Roof decking standards — OSB vs plywood, thickness, code minimums, decking replacement during reroof

Roof decking (also called sheathing) is the structural panel layer beneath the underlayment and shingles — the sheet material that spans between rafters or trusses and provides the substrate for everything above. Two materials dominate current Texas residential construction: oriented strand board (OSB) and plywood. OSB is the most common new-construction material — manufactured by hot-pressing wood strands with adhesives, producing a structurally engineered panel less expensive than plywood per square foot. Plywood is the older traditional material — cross-laminated wood veneers with longer-term nail-holding characteristics and somewhat better moisture tolerance at the panel edges than OSB. Both materials are code-compliant when meeting IRC R503 (engineered wood panels) and R803 (sheathing requirements) specifications; for typical residential rafter spacing of 24 inches on center, the minimum thickness is commonly 7/16 inch OSB or 15/32 inch plywood per the published code references. Older Houston-area homes may have either material; new construction since the early 2000s is overwhelmingly OSB. Long-term performance differences worth understanding: OSB swells more than plywood when exposed to standing water at panel edges, and the swelling is permanent — once OSB has been wet at an edge, the swollen edge does not return to original dimensions on drying. This means OSB roof sections that have leaked at the underlayment level (e.g., from a failed pipe boot — cross-reference KB-D4-014) may develop visible deck swelling that is detectable from inside the attic. Plywood is more forgiving of intermittent wetting events at panel edges. Decking inspection during reroof is a required step — the existing decking should be visually inspected from the deck side once the existing shingles are torn off (cross-reference KB-D4-017 for the tear-off vs overlay context). Sections that must be replaced before the new roof installation: any decking that shows visible water damage or staining patterns, any decking that sounds or feels "spongy" when walked (an installer working on the roof can detect this with foot pressure), any decking with visible cracks or splits, and any decking section with delamination of the OSB strands or plywood veneers. The decking replacement decision is per-sheet — a single 4x8 sheet can be replaced without replacing the surrounding intact decking. The cost is the per-sheet material + installer labor; typical Houston-market range is published in industry-typical trade press but varies enough by year and market that specific dollar figures should track the homeowner's contractor estimate rather than a published rate. The "mid-job decking upcharge" pattern is one of the most common Texas contractor-surprise complaints (cross-reference D8-001 storm chaser patterns, D8-010 red flags in contractor pitches): a contractor signs the homeowner to a low base price for shingle install, then "discovers" significant rotten decking once the tear-off begins, presents a large upcharge that the homeowner has limited ability to negotiate mid-job (the existing roof is already off and the homeowner cannot easily switch contractors). The defensive position is to require the original estimate to specify (a) per-sheet decking replacement cost, (b) an estimated number of sheets contingency built into the base price, and (c) a written process for additional sheets identified during the work (typically photos of the affected sections + homeowner sign-off before replacement). A contractor who refuses to specify per-sheet contingency in the estimate is signaling a potential upcharge scam at minimum and should be re-evaluated against the broader contractor-selection framework (cross-reference D10-001 through D10-014 for the Vfane pillar framework). Texas / Houston relevance: Houston humidity affects OSB long-term, particularly on poorly-ventilated roof decks (cross-reference D3-001) where the deck back-side experiences sustained high humidity. North-facing roof planes with marginal ventilation can develop OSB delamination over 20-30 years even without surface-level shingle failures. [Source: International Residential Code R503 engineered wood panel sheathing requirements; IRC R803 roof sheathing requirements; APA — The Engineered Wood Association OSB and plywood span and thickness specifications; NRCA Roofing Manual roof deck inspection chapter; manufacturer install guides specifying acceptable deck substrates]