This from apawood.org
Question: Plans call for five- ply plywood roof sheathing but the
lumber yard sent out four-ply. They say they won't have five-ply in
stock until next month. What is the difference between the panels?
Answer: In general, building code provisions and APA's use and
application recommendations for floor, wall and roof sheathing do
not make a distinction for panel construction. With the exception of
roof sheathing applied parallel to framing building code and APA
load span tables, treat 3-, 4-, and 5-ply plywood panels equally.
For example, consider a 15/32 or 1/2-inch C-D Exposure 1 (CDX)
plywood meeting the requirements of U.S. Product Standard PS 1-07:
Structural Plywood. For 15/32 and 1/2-inch the Standard provides for
3-, 4-, and 5-ply panel construction.
A 4-ply panel is actually a special case of 3-ply construction. The
4-ply panel is composed of two face veneers or layers, and a cross
band of two parallel-laminated veneers, which create their own
layer. Therefore, 3-ply and 4-ply panels are both 3-layer
In conventional applications where face grain is across supports,
all three constructions of equal Span Rating are intended to do the
same job. This is reflected in APA recommendations and in building
code provisions, where no distinction in ply-layer construction is
made for a given Span Rating. It is recognized, however, that there
may be certain very specialized applications in which a property,
not usually important, assumes particular importance. In these
cases, it is sometimes necessary to specify a particular ply-layer
construction in addition to the thickness and grade.
A typical application of 15/32 and 1/2-inch CDX is roof sheathing.
For example, consider a conventional application where the panel
face grain is across supports. In this case let's say that the
minimum stiffness of 5-ply panel is 100 percent. If so, the minimum
stiffness for 4-ply would be 108 percent and 3-ply would be 121
percent. All building code provisions and APA application
recommendations for roof sheathing are equal for the three
constructions and are based on the minimum, which is 5-ply.
On the other hand, there are some roof sheathing applications in
which the face grain is placed parallel to supports. In this
situation, if we again say that the minimum stiffness of the 5-ply
15/32 and 1/2-inch is 100 percent, the minimum stiffness for the 4-
ply is 65 percent and 3-ply is 28 percent. Unlike when the face
grain is across supports, this difference is significant enough that
ply-layer construction needs to be specified when face grain is
parallel to the supports.
There are several properties of plywood panels that are of interest
to design professionals, depending upon the application. The
relationship between the ply-layer construction of panels varies
depending upon the property of interest. In addition, published
mechanical properties are calculated based on several allowable
panel constructions. Refer to APA Panel Design Specification, Form
D510, for mechanical property design data.