Florida shingle reroof on a 5:12, 130 mph zone.

Code Basis: 2023 FBC-R (R905.1.1, R905.2.7, R905.2.8.3) & FBC-B (1507.1.1, 1507.2.7). HVHZ (Miami-Dade/Broward) has stricter rules—below assumes non-HVHZ.

Allowed Underlayment/Secondary Water Barrier (SWB) Options

For 5:12 slope in 130 mph (Exposure B/C), you need a Self-Adhering Polymer-Modified Bitumen (SAPMB) membrane complying with ASTM D1970 as your SWB. This satisfies both the underlayment and the SWB requirement (R905.2.7.1).

• Option A (Standard): Single layer ASTM D1970 over entire roof deck. No felt/paper needed underneath or on top.
• Option B (Two-Layer): ASTM D226 Type II (No. 30 felt) or ASTM D4869 Type IV fastened per Table R905.2.7.1(1) + ASTM D1970 applied over all eaves, rakes, valleys, hips, ridges, penetrations, and wall intersections (min. 36" wide sheets, 6" laps). Option A is faster and cheaper labor-wise.

Prohibited: Synthetic "roofing underlayments" (ASTM D8257) cannot serve as the SWB in 130 mph zones. They are only approved as underlayment beneath a separate SWB (Option B) or on slopes ≥ 2:12 in lower wind zones.

Fastening Requirements (Critical for 130 mph)

For ASTM D1970 (Option A): Mechanical fasteners are REQUIRED in addition to self-adhesion.

• Fastener: Cap nails (min. 1" cap) or cap staples (min. 1" crown, 16 ga.).
• Penetration: Min. ¾" into deck or through deck thickness.
• Pattern (R905.2.7.1 / Table R905.2.7.1(1)):
• Field: 12" o.c. max (staggered rows).
• Laps (Side & End): 6" o.c. max (staggered from field pattern).
• Edges (Eaves/Rakes): 6" o.c. max.
• Deck: Must be solid 15/32" plywood/OSB (R905.2.1). No skip sheathing.

For Felt (Option B base layer): Cap nails/staples 12" o.c. field, 6" o.c. edges/laps (Table R905.2.7.1(1)). Then roll D1970 over it (self-adhered only, no extra caps needed

For a Florida shingle reroof on a 5:12 slope in a 130 mph wind zone (likely Wind Zone 2 or 3 per FBC), the Florida Building Code (FBC), 8th Edition (2023), governs underlayment and secondary water barrier (SWB) requirements.

Allowed Underlayment Options (FBC R905.1.1, R905.2.8.2):

• Asphalt-saturated felt: Minimum #15 (ASTM D226 Type I) or #30 (Type II).
• Synthetic underlayment: Must comply with ASTM D226, D4869, or D6757 and be listed for use under asphalt shingles (e.g., GAF Deck Armor, Owens Corning Syntech).
• Self-adhering polymer-modified bitumen: Allowed as primary underlayment in high-wind zones if tested per ASTM D1970 and listed for the application.

Secondary Water Barrier (SWB) Requirements (FBC R905.1.1, R905.2.8.2, R905.2.8.3):

Required in High Velocity Hurricane Zone (HVHZ) — which includes Miami-Dade and Broward Counties — and strongly recommended (often required by local amendment) in 130 mph zones outside HVHZ for reroofing.

Allowed SWB Options (FBC R905.2.8.2):

1. Self-adhering modified bitumen membrane (e.g., Grace Ice & Water Shield, GAF WeatherWatch): - Must be applied directly to the roof deck. - Minimum 6-inch overlap at seams. - Fastened only at laps if required by manufacturer (typically self-sealing; no fasteners needed in field). - Critical: Must be continuous over sheathing joints and extend at least 6 inches up vertical surfaces (walls, chimneys).

1. Alternative: Two layers of approved underlayment (e.g., #30 felt + synthetic) may be accepted as SWB in some jurisdictions if installed per manufacturer’s high-wind application — but verify with local building department. Self-adhering membrane is the most reliable and widely accepted method.

Fastening (FBC R905.2.8.2, R905.2.8.3):

• Self-adhering SWB: No fasteners in the field — adhesion to clean, dry, primed deck is sufficient. Fasteners only at terminations (e.g., top edge under flashing) if specified by manufacturer.
• Underlayment (if used as SWB alternative):
• #15/#30 felt: Minimum 1-inch crown staples or 11-gauge roofing nails, spaced max 6” o.c. at edges and 12” o.c. in field (FBC Table R905.1.1(2)).
• Synthetic: Follow manufacturer’s high-wind fastening pattern (often 6”/12