Roof Decking Types in Waukegan: Plywood, OSB, Boards and Span Ratings

The deck is the one part of a roof that was already there before anybody sold you anything. Shingles get chosen, underlayment gets specified, and the decking gets discovered — usually around ten on the first morning, with a crew standing on it and a contract already signed.

Roof decking vs sheathing: same board, two chapters

The trade uses the words interchangeably. The code does not quite. Sheathing is the structural word. IRC Chapter 8 covers roof sheathing — R803.1 for lumber, R803.2 for wood structural panels — and cares about grade, thickness and how far material spans between rafters. Deck is the assembly word. Chapter 9 defines a roof deck as the sloped surface itself, not including its supporting members or vertical supports, and cares what the covering fastens to.

R905.2.1 uses both in one breath: "Asphalt shingles shall be fastened to solidly sheathed decks." Sheathing is the material; deck is the plane; "decking," the word homeowners actually hear, is barely code language at all. Which is why a deck can be adequate as a deck and non-compliant as sheathing — a plank roof holds shingles fine and may still fail a span check.

Plywood vs OSB: the difference shows up wet

Both comply. Panels must conform to DOC PS 1, PS 2, CSA O437 or O325 and carry a grade mark, and the code prefers neither.

The honest difference is how they behave in water. In Professional Roofing, April 2021, NRCA's Mark Graham puts tested thickness swell from oven dry to vacuum soak at 9.4 percent for 1/2-inch plywood and 32.9 percent for 7/16-inch OSB — about 3.5 times greater — and notes swell is especially sensitive to one-sided wetting, which describes every deck that has sat open in the rain.

Two consequences you can observe:

• Edge swell. OSB swells hardest at its cut edges and does not fully recover, so panel joints telegraph as parallel ridges under the shingles. A soaked plywood deck generally comes back; OSB has spent some of its thickness permanently.
• Fastener backout. Graham attributes the ridging and backed-out nails NRCA sees to those moisture-driven dimensional changes. A nail that has walked up an eighth of an inch is a bump, then a hole.

NRCA's stated position, given the choice, is plywood complying with PS 1 — a trade association's recommendation, not code. On a dry-in-a-day job in a dry climate, OSB is a rational purchase.

Roof decking thickness is set by the rafters, not by preference

IRC R803.2.2 caps panel spans at the values in Table R503.2.1.1(1) or APA E30, and the table is indexed by span rating rather than by thickness.

That rating is stamped on every panel. Per APA it is two numbers split by a slash: the left is maximum support spacing in inches for roof use, the right the same for subflooring.

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

Read off Table R503.2.1.1(1) as reprinted with its commentary by the City of Republic, Missouri. A 24/0 panel has a zero on the right: rated to span 24-inch rafters, never rated to be walked on as a floor.

Two things follow. Edge support is a column, not a detail — blocking, tongue-and-groove edges or H-clips buy span, and their absence takes it back. And rafter spacing decides your minimum. Panels as thin as 3/8 nominal are permissible for specific span and load conditions; NRCA recommends a full step above that, 15/32 at 16-inch spacing and 5/8 at 24-inch.

One consequence is visible from your attic. R905.2.5 requires shingle nails to reach not less than 3/4 inch into the sheathing, and where sheathing is thinner than 3/4 inch, fasteners shall penetrate through it. Those nail points overhead are code being followed, not a crew being careless.

Board decking, and the tear-off that becomes a re-deck

Houses built before panels took over are decked in 1x6 or 1x8 laid across the rafters. Lumber has its own table, R803.1: 5/8-inch net thickness at 24-inch rafter spacing, jumping to 1-1/2-inch tongue-and-groove beyond that. The wording is unchanged from the 2015 IRC through the 2024 editions built on it.

Board decks are not automatically a problem — old-growth 1x is frequently better wood than anything sold today. What ends the argument is the gaps. Boards shrink for eighty years, and a nail landing in a gap holds nothing.

Jurisdictions write this down. Denver's Roofing Guide and Checklist, updated April 2024, sets limits no manufacturer may relax: no continuous opening between boards greater than 1/2 inch, and on 1x decking under asphalt shingles no opening greater than 6 square inches, smaller ones permitted to be covered in 26-gauge metal. That is a municipal amendment and your city's number may differ, but the check is universal — and it is the one that turns a re-roof into a re-deck partway through the morning.

Spaced and step sheathing: decks built for something else

Spaced sheathing — skip sheathing to most crews — is a genuine code-recognized deck, not an old shortcut. R905.7.1 and R905.8.1 permit wood shingles and shakes over boards not less than 1 by 4 nominal, spaced on centers equal to the weather exposure so each board lands where the fasteners go. Both sections demand solid sheathing where the January average daily temperature is 25°F or below, on whatever portion of roof needs an ice barrier.

Tile is stricter. R905.3.1 requires solid sheathing under concrete and clay tile, spaced lumber permitted only in Seismic Design Categories A, B and C. Step sheathing is the tile variant, boards running up the slope in stepped courses so each tile head sits at the right height — which is why a tile deck is not a flat plane you can shingle over.

The consequence is blunt. A shake or tile house converting to asphalt is buying an entire new deck. That is a scope item, not an adder, and it belongs in the bid alongside the slope limits for each covering.

When decking actually has to come off

Visible cases are easy: plies separating, sag scalloped between rafters, holes at valleys and chimneys, a surface that gives underfoot. The invisible case decides jobs. The Forest Products Laboratory's Wood Handbook, Chapter 14, sets out the arithmetic: serious decay needs moisture above the fiber saturation point, averaging 30 percent, while air-dried wood at 20 percent or below carries a reasonable margin of safety. Then the sting — in laboratory tests, losses in toughness ran from 6 percent to more than 50 percent by the time 1 percent of the wood's weight had gone, and at 10 percent weight loss most strength losses exceed 50 percent. At that stage decay is "detectable only microscopically." Wood you can see is rotten lost most of what it had a long time ago.

Code closes the loop from the other side. R908.3 requires a replacement to strip down to the deck; R908.3.1.1 forbids recovering over a roof that is water soaked or deteriorated past being an adequate base.

Roof decking replacement cost is a change order, not a price

Decking is quoted after the price is agreed, priced per sheet, and counted by the only people on the roof. Once the covering is back on, nobody can audit the count.

Published guides do not settle the number. Roof-installation.com, dated December 2025, puts an installed sheet at roughly one and a half to three times the bare panel's price at the yard. MyHomePros quotes a per-sheet figure, then prices a one-to-five-sheet job at up to twice its own rate. Both are contractor pricing pages rather than an index, and the spread is not noise: cutting one sheet into a live roof is slow work, sheeting a whole slope is fast, so the same panel legitimately carries two prices depending on how many there are.

What a fair contract does about it:

• A unit price agreed before work starts, in dollars per 4x8 sheet, panel type and thickness named. Not an allowance, not cost-plus, not a figure produced on the day.
• A lower tier above a threshold. Past ten or fifteen sheets you are buying a re-deck and the rate should drop. Most bids omit this clause and it costs most when the count runs.
• Photographs with something in frame for scale, every sheet out and every sheet in, before underlayment covers them.
• Partial sheets counted as partial. A two-foot patch is not a sheet unless your contract says so.

None of it is adversarial. A contractor who already stated the rate has nothing to argue about at four in the afternoon. The wider bid-comparison discipline sits in roof replacement cost.

Roof underlayment replacement cost: cheap sheet, expensive access

Same logic one layer up. Underlayment is a minor material line — Fixr puts felt and synthetic in a low per-square-foot band — but it lives under the covering, so replacing it means removing and reinstating everything above. The ratio tells it better than a total: Fixr's own items put underlayment labor inside a full re-roof at a fraction of a dollar per square foot, and the lift-and-reset labor beneath concrete tile at several dollars — six to twenty times, depending which ends you compare. Which product goes back down is a separate decision, worked through in roof underlayment types.

The credential question in Illinois

Deck replacement is carpentry billed inside a roofing contract, and whatever bar a state sets for roofing was written around coverings — where it set one at all.

Illinois is one of very few states with a roofing license of its own, and there is no dollar threshold - any roofing work for pay requires one. Limited covers residential buildings up to eight units; unlimited adds commercial and industrial. The bond is ten thousand dollars for limited and twenty-five thousand for unlimited, and an exam covering Illinois jurisprudence is required. Working without the license is a Class A misdemeanor, a felony the second time.

Si el equipo encuentra madera podrida debajo de las tejas, pide el precio por hoja antes de que empiecen y pide fotos de cada hoja que cambien.

Bottom line

Read the stamp before arguing about brands: the span rating tells you what your rafter spacing permits, and that is the only thickness question with a right answer. Both panels are legal; OSB is the one that punishes a wet week. And settle the per-sheet rate, the volume tier and the photographs before anyone opens the roof — that is the last hour you hold any leverage over a number you will otherwise simply be handed.

Waukegan Roof Pros does roof replacement, written roof inspections and roof leak repair across Waukegan and northeastern Lake County, and every deck sheet we replace gets photographed before it disappears. Call (888) 603-6399.