There is a line on a roof replacement estimate for wood nobody has seen yet. Every other line is measured: the squares off the roof, the layers counted at the drip edge, the pitch. The decking allowance is a range, and it stays a range until the old shingles come off.
What is under the shingles: roof decking by the age of the house
Roof decks here come in three kinds, and the age of the street is a fair guess. Nearly one in five Eau Claire housing units was standing before 1940, and the frame houses of that era around Randall Park and in the Third Ward usually carry 1-inch boards, a 1x6 up to a 1x12 run across the rafters, the deck usually called plank decking. Plywood was in use as roof sheathing by the 1940s and the post-war boom scaled it, so the ranches and split-levels of the 1950s through the 1970s usually carry plywood. OSB, oriented strand board, arrived in the early 1980s, cheaper with comparable strength, so a roof framed since then may carry either panel. The only way to know is to look.
| House era | Usually under the shingles | The signs it has failed | The minimum an acceptable deck meets |
|---|---|---|---|
| Before 1940 | 1-inch boards, 1x6 to 1x12, gapped by shrinkage | Gaps past 1/4 inch, split or cupped boards, rot along the eave | CertainTeed: nominal 1 inch, no gap over 1/4 inch. GAF: nothing wider than 6 inches, seasoned and dry |
| 1940s through the 1970s | Plywood panels | Plies separating at the edges, dark underside, soft underfoot | All three: at least 3/8 inch, APA rated. CertainTeed: an Exposure 1 or Exterior stamp |
| 1980s on | Plywood or OSB panels | Swollen, fuzzy edges, crumbling around old nail holes, sag between trusses | CertainTeed and Owens Corning: OSB at least 7/16 inch. Owens Corning: APA rated, gaps of 1/8 to 1/4 inch |
How roof decking fails, and what makes a sheet come out
Part of the answer is written down. The shingle makers publish deck minimums, and Wisconsin’s dwelling code, SPS 321.28(3)(a)3, says shingles go on according to the manufacturer’s recommendations, which makes those minimums code for your roof. CertainTeed, GAF, and Owens Corning land on nearly the same floor, and all three add the same conditions: the deck has to be dry, it has to hold a nail, and it cannot be insulation board. GAF puts a number on the nail, 3/4 inch into a wood deck or just through a plywood one, so a sheet that will not hold that nail has failed. Thickness, gap over a quarter inch, board width past 6 inches, nail hold: those get measured, and they are not arguable.
The rest is a call made standing on the deck. GAF’s own wording for a board deck is well-seasoned, with adequate nail-holding capacity and a smooth surface, and CertainTeed hands the wide-board decision to the contractor outright. No manual says how soft is too soft, which is why every sheet gets photographed before it leaves the roof.
Rotted wood is dark in spots, crumbly, spongy underfoot, and smells musty; leaks and condensation are the two common causes. Plywood is layers of veneer, so a wet sheet separates into plies that fan apart at the edge. OSB is compressed strands, and water swells it in thickness, most at the edges where the exposed end grain draws it in, which is why wet OSB shows a fuzzy, thickened edge.
On a house with a history of ice dams, the eave sheets usually go first: meltwater backing up behind the dam at the cold overhang gets under the first courses of shingles. Under the deck, a bathroom fan or dryer duct that ends in the attic wets the sheathing above it. Stopping that at the source is attic ventilation work; here it is only why a tear-off so often finds a bad sheet over the bathroom.
Can the old boards stay, or does the deck get sheeted over?
Rotten boards along the eave come out and get replaced with boards; the question is the rest. Boards shrink as they dry, and a deck laid tight commonly ends up with gaps of half an inch or more. On 6-inch boards with a 5/8-inch gap, a roofing engineer writing for IIBEC put the odds of a nail landing in a joint at nearly one in ten. CertainTeed draws its line at a quarter inch: gaps of 1/8 to 1/4 inch get a double layer of water-resistant underlayment, and anything wider makes the deck unacceptable until plywood or OSB goes over it. GAF holds board decks to 1/8 inch, so the shingle decides how much gap the boards are allowed.
Width matters as much as gap. Boards wider than 6 inches move with humidity more than a shingle tolerates, and it shows as buckled shingles. CertainTeed allows either fix, an overlay of 4x8 panels or a saw cut down the center of every wide board to make two narrow ones, and leaves the call to the contractor. A wide-board deck is one of the few things the attic shows before the shingles come off.
The deck under an old cedar shake roof is different: skip sheathing, also called spaced sheathing, is 1x4 or 1x6 boards spaced to match how much of each shake shows. Asphalt shingles cannot go on that. Owens Corning accepts a spaced deck only on boards at least 6 inches wide, 3/4 inch thick, with gaps no wider than a quarter inch, which a shake deck never meets. It gets a solid layer, 3/8 plywood or 7/16 OSB run long edge up the slope, and the old boards stay unless a piece is decayed, broken, or warped. Whether the old shingles can stay at all is a roof-over versus tear-off question.
How the decking allowance on the estimate is spent or handed back
An allowance is a number set aside for work that might be needed, and on an older house the decking is the line that gets one.
What sizes it is not a guess at how much rot is up there. It is the roof: the squares going on, how many layers are coming off, the era and the deck type, and whatever the attic showed. On a lumber-era house the allowance typically runs $1,500 to $6,000, depending on how many sheets go in. A full sheet-over, for boards gapped past a quarter inch or wider than a 1x6, is a different scope than swapping sheets, and CertainTeed’s manual says to find that out before the estimate goes out, price it, and tell the customer why. The sheet-over lands on the estimate, and the allowance stays what it is for: sheets nobody can see yet.
Where there is attic access, the allowance narrows before the tear-off: the underside shows dark staining where water has been, daylight where a board has split, insulation matted flat under an old leak, and any bath, dryer, or stove vent that ends in the attic, which APA, the Engineered Wood Association, says to correct along with the deck. A pre-war board deck usually spends some of its allowance; a mid-century plywood roof sometimes does; a house built after 2000 rarely does.
No decking comes out before you have seen the photo of it. The allowance is spent only against the sheets in those photos, and whatever is left of it is handed back when the job closes out.
Will the new roof decking meet code?
The state code sets no thickness of its own for plywood or OSB. SPS 321.27(9) puts a panel’s allowable loads and spans on the grade stamp printed on the panel, so the stamp and your rafter spacing decide what can go on. On a stamp like 24/16, the first number is the roof span in inches with the edges supported, and that is the one that matters. APA’s roof construction guide, Table 33, gives the rest: a 7/16 panel stamped 24/16 spans 24 inches with or without edge support, while a 3/8 panel stamped 24/0 spans 24 inches only with edge support and drops to 19.2 inches without. That is when panel clips, H-clips in the trade, are required: one per span, whenever the stamp’s unsupported span is shorter than the framing spacing. A 24/0 panel on 24-inch trusses needs a clip in every span; a 24/16 panel does not.
Every panel edge and end gets a 1/8-inch gap so it can swell without buckling; Owens Corning caps the gap at 1/4 inch. Nailing follows the code’s schedule: panels every 6 inches at the edges and 12 inches over the supports between, boards face-nailed at every rafter with two 8d nails for a 1x6 or 1x8, three for anything wider.