What is roof decking?
Roof decking is the structural sheathing (typically plywood or OSB) that forms the base layer of a roof, to which underlayment and shingles attach.
The decking is the wooden layer that forms your roof's foundation. Usually made of plywood or oriented strand board (OSB), it sits directly on your home's rafters or trusses and serves as the surface to which underlayment and shingles are fastened. This sheathing carries the weight of your entire roof system and protects the structure underneath from weather exposure.
When decking fails, you risk far more than a roof leak. Water that reaches rotted decking spreads rot into the rafters and framing, then into wall cavities and attic spaces. Soft spots under foot, visible sagging, or water stains on attic framing are all warning signs that decking has deteriorated. Early replacement before rot reaches structural members is far more affordable than dealing with frame damage.
Most homes use half-inch to three-quarter-inch plywood or OSB for this layer. The thickness depends on rafter spacing and local building codes. When you hire a contractor for roof replacement or installation, the decking condition directly affects whether underlayment and shingles will perform as intended. If existing decking is sound, it can often be reused. If it shows signs of rot, splitting, or compression, replacement is necessary to ensure a long-lasting roof system.