Plywood vs. OSB Truss Repairs

The building codes prohibit the cutting, notching drilling, or otherwise altering of truss members or components without written approval from a registered design professional. When specifying the material in a repair detail, the Building Designer or Truss Designer has a number of options including plywood or oriented strand board (OSB) gussets, metal nail-on plates, lumber scabs or repair frames. The following question examines two commonly-used products.

Question

Which is better to use for truss repairs, plywood or OSB?

Answer

Plywood and OSB are two types of wood structural panel products commonly specified and used to repair damaged joints in metal plate connected wood trusses. There are several factors to consider before deciding which product to use.

First, everyone involved in the repair process needs to understand the difference between the two products. The 2009 International Building Code^® (IBC^®) defines plywood as:

Similarly, the IBC defines OSB as:

It’s not uncommon to hear people refer to a wood structural panel as “plywood,” but after further discussion it’s clear they’re actually talking about OSB. Make sure there is a clear understanding of the differences between the two, especially if the repair requires a specific product.

Second, Section 2303.1.4 of the 2009 IBC requires that wood structural panels used in structural applications, such as truss repairs, must conform to the requirements provided in the U.S. Department of Commerce/National Institute of Standards and Technology standards (DOC), PS 1 or PS 2. More specifically, Section 2303.1.4 states:

Structural plywood may be qualified using the requirements of PS 1, Structural Plywood, or PS 2, Performance Standard for Wood-based Structural-use Panels, whereas structural OSB is qualified under PS 2. The standard used to qualify a product is included as part of the trademark on the panel (see Figure 1). Only panels conforming to PS 1 or PS 2 should be used.

The building codes provide allowable span and load information for wood structural panels used in typical floor, roof and wall sheathing applications. For truss repairs, specific design capacities, such as allowable tensile strength and shear through-the-thickness, are required. Design capacities for various panel grades and span ratings are available from qualified panel testing and grading agencies (e.g., APA–The Engineered Wood Association, TECO, etc.) and are also provided in Chapter M9 of the ASD/LRFD Manual for Engineered Wood Construction (2005), published by the American Wood Council (AWC) and the American Wood Council of the American Forest & Paper Association (AF&PA). The design capacities in these documents are based on tests of panels manufactured in accordance with PS 1 and PS 2, which bear the trademark of a qualified inspection and testing agency (see Table 1 and 2 for examples).

These tables show there are generally only slight differences between the design capacities for structural plywood and OSB for a given span rating. An exception, however, occurs with shear through-the-thickness capacities, where OSB provides considerably higher values. Based on this difference, if a truss repair specifies only OSB, plywood should not be substituted without written permission from the registered design professional who prepared the truss design repair drawing.

Another design capacity to consider is dowel bearing strength, which determines the allowable lateral resistance provided by the nails, screws or bolts that attach the wood structural panel to the truss. Table 11.3.2B of the National Design Specification^® for Wood Construction (NDS^®) lists the dowel bearing strengths for various wood structural panels (see Table 3). All grades of OSB, Structural 1 and Marine plywood are assigned the same dowel bearing strength, whereas all “other grades” of plywood have a lower dowel bearing strength. Since the majority of plywood used for construction applications in the U.S. is NOT Structural 1 or Marine, lower lateral resistance values (i.e., more fasteners) typically must be used for a repair using plywood than for one using OSB.

To pose a question for this column, call the SBCA technical department at 608/274-4849 or email technicalqa@sbcmag.info.