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Roof Sheathing Considerations
Roof Sheathing Installation in
High Wind Areas
 Keeping
the roof sheathing on the roof trusses is extremely
important during a windstorm like a tornado or
hurricane. The connection of the roof sheathing to the
roof structure is one of those vulnerable points in a
house that has been known to fail during high wind
events, which can cause major damage to the house.
Losing the roof sheathing can cause the following
problems:
- It allows rainwater to enter the house
through the openings left by the missing
roof sheathing panels. This increases the
water damage to the interior finishes and
house contents.
- Because the sheathing is often used to
horizontally "brace" the roof trusses, if it
is blown off, the trusses no longer are
adequately braced and can collapse into the
house.
- Any opening left by missing roof
sheathing can allow the attic space to
become pressurized by the wind. The
resulting forces can exceed the design
strength of the hurricane straps (roof
structure-to-top of wall connectors) causing
them to fail. In many cases when this
happens, the entire roof blows off the house
in one large section.
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Why
is Roof Sheathing Prone to Damage in High Winds?
Roof sheathing is especially vulnerable to wind
damage because the connection of the roof
sheathing to the roof structure is typically
made with nails oriented vertically. The nails
are oriented in the same direction as the uplift
forces acting on them. This places the nails in
what is called direct withdrawal.
The nails oriented this way are held in place
by the friction created between the nail shank
and the roof structural members the nail is
driven into. The force needed to overcome this
friction is much less than the force required to
shear the nail in half. |
Uplift Forces Acting on Sheathing Nail
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The type of nail used and how it is installed
can affect its resistance to the uplift forces
during a windstorm. Here are some ways the
uplift resistance of the sheathing to roof
structure connection can be affected:
- The shorter the nail shank, the
weaker the connection.
- The smaller diameter of the nail
shank, the weaker the connection.
- The wider the spacing between
sheathing nails the weaker the
connection.
- Thinner roof sheathing may
increase the potential of the roof
sheathing panel being pulled off
over the sheathing nail.
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Nail Terms
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What Do the Building Codes
Require?
Selecting an adequate nailing pattern for roof sheathing
depends on the uplift forces on the roof and the shear
forces imposed on the sheathing when used as a roof
diaphragm. The building codes generally specify a
minimum nailing pattern. However, this is a minimum. The
design (shape of the house and roof) can affect the type
of uplift forces acting on the roof sheathing.
Pre-Hurricane Andrew Dade and Broward County
Requirements: Before Hurricane Andrew, Dade and
Broward Counties required:
| Minimum Roof Sheathing
Thickness: |
1/2" Nominal (15/32") |
| Min. Sheathing Nail Spacing: |
6" on center at panel edges
12" on center in the middle of the panel |
| Min. Sheathing Nail Size: |
6d common nails for 1/2"
sheathing
8d common nails for 5/8", 3/4", & 7/8" sheathing
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In their assessment of damage after Hurricane Andrew,
the Federal Emergency Management Agency (FEMA) said,
"The roof framing systems observed were composed
typically of prefabricated light wood trusses and
plywood sheathing. While the trusses were found to have
performed well under the wind forces, the connection of
the sheathing to the trusses was inadequate."
In the same FEMA assessment, it was noted that many of
the roof structures were relying on the roof sheathing
for lateral bracing. In the report they said, "It is the
opinion of the assessment team that reliance on
sheathing for truss-roof bracing, and the corresponding
loss of the sheathing, was a major cause of the total
damage of the building systems."1
In later tests made by the plywood industry, it was
found that the code-required nailing was sufficient to
withstand the wind loads required by the building codes
prior to Hurricane Andrew. However, the nailing pattern
was very marginal. It was found that removing one nail
from this nailing pattern reduced the resistance of the
sheet to withdrawal (uplift) by a large percentage. This
highlighted the importance of making sure that all nails
penetrate the supports below the plywood.
Post-Hurricane Andrew Dade and Broward County
Requirements: With the adoption of the 1993 edition
of ASCE-7 wind load criteria by the building codes in
Dade and Broward Counties, the nailing pattern required
by the codes before Hurricane Andrew became inadequate.
The Dade and Broward Building Codes following Hurricane
Andrew required the following minimums:
| Minimum Roof Sheathing
Thickness: |
5/8" Nominal (19/32")
Oriented Strand Board is not allowed |
| Min. Sheathing Nail Spacing: |
6" on center at panel edges
6" on center in the middle of the panel |
| Min. Sheathing Nail Size: |
8d common nails for 5/8"
sheathing
10d common nails for 3/4" sheathing |
The 1996 Dade and Broward County Codes require 8d common
nails (.131" diameter x 2 1/2" long) for sheathing up to
a nominal 5/8" thickness and 10d common nails (.148"
diameter x 3" long) for nominal 3/4" thick plywood,
because the thicker sheathing reduces the depth of
penetration of the nail into the roof structural member
below. Also important: staples are not allowed.
The new nail spacing is 6" on center for the entire
sheet of plywood except that the plywood must be nailed
at the gable end with 4" on center spacing.
New Statewide Florida Building Code Requirements:
Florida is in the process of writing a new statewide
building code. Broward and Dade Counties will probably
maintain their current roof sheathing requirements.
Outside of those counties the new code minimum
requirements for roof sheathing, according to the Third
Draft (Feb. 2000), are:
| Minimum Roof Sheathing
Thickness: |
1/2" Nominal (15/32")
May be plywood or OSB |
| Min. Sheathing Nail Spacing: |
6" on center at panel edges
12" on center in the middle of the panel |
| Min. Sheathing Nail Size:
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6d common nails for 1/2"
sheathing
8d common nails for 5/8" sheathing
16-gauge Wire Staples are also acceptable |
The minimum staple spacing is 4" on center at the edges
and 8" on center in the middle of the middle of the
sheathing panel.
The minimum requirements for the state of Florida, if
unchanged from the most recent code draft, will be equal
to or lower for most of Florida (outside Broward and
Dade Counties) than what was required in Broward and
Dade Counties before Hurricane Andrew. If another
hurricane with a magnitude of a Hurricane Andrew hits
Florida other than in Broward or Dade Counties, adopting
these reduced nailing requirements could result in the
same types of damage to roof sheathing seen during
Hurricane Andrew.
Problems to Watch Out For When
Installing Roof Sheathing
Much has been learned from the experiences from
Hurricane Andrew. Contractors installing roof sheathing
should be aware of the following potential problems:
- If gun nailing is used, the chance that the
nails may miss the roof structural member below
increases, a critical component in roof sheathing
failures in past hurricanes. The bottom of the roof
sheathing can be easily inspected for driven nails
that have missed the roof structural member, and it
is important to re-nail any sections that have been
missed.
- If gun nails are used, there are several things
to be considered. Power-driven nails generally
have a smaller diameter shank and are sometimes
shorter than common nails. Power driven nails
are allowed by the code. However, if their diameter
and length do not meet or exceed the dimensions
given by the code, the nail spacing must be reduced
accordingly to compensate for the smaller size.
- Some types of power nail guns use "clipped head"
nails (also known as modified round-head nails and
P-nails). These are nails with heads that have a
portion of the head removed so the nails can be
packaged tightly together and still be driven.
Because the heads are smaller, it is easier for the
roof-sheathing panel to be pulled off over the nail
head in high winds. Gun-driven nails are available
with full heads. These should be used if nail guns
are the chosen method of installation. The building
codes in Broward and Dade Counties require
roof-sheathing nails to have full heads.
- It is important to understand how the wind in a
hurricane creates uplift forces on the roof. Some
parts of the roof will have greater uplift forces
exerted on it than other parts -- typically roof
edges and at roof ridges. Closer nail spacing may be
required in those locations.
On the Good Neighbor HouseŽ, the nail spacing
required to resist 156-mph winds at the roof edges
and ridges was 2" on center. The building code
indicates a minimum nail size and spacing. This is a
minimum and is allowed to be changed to increase the
wind resistance of the roof sheathing to roof
structure connection.
- Make sure the contractor is using the correct
nail. Not all nails are equal. An 8d nail can be a
box nail, common nail, or power driven nail. An 8d
box nail has a smaller shank diameter than an 8d
common nail. Power driven 8d nails have a smaller
shank diameter and are shorter than an 8d common
nail. The code requirements are based on common nail
sizes. Any reduction in this lowers its uplift
resistance capabilities.
The strength of the roof sheathing to roof structure
connection is in the friction created between the
roof structural member and the surface area of the
nail shank driven into it. Using a smaller diameter
nail or shorter nail reduces the connection's
resistance to uplift.
Look at the box for the nail description. If it is
not a common nail, ask questions. Take a few of the
nails to the hardware store and compare them to a
common nail.
- As soon as the sheathing nailing has been
confirmed to be installed correctly, install the
underlayment for the roof covering as soon as
possible. The underlayment, which is typically black
building paper, can protect the roof sheathing from
rain. Rain can cause the sheathing to delaminate or
swell.
Because the roof sheathing-to-roof structure connection
is so critical to the survivability of the entire house
during a hurricane or tornado, special attention must be
given to the materials used and installation methods. It
is imperative that your contractor is aware of this,
knows the steps that must be followed for good
installation, and understands how deviating from code
minimums or specifications can affect the performance of
this critical connection.
1Building
Performance: Hurricane Andrew in Florida; Federal
Emergency Management Agency; Federal
Insurance Administration; December 21, 1992; p.22
State FarmŽ
believes the information contained in the Good Neighbor
House is reliable and accurate. We cannot, however,
guarantee the performance of all items demonstrated or
described in all situations. Always consult an
experienced contractor or other expert to determine the
best application of these ideas or products in your
home.
01-05-2001
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