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R908.7 Wind Mitigation

2020 FBC - Residential, 7^th edition > 9 Roof Assemblies > R908 Existing Roofing > R908.7 Wind Mitigation

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When a roof covering on an existing structure with a sawn lumber, wood plank or wood structural panel roof deck is removed and replaced, the following procedures shall be permitted to be performed by the roofing contractor:

Roof-decking attachment shall be as required by Section R908.7.1.
A secondary water barrier shall be provided as required by Section R908.7.2.
Exception: Structures permitted subject to the Florida Building Code are not required to comply with this section.

R908.7.1 Roof Decking Attachment for Existing Structures With Wood Roof Decks

Fastening for sawn lumber, wood plank or wood structural panel roof decks shall be in accordance with Section R908.7.1.1 or R908.7.1.2 as appropriate for the existing construction. 8d nails shall be a minimum of 0.113 inch (2.9 mm) in diameter and shall be a minimum of 2¹/₄inch (57 mm) long to qualify for the provisions of this section for existing nails regardless of head shape or head diameter.

R908.7.1.1

Roof decking consisting of sawn lumber or wood planks up to 12 inches wide and secured with at least two nails (minimum size 8d) to each roof framing member it crosses shall be deemed to be sufficiently connected. Sawn lumber or wood plank decking secured with smaller fasteners than 8d nails or with fewer than two nails (minimum size 8d) to each framing member it crosses shall be deemed sufficiently connected if fasteners are added such that two clipped head, round head, or ring shank nails (minimum size 8d) are in place on each framing member it crosses.

R908.7.1.2

For roof decking consisting of wood structural panels, fasteners and spacing required in columns 3 and 4 of Table R908.7.1.2 are deemed to comply with the indicated design wind speed range. Wood structural panel connections retrofitted with a two part urethane based closed cell adhesive sprayed onto the joint between the sheathing and framing members are deemed to comply provided testing using the manufacturer's recommended application on panels connected with 6d smooth shank nails at no more than a 6-inch edge and 12-inch field spacing demonstrate an uplift resistance of a minimum of 200 psf.

Supplemental fasteners as required by Table R908.7.1.2 shall be ASTM F1667 classification RSRS-01 ring shank nails with the following minimum dimensions:

0.113-inch nominal shank diameter.
Ring diameter a minimum of 0.012-inch greater than shank diameter.
16 to 20 rings per inch.
A minimum 0.280-inch full round head diameter.
Ring shank to extend a minimum of 1¹/₂inches from the tip of the nail.
Minimum 2¹/₄inch nail length.

TABLE R908.7.1.2

SUPPLEMENT FASTENERS AT PANEL EDGES AND INTERMEDIATE FRAMING

EXISTING FASTENERS	EXISTING SPACING	V_asd 110 MPH OR LESS SUPPLEMENTAL FASTENER SPACING SHALL BE NO GREATER THAN	V_asd GREATER THAN 110 MPH SUPPLEMENTAL FASTENER SPACING SHALL BE NO GREATER THAN
Staples or 6d	Any	6 inches o.c.^b	6 inches o.c.^b
8d clipped head, round head, smooth or ring shank	6 inches o.c. or less	None necessary	None necessary
8d clipped head, round head, smooth or ring shank	Greater than 6 inches o.c.	6 inches o.c.^a	6 inches o.c.^a

For SI: 1 inch = 25.4 mm.

Maximum spacing determined based on existing fasteners and supplemental fasteners.
Maximum spacing determined based on supplemental fasteners only.
V_asd shall be determined in accordance with Section 1609.3.1 of the Florida Building Code, Building or Section R301.2.1.3 of the Florida Building Code, Residential.

R908.7.2 Roof Secondary Water Barrier for Existing Structures With Wood Roof Decks

A secondary water barrier shall be installed using one of the following methods when roof covering is removed and replaced.

In HVHZ regions:
1. All joints in structural panel roof sheathing or decking shall be covered with a minimum 4 inch (102 mm) to 6 inch (153 mm) wide strip of self-adhering polymer modified bitumen tape applied directly to the sheathing or decking. The deck and self-adhering polymer modified bitumen tape shall be covered with one of the underlayment systems approved for the particular roof covering to be applied to the roof.
2. The entire roof deck shall be covered with an approved asphalt impregnated 30# felt underlayment or approved synthetic underlayment installed with nails and tin-tabs in accordance with Section 1518.2, 1518.3 or 1518.4 of the Florida Building Code, Building. (No additional underlayment shall be required over the top of this sheet.) The synthetic underlayment shall be fastened in accordance with the manufacturer's recommendations.
Outside the High-Velocity Hurricane Zone:
1. Underlayment shall comply with Section R905.1.1 of the Florida Building Code, Residential.

Exceptions:

Roof slopes < 2:12 having a continuous roof system shall be deemed to comply with Section R908.7.2 requirements for a secondary water barrier.
Clay and concrete tile roof systems installed as required by the Florida Building Code, Residential are deemed to comply with the requirements of Section R908.7.2 for Secondary Water Barriers.