Oriented strand board (OSB) OSB Grading Guide Panel Selection, Handling and Storage
OSB panels are available in three grades, identified by a
designation of the intended end-use. Sheathing
(SHEATHING SPAN®) is intended for use as covering
material for roofs, subfloors, and walls. Structural I
sheathing (Structural I SHEATHING SPAN) meets
additional requirements for cross-panel strength and
stiffness, as well as requirements for racking shear; it serves
in panelized roof systems, diaphragms, and shearwalls.
Single-floor (FLOOR SPAN®) is used as a combination
subfloor and underlayment and may be used under several
different types of finish flooring as well as subflooring in
a two-layer floor system with underlayment. SPCL-PNL®
meets HUD requirements for floors designed in
manufactured home construction. Table 1 contains
application recommendations for each of these panel grades.
Bond classification is related to the moisture resistance of
the glue bond under intended end-use conditions and does
not relate to the physical (e.g. erosion, ultraviolet, etc.) or
biological (i.e. mold, fungal decay, insect, etc.) resistance
of the panel*. OSB panels in conformance with PS 2 must
meet the bond classification requirements for Exposure 1.
Note: An Exterior bond classification is not available
with OSB products.
Exposure 1 is defined in PS 2 as a bond classification for
panels that are suitable for uses not permanently exposed to
the weather. Panels classified as Exposure 1 are intended to
resist the effects of moisture on structural performance due to
construction delays or other conditions of similar severity*.
* U.S. Department of Commerce, National Institute of Standards and
Technology, Voluntary Product Standard PS 2 — Performance
Standard for Wood-Based Structural-Use Panels
Span rating numbers for SHEATHING SPAN and
FLOOR SPAN indicate the maximum spacing of supports
over which the panels should be placed.
The span rating on SHEATHING SPAN panels appears as
two numbers separated by a slash (e.g., 32/16 or 48/24). The
first number is the maximum on center (o.c.) support
spacing in inches for roof sheathing. The second number is
the maximum o.c. support spacing when the panel is used
for subflooring. A panel marked 32/16 may be used for roof
sheathing over supports spaced up to 32 inches o.c. or for
floor sheathing over supports spaced up to 16 inches o.c.
The span rating on FLOOR SPAN panels appears as a single
number (e.g., 20 o.c.). FLOOR SPAN panels are designed
for single-floor applications and are manufactured with span
ratings of 16, 20, 24, 32, and 48 inches o.c.
OSB used in structural sheathing applications in the U.S.
must meet the requirements contained in the Department
of Commerce’s Voluntary Product Standard PS 2
“Performance Standard for Wood-Based Structural-Use
Panels.” In Canada, panels must comply with the
Canadian Standards Association’s CAN/CSA O325
“Construction Sheathing” or CSA O437 “OSB and
Waferboard.” Please refer to CSA-O325 for a description
of span ratings used in Canada.
SIZES AND AVAILABILITY
Grademark-specified OSB panel thicknesses range from
1/4 inch to 1-1/4 inch. Not all thickness are readily
available. Check with suppliers to determine availability.
Standard dimensions are nominal 4 feet by 8 feet. Because
OSB is made with wood, which will expand or contract
when subjected to changes in temperature and relative
humidity, a plus 0/minus 1/8-inch tolerance on width and
length is permitted for panels manufactured in
conformance with PS 2 to allow for possible expansion
after installation. In addition, a tolerance of ±1/32-inch is
allowed on the grademark-specified thickness of 13/16-
inch and less and ±5% of the grademark-specified
thickness for panels thicker than 13/16-inch. Some mills
also produce specialty sizes (e.g., smaller panels for the
Japanese market or longer panels for vertical applications
on walls, such as panels 4 feet wide by 9 or 10 feet long)
and “jumbo” panels, such as required as skins for structural
insulated panels. Panels 19/32-inch thick and thicker are
manufactured with a square edge or tongue-and-groove
(T&G) edge.Unlike plywood, most OSB T&G panels are manufactured to give a full 48-inch face. However, due to manufacturing limitations, some T&G panels have a 47-1/2-inch net face width. In some areas, certain panel thicknesses or span ratings may be difficult to obtain. OSB suppliers can verify the availability and details of the products they carry. To order OSB panels, the specifier should designate thickness, grade, span rating, bond classification, dimensions, “square edge” or “T&G” (as desired for thicker floor panels), and number of pieces. For example: 7/16-inch SHEATHING SPAN, 24/16 Exposure 1, 48 x 96 inches, square-edge, 200 pcs. The specifier is wise to identify an alternate panel in the specifications in the event that a certain dimension is not readily available. Fig. 1 illustrates a typical gradestamp that is applied to a certified panel, while Table 2 contains metric equivalents for panel thicknesses.