What is the difference between plywood sheathing and OSB?

Both are structural panels rated for equivalent sheathing applications, but they differ in construction. Plywood uses peeled veneer layers cross-laminated with adhesive. OSB uses oriented wood strands pressed with resin. Plywood handles moisture and repeated wetting better; OSB is less expensive and uses wood fiber more efficiently.

What does CDX plywood mean?

CDX stands for C-grade face veneer, D-grade back veneer, and X (Exterior) glue. The X in CDX refers to the adhesive, not the panel's exposure rating. CDX is not rated for permanent exposure to weather; it is a common wall and roof sheathing grade designed for temporary construction moisture.

Can I use plywood sheathing on both walls and roofs?

Yes. The same structural sheathing panels are approved for walls, roofs, and floors as long as the panel's span rating matches the framing spacing. Always verify the panel stamp against the span requirements for your application.

Why does plywood delaminate?

Delamination occurs when the adhesive bond between veneer layers fails, usually from prolonged moisture exposure, freeze-thaw cycling, or low-quality glue in the original manufacturing. Interior-grade plywood can delaminate quickly if used in wet locations; always use Exterior-rated glue panels for structural sheathing.

What thickness plywood is used for wall sheathing?

Most residential walls with studs 16 inches on center use 7/16-inch or 1/2-inch plywood. Walls with studs 24 inches on center typically require 1/2-inch or 5/8-inch panels. Engineered shear walls may specify Structural 1 plywood in a thickness determined by the shear wall schedule.

Plywood Sheathing — Structural Wall, Roof, and Floor Panels

A plywood sheathing panel is a structural wood panel made from thin wood veneers bonded in alternating grain directions, used on walls, roofs, and floors to provide lateral strength, a nailing base, and a flat substrate for exterior finishes.

Plywood Sheathing diagram — labeled parts and installation context

What It Is

Plywood is manufactured by peeling thin veneer sheets from a log, then stacking them with each layer's grain perpendicular to the one below. An odd number of layers — typically 3, 5, or 7 plies — is used so the face and back grain run in the same direction. The assembly is pressed with waterproof adhesive under heat and pressure at approximately 250 degrees Fahrenheit. This cross-laminated structure distributes loads in both directions and resists splitting from fasteners near panel edges far better than solid wood.

The face veneer is sanded or textured depending on the application. Structural sheathing grades use B or C-grade face veneers, which are not cosmetically perfect but are free of defects that would compromise structural performance. The back veneer may be a lower grade. Core veneers use the lowest-grade material compatible with the glue bond. Standard panels measure 4 feet by 8 feet, though 4-by-9 and 4-by-10 sheets are available for tall-wall applications that reduce the need for horizontal blocking.

Plywood panels are graded and stamped with a span rating, veneer grade, glue exposure classification, and mill number. APA Structural 1 plywood has the highest racking and shear values and is used in engineered shear walls. The span rating appears as two numbers separated by a slash — for example, 32/16 means the panel can span 32 inches on a roof and 16 inches on a floor when used as a subfloor.

Types

CDX plywood (C-face, D-back, Exterior glue) is the standard wall and roof sheathing grade and is the most commonly specified panel in residential construction. T&G (tongue-and-groove) plywood panels are used in floor systems where the interlocking edge transfers loads between adjacent panels without requiring blocking at the unsupported joint. Rated Sheathing panels are the APA designation for general structural use on walls, roofs, and subfloors.

Structural 1 panels have stronger racking values for engineered shear wall applications, with all plies oriented in the same species group to maximize cross-grain stiffness. Fire-retardant-treated plywood is chemically impregnated under pressure and is required in certain multifamily and commercial assemblies where the code calls for a Class A flame spread rating.

Where It Is Used

Plywood sheathing is used on exterior walls to resist racking, on roofs as a nail base for shingles and underlayment, and on floors as a structural subfloor. It is also used as backing board for grab bars and heavy hardware when a full-thickness substrate is needed. In high-seismic zones such as Seismic Design Categories D through F, structural plywood shear walls are a primary lateral force-resisting element per IBC Section 2305.

Wall sheathing panels are nailed with 8d common nails at 6 inches on center along panel edges and 12 inches on center at intermediate supports. Roof sheathing follows similar nailing schedules but may use closer spacing in high-wind zones as specified by the local wind speed map and the building code nailing table.

How to Identify One

Plywood has a smooth or lightly textured face with visible veneer grain and a layered edge showing alternating wood grain directions. Its edges are more uniform and dense than OSB, which displays a mosaic of compressed wood strands. The grade stamp on the face identifies the product and span rating. Structural 1 panels are explicitly marked with the Structural 1 designation adjacent to the span rating.

Replacement

Plywood sheathing damaged by rot or delamination must be replaced to restore the structural integrity of the wall, roof, or floor assembly. Wall panels require removing the exterior siding and any water-resistive barrier. Roof panels require removing shingles and underlayment down to the damaged section. Floor panels are cut and replaced to land on joist centers so that each cut edge has bearing.

Replacement panels must match the original thickness, span rating, and grade to maintain the engineered capacity of the assembly. In shear wall applications, the nailing schedule must be duplicated exactly because the shear capacity of the wall is directly tied to the nail size, spacing, and edge distance. A common mistake is substituting OSB for plywood without verifying that the shear wall design allows it — both products are listed for shear walls, but the allowable shear values differ by panel type in the IBC shear wall table.