Roof decking is the flat wood layer your shingles are nailed to. It sits on top of the rafters or trusses and under the underlayment. Most people also call it sheathing. On most Columbia homes it’s sheets of plywood or OSB, and it’s what holds the whole roof together.
What is roof decking made of?
Most roof decking built today is plywood or OSB (oriented strand board). Plywood is thin wood layers glued in alternating directions. OSB is wood strands pressed together with moisture-resistant glue. Older homes sometimes have plank decking, which is rows of solid boards.
Is one better? Not by much. APA, the Engineered Wood Association, says both are “manufactured according to a set of standards that assure very similar performance when used in the intended applications” (APA, Plywood or OSB). What matters more is the right thickness for your rafter spacing.
Every rated panel carries a stamp. The span rating on that stamp is two numbers with a slash. The first number is the “maximum support spacing for roof sheathing” in inches (APA). Lumber sheathing needs a minimum thickness of 5/8 inch under the state code (SC Residential Code 2021, R803.1).
Why does roof decking matter?
Your shingles are only as good as what’s under them. South Carolina’s code puts it plainly: “Asphalt shingles shall be fastened to solidly sheathed decks” (SC Residential Code 2021, R905.2.1).
Nails need solid wood to bite into. Rotted wood won’t hold them. The shingles may look fine on install day. Then the first big summer storm lifts them right off.
How does roof decking go bad in Columbia?
Water is the enemy, and our climate gives it plenty of chances. Columbia averages 70.2% relative humidity and 45.24 inches of rain a year (Wikipedia climate data, Columbia, SC).
Decking fails slowly. A small leak at a vent boot or a bad piece of flashing drips for months. The wood soaks it up. It swells, softens and starts to rot. Poor attic airflow makes it worse, because warm, damp air condenses on the underside of the boards.
The panels aren’t built to sit wet. APA rates most roof sheathing Exposure 1, meaning “suitable for uses not involving long-term exposure to weather” (APA).
What do you notice from the ground? A roofline that dips or sags between rafters. Water stains on the ceiling. From inside the attic, you may see dark stains or daylight around the nails. These overlap with the signs you need a new roof, so don’t climb up to check. Let someone trained walk it.
Do I need new roof decking when I replace my roof?
Usually not all of it. Most of the time only the sheets that are soft, rotted or delaminated get replaced.
Nobody knows the full answer until the old shingles come off. A good roofer walks the bare deck, finds the soft spots and shows you before anything gets covered up. It’s one step in how long a roof replacement takes.
Can you skip the tear-off and shingle over the top? Sometimes, but the code limits it. South Carolina doesn’t allow a roof recover where the existing roof “is water soaked or has deteriorated to the point that the existing roof or roof covering is not adequate as a base.” It also bans it where the roof already “has two or more applications of any type of roof covering” (SC Residential Code 2021, R908.3.1.1). A layover also hides the deck. You lose the one chance to check it.
If you’re weighing a fix against a full roof, our roof repair vs replacement breakdown helps.
Frequently asked questions
What is roof decking called besides decking? Sheathing, or just the roof deck. Both mean the same wood layer.
How long does roof decking last? There’s no set clock on it. A shingled roof here lasts between 15 and 40 years (The Red Shirt Guys FAQ). Dry decking can often be reused at replacement. Wet decking rots much sooner.
Is OSB roof decking worse than plywood? Not when it’s used as intended. APA says the two perform very similarly. Correct thickness and keeping it dry matter more.
Can a roof inspection find bad decking? Often, yes. Sagging lines, attic stains and soft spots underfoot all point to it. Our roof inspection checklist covers what we look for.
Who should look at your roof decking?
Someone who’ll show you what they find. The Red Shirt Guys have over twenty years of experience inspecting and replacing roofs in Columbia, and we’re veteran-owned. If your roofline dips or your ceiling is stained, book a free roof estimate or call 803.243.3968. We’ll tell you straight whether the deck needs work.
Not sure what your roof needs?
Get a free, no-pressure roof inspection from The Red Shirt Guys.
Request a free inspectionor call 803-243-3968