EstimatorKitSubfloor, wall and roof sheathing
Sheathing by the course, not by division
For a floor, a wall or a roof plane this returns the sheets of plywood or OSB to buy, counted the way they are actually laid: so many panels to a course, so many courses, and the offcut left at the end of each row. It also reads the span rating back against your framing spacing and counts the clips or blocking that every joint landing between two joists has to have. Tongue-and-groove panels are counted at their real 47 1/2 in coverage, which is 31.7 sq ft rather than 32.
- Floor, wall and roof
- 6 span ratings
- T&G at 47 1/2 in
Stamped 48/24. The number that matters is on the panel, and it is a span rating rather than a thickness.
Same span rating either way — it changes the weight, not the table.
Covers 47.5 in, not 48 — the net coverage width printed on a tongue-and-groove Sturd-I-Floor panel: the tongue laps into the groove beside it, so 48 in of panel covers 47 1/2 in of deck.
Across means the long edge crosses the supports. That is the orientation the span rating is written for.
Panels to buy, laid out course by course
26 sheets
- Laid out, before the allowance
- 24
- Allowance for cuts
- +5%
- Every course cut from fresh
- 24
- Area divided alone would say
- 22
- Surface
- 672 sq ft
- Each panel covers
- 31.7 sq ft
- Weight on the truck
- 1,993 lb
How the courses fall
3 panels fill a course, 8 courses cover the surface. Cutting each course from fresh panels takes 24; carrying the piece left at the end of one course into the start of the next brings it to 24, and that is the figure above.
The run divides exactly into whole panels, so no course ends in a cut and every end joint already lands on framing. Stagger it by starting alternate courses with a panel cut in half, and put both halves in the deck. The final course overshoots the far edge by 3' 8" and gets ripped down to 3 1/2".
What the stamp says against 16 in framing
23/32 in stamped 48/24 is listed to 24 in of joist spacing. Your framing is at 16 in, which is inside that.
This is the published span-rating ladder reproduced for reference. It is not an approval: the stamp on the bunk you are buying and the code your inspector enforces are what decide, and anything carrying a point load or an engineered detail is a question for the designer.
The tongue is the edge support, which is why a T&G deck needs no blocking between joists and a square-edge one does. What it costs is coverage — 47 1/2 in a panel rather than 48 — and the bill only lands when that tips the course count. This run takes 8 courses, against 7 in square-edge board — the extra one is a 3 1/2" strip whose whole job is to close a gap the tongues opened.
Add 23 tubes of subfloor adhesive: 672 linear feet of bead down every joist and into every groove, at 30 ft a 10 oz cartridge, a working figure for a 1/4 in bead rather than a published one. The yield printed on the cartridge wins, and it moves with bead size and how many beads a joist gets — a 3/8 in bead roughly halves this, and a 28 oz sausage runs about 84 ft. Glue is what stops a floor squeaking — the nails alone do not.
Sheet weight at 40 lb per cubic foot — a working figure for oriented strand board, not a published standard, roughly a sixth heavier than plywood of the same thickness, varying with resin content and moisture. One 23/32 in sheet is 77 lb, so this order is 1 tons of panel to move.
| Thickness | Stamp | Roof, edges supported | Roof, unsupported | Subfloor | Sturd-I-Floor T&G |
|---|---|---|---|---|---|
| 3/8 in | 24/0 | 24 in | 20 in | not rated | — |
| 7/16 in | 24/16 | 24 in | 20 in | 16 in | — |
| 15/32 in | 32/16 | 32 in | 28 in | 16 in | — |
| 19/32 in | 40/20 | 40 in | 32 in | 20 in | 20 oc |
| 23/32 in | 48/24 | 48 in | 36 in | 24 in | 24 oc |
| 7/8 in | Sturd-I-Floor 32 oc | — | — | 32 in | 32 oc |
Rounded panel weights at the chosen density: 3/8 in 40 lb · 7/16 in 47 lb · 15/32 in 50 lb · 19/32 in 63 lb · 23/32 in 77 lb · 7/8 in 93 lb for a 4 by 8.
Why area divided by 32 is the wrong number
Panels go on in courses, and a course is finished by whatever is left of the last sheet in it. That is the whole reason a sheathing order cannot be a division. A 25 ft run takes four panels to a course because the fourth one covers a single foot, and the 7 ft it leaves behind is only useful if it is long enough to open the next course — which, at 4 ft or more, is exactly how a good layout buys its stagger for nothing. End joints have to break on different framing members from one row to the next, and the cheapest way to get that is to cut one panel in half and start alternate courses with a half, so both pieces of the cut sheet go into the deck. A calculator that hands back an area divided by 32 has told you the floor of the order and left you to discover the rest at the yard. The framing under it is a separate count: studs come out of the stud calculator and a sloped run out of the rafter length calculator, which is the number to put in the run field here rather than the footprint.
Thickness is the part people guess at, and there is nothing to guess: every wood structural panel carries a span rating, and it is the rating rather than the fraction that the code reads. A stamp like 40/20 means the panel is listed over roof supports at 40 in and floor joists at 20 in, and the roof half of it comes with a condition — the wider number assumes the long panel edges are supported, by a tongue, by clips or by blocking, and it drops by several inches when they are not. That is why the same 15/32 in sheet is fine on rafters at 24 in and marginal at 32. Two further things travel with the rating and get missed. It is written for the long dimension crossing the supports, so turning a panel to suit a room puts it on its weak axis with a number that no longer applies. And it is a published table, not an approval: the stamp on the bunk in front of you and the inspector reading your local code are what decide, and anything carrying a point load belongs with the designer. Nailing the panel down is its own schedule, with edge spacing that matters more than the nail does, and that lives on the nail size chart.
Two small numbers decide whether the deck stays flat. The first is the 1/8 in space APA asks for at panel ends and edges, which is also why a square-edge sheet is milled a shade under 4 by 8 — gapped properly, panel plus space adds back up to the 48 in framing module and the layout stays true across the building. Skip it and a deck that gets rained on before the roof is dry buckles at a joint, and the ridge telegraphs through the finish floor for the life of the house. The second is the half inch a tongue-and-groove panel gives up to the lap: it covers 47 1/2 in of deck, not 48, which is about a percent on the count and buys the edge support a square-edge floor has to block for. Glue completes it — a bead down every joist and into every groove is what stops a floor squeaking, and the nails on their own do not. Once the shell is closed in, the same 4 ft module runs on inside, but counted for joints rather than for span, on the drywall calculator, and the pitch that set the roof geometry in the first place is the same rise over run the rafter length calculator steps off.
Sheathing questions
How many sheets of plywood does a 24 by 32 ft subfloor take?
Twenty-four, and every way of counting agrees because both dimensions are exact multiples of the panel: six courses of four 8 ft sheets, 768 sq ft at 32 sq ft a sheet. That agreement is the exception. Shift the same deck to 25 by 33 and the three counts separate — 26 by division, 29 when the piece left at the end of a course is carried into the start of the next, and 35 if every course is cut from fresh panels — because a course that overruns the last full sheet by a foot still costs a whole sheet. The 29 is the honest order and the 26 is the floor it can never go below.
What thickness of subfloor goes over joists at 16 in on center?
Any panel whose span rating carries a second number of 16 or better, which starts at 7/16 in stamped 24/16 and is more often 19/32 or 23/32 in practice. The rating is what the code reads, not the fraction: a panel is tested and stamped as an assembly of veneers or strands, so two sheets of the same thickness from different mills can carry different ratings. Most builders go two steps above the minimum on a floor because deflection between joists is felt underfoot long before anything is structurally in question, and because tile and stone set over a bouncy deck crack at the grout line.
Does tongue-and-groove subfloor really cover less than 32 square feet?
Yes — the tongue laps into the groove of the panel beside it, so a T&G sheet covers 47 1/2 in of deck rather than 48. That is 31.7 sq ft against 32, about one panel in every ninety, and it is invisible in a calculator that multiplies sheets by 32 until the last course comes up short and somebody drives back to the yard for one sheet. What the lap buys is edge support along the long joints, which is exactly the thing a square-edge deck has to add blocking to get.
What are H-clips, and does a roof deck need them?
They are small metal clips that slide onto the long edge of a roof panel and hold it level with the panel above, one in every rafter bay along the joint. Whether they are needed comes off the span rating, which lists two roof numbers: the wider one applies when panel edges are supported, the narrower one when they are not. A 15/32 in panel stamped 32/16 covers rafters at 32 in with support and around 28 without, by APA's roof-span table, so on rafters at 24 in you may skip the clips and on rafters at 32 in you may not. Blocking cut between the rafters does the same job and gives you something solid to nail the edge to.
Is OSB as good as plywood for sheathing?
For span and stiffness they are interchangeable, because both are rated by the same stamp and neither is listed by species. They part company at the edges and in the rain: OSB swells more at a cut edge once it is wet and does not fully come back, so a deck that is going to sit open through a wet season is worth building in plywood, while a roof that gets dried in the same week is not. By the working densities this page uses, OSB is also about a sixth heavier at the same thickness, which is 77 lb against 65 for a 23/32 in sheet — noticeable on the twentieth one up a ladder.