Specifying Structural Wood Panels

For structural uses such as sub-flooring  and sheathing, wood panels may be specified either by thickness or by  span  rating. The span rating is determined  by laboratory load testing and is given  on the gradestamp on the back of the  panel, as shown in Figures 3.33 and  3.34. The purpose of the span rating  system is to permit the use of many  different species of woods and types  of panels while achieving the same  structural objectives. Under normal  loading conditions, any panel with  a span rating of 32/16 may be used  as roof sheathing over rafters spaced  32 inches (813 mm) apart or as sub-flooring over joists spaced 16 inches  (406 mm) apart. The long dimension of the sheet must be placed perpendicular to the length of the supporting members. A 32/16 panel may be  plywood, composite, or OSB, may  be composed of any accepted wood  species, and may be any of several thicknesses, so long as it passes the  structural tests for a 32/16 rating.

Figure 3.33 Typical gradestamps for structural wood  panels. Gradestamps are found on the
back of each panel. (Courtesy of APA–The Engineered Wood Association)

Figure 3.34 A guide to specifying structural panels. Plywood panels for use in wall paneling, furniture, and other applications where  appearance is important are graded by the visual quality of their face veneers rather than their structural properties.

The designer must also select from  three  exposure durability classifi  cations  for structural wood panels: Exterior,  Exposure 1, and Exposure 2. Panels marked Exterior are suitable for  use as siding or in other applications  permanently exposed to the weather. Exposure 1 panels have fully water- proof glue but do not have veneers of  as high a quality as those of Exterior panels; they are suitable for structural  sheathing and sub-flooring, which will  be protected from the weather once  the building is finished, but must often endure long periods of wetting during  construction. Exposure 2 is suitable  for panels that will be fully protected  from weather and will be subjected  to only a minimum of wetting during  construction. About 95 percent of  structural panel products are classified as Exposure 1.

For plywood panels intended as finish surfaces, the quality of the face  veneers is of obvious concern and  should be specified by the designer  (Figure 3.32). For example, A-B  plywood (with an A grade veneer on  one face and a B grade veneer on the  other) might be specified for cabinet construction, while less expensive  C-D plywood is adequate for exterior  sheathing that will be concealed in  the finished construction. For higher-quality architectural woodwork,  fine  flitch-sliced hardwood face ve- neers may be selected rather than  rotary-sliced softwood veneers and  the matching pattern of the veneers specified.

Figure 3.32 Veneer grades for softwood plywood. Ply- wood with A grade face veneer is costly and
limited in availability. It is used only in applications demanding the highest quality appear-
ance. Plywood with B face veneer is used for  concrete formwork construction and for less
demanding appearance applications. Plywood  with C-plugged face veneer is used for under-
layment and combination subfl  ooring/under- layment, where an especially smooth substrate
is required to avoid telegraphing irregularities  through the fi nish fl ooring material. Plywood
for sheathing and subfl ooring is most com- monly specifi ed as “CDX,” consisting of a C
grade face, a D grade face, and constructed  with exterior glue (the panel is installed with
the C face oriented toward the weather). C  grade veneer is also the lowest grade permitted
in any ply of an Exterior rated panel, that is, a  siding panel intended for permanent exposure
to the weather.

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