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 constructions.

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.