Roof Decking: Plywood vs OSB

Plywood Roof Decking

CDX plywood is the traditional roof decking material. It is made from thin wood veneers layered in alternating grain directions and bonded with waterproof adhesive. The "C" and "D" refer to the face veneer grades (C is the better face, D is the back), and "X" indicates exterior-grade glue. Standard residential roof plywood is 15/32 inch or 1/2 inch thick in 4-by-8-foot sheets. Plywood's cross-laminated construction gives it good dimensional stability. When exposed to moisture, plywood swells relatively evenly across its surface and returns close to its original dimensions when it dries. Delamination (separation of the veneer layers) occurs with prolonged water exposure but takes longer to develop than the edge swell problems that affect OSB. Plywood is slightly more expensive than OSB and accounts for a smaller share of new residential construction but remains preferred by some builders and in regions with high moisture exposure.

OSB Roof Decking

Oriented strand board (OSB) is engineered from compressed wood strands (typically aspen, poplar, or southern pine) bonded with resin under heat and pressure. The strands are oriented in layers, with surface strands aligned in one direction and core strands perpendicular, providing structural strength similar to plywood. OSB is the dominant decking material in new residential construction because it costs less than plywood and offers consistent performance across the full sheet with no knots, voids, or weak spots. The standard thickness for residential roof decking is 7/16 inch or 15/32 inch. The primary weakness of OSB is its moisture behavior. When OSB edges are exposed to water, they swell permanently. Unlike plywood, OSB does not return to its original dimensions after drying. This edge swell creates ridges visible through the shingle surface, commonly called "ghosting" or "telegraphing."

Moisture Performance and Ventilation

The moisture behavior difference between plywood and OSB is the most important factor in the choice between them. Both materials perform well in a properly built and ventilated roof assembly where the decking stays dry. Problems arise when leaks, ice dams, or condensation expose the decking to moisture. Plywood tolerates occasional wetting better because its cross-grain construction limits directional swelling and it recovers more fully when dried. OSB absorbs moisture through the edges and at any cut or damaged areas, and the resulting swell is permanent. Proper attic ventilation is critical for both materials. A poorly ventilated attic traps moisture that condenses on the underside of the decking, particularly in cold climates. This sustained condensation degrades both plywood and OSB over time, but OSB shows damage sooner because of its edge swell characteristics.

Decking and Insurance Claims

Roof decking condition is discovered during the tearoff phase of a roof replacement. It is one of the most frequently supplemented items on insurance claims because the damage is hidden under the shingles and underlayment and cannot be fully assessed from the initial exterior inspection. When decking damage is found, the contractor should stop work, photograph the affected areas with a tape measure showing the extent, and submit a supplement to the adjuster before proceeding. Xactimate prices decking replacement by the square foot, with separate line items for plywood and OSB at their respective thicknesses. The number of damaged sheets, their locations, and the cause of the damage (storm-related leak, long-term moisture, prior unrepaired leak) should all be documented. Storm-related deck damage, such as rot from a leak caused by wind-displaced shingles, is covered. Pre-existing rot from a decade-old plumbing vent leak is generally excluded as maintenance neglect.