What roof decking is

The decking is the roof’s skin of wood: panels or boards nailed across the rafters or trusses, with the underlayment and roofing laid over it. Asphalt shingles go only on a solidly sheathed deck (IRC R905.2.1). The guide to the parts of a roof shows it under the underlayment, and the roof rafters guide the framing under it.

Plywood, OSB or boards

Decking What it is
Plywood Thin wood veneers glued in cross layers
OSB Oriented strand board: wood strands pressed with resin
Board or plank Lumber boards, the older way, solid or spaced

Plywood and OSB are both wood structural panels to the code: made to standard DOC PS 1, PS 2, CSA O325 or O437 and grade-marked by an approved agency (IRC R803.2.1). Lumber decking has its own thickness table: 5/8 inch net for rafters 24 inches apart (Table R803.1). Spaced boards are for wood shingles and shakes (R803.1).

How thick a panel

A panel’s span rating, stamped on it as two numbers such as 32/16, gives the farthest apart the supports can be for a roof (the first number) and a floor (the second). The code’s roof spans (Table R503.2.1.1(1)):

Span rating Panel thickness Roof span with edge support Without edge support
24/0 3/8 in 24 in 20 in
24/16 7/16 in 24 in 24 in
32/16 15/32 or 1/2 in 32 in 28 in
40/20 19/32 or 5/8 in 40 in 32 in
48/24 23/32 or 3/4 in 48 in 36 in

Edge support means blocking, panel edge clips or tongue-and-groove edges between the rafters. The spans assume a dead load of 10 psf; heavier roofing lowers the live load the panel can carry. Panels run with their long side across the rafters, and can’t overhang a gable end more than 9 inches unless gable framing holds them (R803.2.3).

When decking gets replaced

On a reroof, the old roofing comes off down to the deck, so soft, rotted or delaminated sheets show. Those are cut out and replaced, priced by the sheet or the square foot, before the new underlayment goes on. On an insurance job, decking found at tear-off is the kind of item a supplement asks for. Spaced plank decks, left from an old wood shingle roof, are sheathed solid before shingles go on (IRC R905.2.1).

Floors use the same panels: the subfloor replacement cost guide gives the floor spans and prices.

Common questions

Is OSB or plywood better for roof decking?

Both are wood structural panels the code allows when they carry the right grade mark and span rating (IRC R803.2). Pick by the span rating your rafter spacing needs.

How thick should roof decking be?

For rafters or trusses 24 inches apart, a 7/16 inch 24/16 panel spans them with or without edge support, and a 3/8 inch 24/0 panel with edge support (IRC Table R503.2.1.1(1)).

How many sheets of decking does a roof need?

A 4 by 8 sheet covers 32 square feet. Divide the roof’s area by 32 and add some for cutting. The roofing calculator gives the roof’s area.

Is decking part of a roof estimate?

Replacing damaged decking is a line of its own, priced by the sheet or square foot for what’s found when the roof comes off.