Short Answer
Roof decking is the structural surface beneath the roofing materials. Common materials include plywood, OSB, and other approved structural panels. The roofing contractor should evaluate the deck during replacement.
Detailed Explanation
Roof decking is the flat structural surface that roofing materials are fastened to. It's typically made of plywood or oriented strand board (OSB) panels that are nailed to the roof's framing (rafters or trusses). Everything in the roofing system — underlayment, flashing, shingles — depends on a solid, sound deck for proper attachment and performance. If the deck is soft, rotted, delaminated, or structurally compromised, the new roofing system won't perform properly regardless of the quality of the materials installed above it. That's why a responsible contractor always inspects the deck after the old roofing is removed and before the new system is installed.
Why It Matters
A compromised deck is like building a house on a weak foundation — no matter how good the materials above are, the system won't perform properly.