Roof Sheathing Installation
Home Builder’s Guide to Coastal Construction
FEMA 499/June 2005
Technical Fact Sheet No. 18

Purpose: To provide information about proper roof sheathing installation,
emphasize its importance in coastal construction, and illustrate fastening
methods that will enhance the durability of a building in a high-wind area.

Key Issues
--Insufficient fastening can lead to total building failure in a windstorm.
--Sheathing loss is one of the most common structural failures in hurricanes.
--Fastener spacing and size requirements for coastal construction are typically
different than for non-coastal areas.
--The highest uplift forces occur at roof corners, edges, and ridge lines.
--Improved fasteners such as ring shank nails increase the uplift resistance of
the roof sheathing.

Sheathing Type
Typically, 15/32-inch or thicker panels are required in high-wind areas.
Oriented Strand Board (OSB) or plywood can be used, although plywood will
provide higher nail head pull-through resistance. Use panels rated as “Exposure
1” or better.

Sheathing Layout
Install sheathing panels according to the recommendations of the Engineered Wood
Association (APA). Use panels no smaller than 4 feet long. Blocking of
unsupported edges may be required near gables, ridges, and eaves (follow design
drawings). Unless otherwise indicated by the panel manufacturer, leave a 1/8-
inch gap (about the width of a 16d common nail) between panel edges to allow for
expansion. (Structural sheathing is typically cut slightly short of 48 inches by
96 inches to allow for this expansion gap – look for a label that says "Sized
for Spacing.") This gap prevents buckling of panels due to moisture and thermal
effects, a common problem.

Fastener Selection
An 8d nail (2.5 inches long) is the minimum size nail to use for fastening
sheathing panels. Full round heads are recommended to avoid head pull-through.
Deformed-shank (i.e., ring- or screw-shank) nails are required near ridges,
gables, and eaves in areas with design wind speeds over 110 mph (3-second gust),
but it is recommended that deformed shank nails be used throughout the entire
roof. If 8d “common” nails are specified, the nail diameter must be at least
0.131 inch (wider than typical 8d pneumatic nails). Screws can be used for even
greater withdrawal strength, but should be sized by the building designer.
Staples are not recommended for roof sheathing attachment in high-wind areas.

Fastener Spacing
It is extremely important to have proper fastener spacing on all panels. Loss of
just one panel in a windstorm can lead to total building failure. Drawings
should be checked to verify the required spacing; closer spacing may be required
at corners, edges, and ridges. Visually inspect work after installation to
ensure that fasteners have hit the framing members. Tighter fastener spacing
schedules can be expected for homes built in high-wind areas. Installing
fasteners at less than 3 inches on center can split framing members and
significantly reduce fastener withdrawal capacity, unless 3-inch nominal framing
is used and the nailing schedule is staggered.

Ridge Vents
When the roof sheathing is used as a structural diaphragm, as it typically is in
high-wind and seismic hazard areas, the structural integrity of the diaphragm
can be compromised by a continuous vent (see figure below left). Maintain ridge
nailing by adding additional blocking set back from the ridge, or by using vent
holes (see figure below right). Verify construction with a design professional.

Graphic: method for maintaining a continuous load path at the roof ridge by
nailing roof sheathing.

Graphic: Holes drilled in roof sheathing for ventilation—roof diaphragm action
is maintained.

Ladder Framing at Gable Ends
Use extra care when attaching a ladder-framed extension to a gable end. Many
homes have been severely damaged by coastal storms because of inadequate
connections between the roof sheathing and the gable truss. The critical
fasteners occur at the gable-framing member, not necessarily at the edge of the
sheathing. Nailing accuracy is crucial along this member. Tighter nail spacing
is recommended (4 inches on center maximum).

Common Sheathing Attachment Mistakes
Common mistakes include using the wrong size fasteners, missing the framing
members when installing fasteners, overdriving nails, and using too many or too
few fasteners.

Additional Resources
Engineered Wood Association (www.apawood.org)