The Roof Deck: What's Under Your Shingles

Roof decks on homes around here are plywood or OSB (oriented strand board) panels nailed across the rafters. Both are engineered wood; plywood is layered veneers, OSB is compressed strands. Modern OSB earned its place as the common choice, plywood tolerates getting wet a little more gracefully, and a sound roof system keeps either one dry, which is the actual point.

The deck's job description: hold the nails, carry the loads (crew, snow, the occasional branch), and give the waterproofing layers a flat, solid base.

Decking fails one way: moisture, sustained. A slow leak at a flashing point, years of attic condensation, or standing water working through a valley keeps a patch of wood damp until rot sets in. OSB swells and crumbles; plywood delaminates; either way the wood stops holding nails.

From inside the attic, trouble looks like staining, soft spots, or daylight. From on the roof, a spongy feel underfoot. By the time decking telegraphs through to the shingle surface as a visible sag, the moisture has been at work for a long while.

Decking hides under every layer above it, so its condition is genuinely unknown until tear-off. That's not a contractor dodge; it's physics. What separates honest outfits is how the unknown is handled: per-sheet replacement pricing in writing before the job, photos the moment anything soft turns up, and a conversation before the work continues.

(Where decking fits in the cost picture, and what tear-off day looks like.) Most roofs need a few sheets or none; roofs that leaked for years need more. The bill should follow the photos, never precede them.

Everything in roof maintenance is ultimately deck protection: moss control, clear valleys and gutters, breathing ventilation, and leaks fixed while they're small. The deck never wears out from age alone; it only ever loses to moisture you could have evicted.

Wondering about yours? An attic check is part of every free inspection where access allows.