What is Roof Decking?
The roof deck (also called roof decking or roof sheathing) is the structural wood layer of a roof, the flat surface that the underlayment and shingles are nailed to. It spans across the rafters or trusses and gives the roof its rigid load-bearing skin. On a typical residential roof, the deck is made of 4-by-8 sheets of CDX plywood or OSB (oriented strand board), usually 1/2 inch or 5/8 inch thick. When the deck is rotted or broken, the shingles above cannot do their job no matter how new they are.
A cut-away view of a residential roof. The cyan plywood surface is the roof deck, sitting between the rafters below and the shingles + underlayment above.
What is the decking on a roof made of?
Almost always wood, in one of two forms. CDX plywood is the premium choice: 4-by-8 sheets of 5-ply or 7-ply softwood plywood, glued with exterior adhesive, holding nails well and resisting moisture for decades. OSB (oriented strand board) is the budget alternative: large wood strands compressed and glued into the same sheet size. OSB is meaningfully cheaper but swells, delaminates, and loses nail-holding capacity when wet.
Standard thickness for residential roofing is either 1/2 inch (15/32) or 5/8 inch (19/32). The IRC under R803 requires roof sheathing strong enough to support a 20-pound-per-square-foot live load plus the dead load of the roofing materials; either common thickness meets that requirement at standard rafter spacing.
Is roof decking the same as plywood?
Not exactly. Plywood is one common material used to make a roof deck, but it is not the only one. Most residential roof decks built since the 1980s are either plywood OR OSB. Older homes (1960s and earlier) sometimes have decks built from 1-by tongue-and-groove pine planks running across the rafters rather than sheet goods. On commercial buildings, metal decking is common; on some custom homes, you will see cementitious panels.
So “roof decking” is the general term for the structural skin, and “plywood” is one possible material it is made from. When a roofing contractor mentions “decking replacement,” they are usually replacing plywood or OSB sheets that have rotted; they may also be re-sheathing over old plank decking.
What is another name for roof decking?
Three terms for the same thing, used interchangeably by contractors, manufacturers, and inspectors:
- Roof decking, the most common homeowner-facing term.
- Roof sheathing, the term used in the International Residential Code (R803) and on architectural drawings.
- Roof deck, used interchangeably with “roof decking,” especially in commercial roofing.
If you see “sheathing replacement” on a roofing quote, that means the same thing as “decking replacement.” Both refer to swapping out the plywood or OSB underneath the shingles.
Is roof decking expensive to replace?
Material cost for a 4-by-8 sheet of 1/2-inch CDX plywood runs $40 to $60 (and 5/8-inch is $55 to $80) at residential roofing supply houses in Northern Virginia and Maryland. OSB is roughly $5 to $10 cheaper per sheet but is rarely used on premium installs.
Installed cost, including labor and material, runs $80 to $130 per sheet replaced ($2.50 to $4.00 per square foot). A typical roof replacement involves replacing 0 to 4 sheets in the eaves, valleys, or around penetrations where the existing deck has rotted. Whole-deck replacement is rare on roofs under 40 years old and would cost roughly the same as a full re-roof of just the structural layer.
Most roofing contractors include “up to 4 sheets” of decking replacement in the base price and charge $90 to $130 per additional sheet found rotted during tear-off. Watch for contractors who quote zero allowance and then charge $200 per sheet on the job; that is a known sandbag pattern.
What DreamHome installs
DreamHome replaces rotted decking with 5/8-inch CDX plywood in all cases (never OSB), nailed off to IRC R803 spec on top of the existing or new rafter framing. The first four sheets are included in the base price on every full roof replacement quote; additional rotted sheets discovered during tear-off are billed at the rate documented on the quote with photo documentation of each replaced sheet. There are no surprise charges; the rate is locked in writing before the job starts.
Red flags on someone else’s roof
- OSB used in place of plywood without disclosure. Budget contractors swap OSB for plywood to save $5 to $10 per sheet. OSB swells when wet and holds nails worse. Ask explicitly which material is being installed.
- Soft spot or sag visible when walking on the roof. Indicates rotted decking under the shingles. From the ground, look for a visible dip in the roof line; that is rot, not just an old roof.
- Zero decking-replacement allowance in the quote. Every old roof has at least some rotted decking; quotes that hide this cost will charge for it later.
- No photo documentation of replaced sheets. If the contractor finds 8 rotted sheets and bills for them, they should have a photo of each one. No photos, no extra charges.
- 1/4-inch plywood used. Below code minimum. Indicates a contractor cutting corners on raw materials.