a roof. Smooth or short nails, spaced widely, can let sheathing panels peel off, and a roof without its deck has lost its main protection. IBHS says switching from smooth nails to 8d ring-shank nails and tightening the spacing can double the uplift the roof withstands (FORTIFIED Roof). Ring-shank nails grip the wood along the shaft rather than only at the head.

Secondary water resistance. If the covering is stripped by wind, the felt or underlayment beneath it is the only barrier, and ordinary felt leaks at every seam and nail hole. A sealed deck adds tape, a fully adhered membrane or foam at the seams. IBHS says that sealing the seams of the roof deck can reduce water intrusion by up to 95%, and that for every inch of rain on an exposed deck "the equivalent of nine bathtubs of water can pour in" (FORTIFIED Roof). Those are IBHS's testing results, and the 95% is a maximum.

Insurers care because roof damage dominates hurricane losses. IBHS says as much as 90% of catastrophe-related insurance claims include roof damage (FORTIFIED FAQ). A roof that loses shingles but keeps its deck and stays sealed produces a repair claim, not a gutted interior.

What Florida requires on a re-roof

Florida law pushes these upgrades into roof replacements on older homes. Fla. Stat. 553.844 directs the Florida Building Commission to require, for site-built single-family homes built before the Florida Building Code, that a roof replacement incorporate secondary water barriers and the strengthening or correction of roof-decking attachment and fasteners (Fla. Stat. 553.844). The same statute adds roof-to-wall connection upgrades in the windborne debris region for homes insured or valued at $300,000 or more, capped at about 15% of the cost of the re-roof.

The Existing Building Code carries out the statute, in Section 706.7 as the Florida Building Commission's analysis of changes for the sixth edition numbers it. A city building department's published Q&A (the City of Sebastian's, which does not show a date) summarizes the nailing rule: if the existing deck is fastened with staples or 6d nails, the entire roof deck is re-nailed with 8d ring-shank nails spaced no more than 6 inches on center; a deck already nailed with 8d ring-shank nails at 6 inches or closer needs nothing more. The same Q&A lists three ways to meet the secondary water barrier rule:

1. A self-adhering polymer-modified bitumen underlayment (ASTM D1970) over the entire roof.
2. A strip at least 4 inches wide of that material, or 3¾-inch AAMA 711 flashing tape, over every joint in the deck, with a code-approved underlayment over the whole roof.
3. Two layers of felt or approved synthetic underlayment installed shingle-fashion with 19-inch laps and cap nails.

Section numbers, edition dates and local amendments change, and Miami-Dade and Broward have their own high-velocity rules. Ask your building department what applies at your address and permit date. A permit usually triggers inspections: the Sebastian Q&A lists a sheathing inspection of the re-nailing, a dry-in inspection and a final, plus an affidavit from the permit holder because the inspector sees only part of the nailing. Keep those records.

What FORTIFIED requires

For a new roof, the 2025 FORTIFIED Home Standard (applicable to projects permitted on or after November 1, 2025) sets the bar higher than the code in several ways (2025 FORTIFIED Home Standard):

• Deck attachment: full round-head ring-shank nails meeting ASTM F1667 (RSRS-01, 0.113 inch by 2 inches) spaced no more than 4 inches on center across all areas of wood structural panel sheathing.
• Sealed roof deck: one of several methods, such as fully adhered flashing tape over the seams plus a code underlayment, two layers of felt or synthetic underlayment, or a self-adhered membrane. The standard warns that tape must be fully adhered without wrinkles and says primer may be needed on OSB.
• Existing roofs: a home whose shingles are in good condition and have at least 5 years of useful life left, with adequate attic access, can be sealed and re-fastened from the attic by applying a 1.5- to 3-inch fillet of a two-component closed-cell spray polyurethane foam adhesive at all sheathing joints, framing intersections and valleys. The adhesive must be tested to a minimum design uplift of 110 psf, be applied by a trained applicator and come with documentation on the contractor's letterhead. The method is not permitted over wood-board decking.

IBHS estimates that re-roofing a 2,000-square-foot home to FORTIFIED Roof typically adds $1,000 to $3,000 plus a verification fee (FORTIFIED FAQ). For program details, discounts and grants, see The FORTIFIED Home standard and insurance discounts and state home hardening grant programs.

How the insurer sees it

On Florida's inspection form, deck attachment is scored by the weakest nailing found, and SWR is a yes-or-no answer that requires a self-adhering underlayment or a foam adhesive barrier, not ordinary felt. Unknown answers earn the least credit. Credits go on the hurricane portion of the premium under Fla. Stat. 627.0629, and states such as Alabama set benchmark discounts for FORTIFIED levels. For credit size, ask your agent for the insurer's table.

The evidence you need depends on the state's form, but it is nearly always the same documents: the roofing permit, the final inspection, the roofer's invoice that names the nails and the underlayment, and dated photographs taken before the shingles went on. Without photographs of the deck, an attic inspector may see only the underside and score you low.

What to put in a roofing bid

Say two bids arrive on a hypothetical older Florida home. Bid A is the standard replacement with felt. Bid B costs more and specifies re-nailing and a sealed deck. The comparison is easier when the bids list the same items.

After a covered loss

If wind damage forces a roof replacement under a homeowners policy, the Florida upgrades above are code requirements, and ordinary replacement cost does not include building code upgrades. Whether the policy pays depends on ordinance or law coverage; the HO-3 provides a limited amount (10% of Coverage A) and insurers' forms vary. Some states also require insurers to offer an endorsement that pays to upgrade a damaged roof to FORTIFIED, covered in The FORTIFIED Home standard and insurance discounts. For how roof claims are paid generally, see will insurance pay for a roof replacement?.

Key takeaways

• Deck attachment is how the roof sheathing is nailed; secondary water resistance is sealing the seams under the shingles. Both are cheapest to upgrade during a re-roof.
• Florida law requires re-roofs of pre-code single-family homes to strengthen deck attachment and add a secondary water barrier; the nailing rule in one city's Q&A is 8d ring-shank nails at 6 inches where the existing deck has staples or 6d nails.
• FORTIFIED Roof asks for 8d ring-shank nails at 4 inches and a sealed roof deck, and allows attic spray foam on some existing roofs.
• IBHS reports that sealing the deck can cut water intrusion by up to 95% and that better nailing can double uplift resistance.
• Proof is paper: permit, sheathing inspection, invoice naming the products, and photographs before the covering goes on.