Plywood and OSB Thickness: What the Fraction on the Sheet Really Means

The fraction on the stamp is not a measurement

Every other chart on this subject is built on a subtraction. Three quarters is really 23/32. A half is really 15/32. Take a 32nd off and you have the truth.

That was a fair description of the world before 2010. It is not one now, and the document that changed it says so in plain language. Panels are labeled with a Performance Category, which PS 1-19 defines as "a panel designation related to the panel thickness range". A range. Not a thickness, not a thickness minus a sanding allowance — the name of a band that a sheet has to land inside.

The code went along with it. IRC section R503.2.1 states that the Performance Category value "shall be used as the 'nominal panel thickness' or 'panel thickness' wherever referenced in this code." So when a code table asks for a 23/32 panel, it is asking for a panel from that category, and the actual sheet under your knee could be anywhere in the band.

The first table above is that band, for every category in both standards. It was not transcribed. It was regenerated from the two tolerance rules the standards state and then checked against all 32 published cells, because a table you retype is a table you can retype wrong.

The ranges overlap, and it changes the answer to the most-asked question

Here is the part that no reprint carries, and it falls straight out of the table.

23/32 CAT is allowed to run from 0.688 to 0.750 inches. 3/4 CAT is allowed to run from 0.719 to 0.781. Those two bands share 0.031 of an inch — a full 32nd — of common ground. A 23/32 sheet at the top of its category measures 0.750. A 3/4 sheet at the bottom of its measures 0.719.

So a sheet marked 23/32 can be thicker than a sheet marked 3/4, and both are perfectly compliant.

It is not a quirk of that one pair, and it is not universal either — it depends on the step between the two categories. Seven pairs in the standard sit exactly a 32nd of an inch apart, and every one of them overlaps by a full 32nd: 5/16 and 11/32, 11/32 and 3/8, 7/16 and 15/32, 15/32 and 1/2, 9/16 and 19/32, 19/32 and 5/8, and 23/32 and 3/4. The reason is arithmetic rather than policy: a tolerance of plus and minus a 32nd makes each band a 16th of an inch wide, so two bands a 32nd apart cannot help but share half their width.

Those seven pairs are, unhelpfully, exactly the pairs anyone ever compares. Where the step is a 16th instead — 3/8 and 7/16, 1/2 and 9/16, 3/4 and 13/16 — the bands meet edge to edge without overlapping, and between 5/8 and 23/32 there is a genuine 32nd of daylight where no category lives at all.

Which means the honest answer to "which is thicker, 23/32 or 3/4?" is: on average the 3/4, but not reliably, and if a 32nd of an inch decides whether your countertop sits flat or your door bottoms out, put a caliper on the actual sheet. The stamp cannot tell you.

Where the sanding story goes wrong

The folk explanation for why sheathing comes in 32nds is that the mill sands a 32nd off a half-inch panel. It is a tidy story and the standard contradicts it twice.

First, 15/32 and 1/2 are two separate categories, both listed, both legitimate, and a mill qualifies its product to whichever one it makes. Neither is a degraded version of the other.

Second, look at what sanding actually does to the numbers. Sanded plywood gets a tighter tolerance, plus or minus 1/64 instead of 1/32, wrapped around the same nominal. The thinnest legal unsanded 15/32 panel is 0.438. The thinnest legal sanded one is 0.453 — a 64th higher, not lower. Sanding buys precision, not thinness.

What is true, and is all we will claim, is that structural sheathing is largely sold in the 32nd categories while sanded goods tend to come in the friendly fractions. Why an individual mill picks one over the other is a manufacturing decision, and we are not going to invent a reason for it.

What the span rating actually promises

Thickness is the wrong question anyway. The number on that stamp that governs whether the panel is legal on your building is the span rating, and it is the one thing on the sheet that no chart in the search results explains.

APA's own definition: on rated sheathing it appears as two numbers with a slash, and "the left-hand number denotes the maximum recommended spacing of supports when the panel is used for roof sheathing" while "the right-hand number indicates the maximum recommended spacing of supports when the panel is used for subflooring" — in both cases with the long dimension of the panel running across three or more supports. A panel marked 32/16 goes over rafters up to 32 inches apart, or joists up to 16.

That is why the third table above is arranged backwards from every other chart. You do not walk into a yard knowing you want 19/32. You walk in knowing your joists are at 16 or your trusses are at 24, and the panel follows from that.

It also explains why 7/16 OSB is what you see on so many roofs. A 24/16 rating covers rafters at 24 inches on center with edge support or without it, so no clips, no blocking and no tongue-and-groove are needed to reach the spacing most trussed roofs are built at. Step up to 32-inch supports and the 32/16 panel gets you there only with edge support: 32 inches with, 28 without.

Why a span rating of 20 means 19.2 inches

The oddest line in the standard is also the most useful. PS 1-19 states that "as a matter of convention, a span rating of 20 is designated for spans of 19.2 inch."

Nineteen point two is not an arbitrary number. Ninety-six divided by five is 19.2, so it is one fifth of an 8-foot sheet, and it is the only spacing between 16 and 24 inches that divides a panel exactly. Frame at 19.2 and every sheet still breaks on a joist. Frame at 18 or 20 and it does not.

It turns up mostly with engineered joists, where it recovers material against 16 inches on center without the deflection you take at 24. If you have ever seen a tape measure with a small diamond every 19.2 inches, that is what the diamond is for.

Sized for Spacing, and the inch that goes missing

APA recommends a 1/8 inch gap at every panel end and edge. Panels take on moisture and grow; without the gap they buckle, and roofs are where the callbacks come from.

Now do the arithmetic that the gap forces. A true 48-inch panel plus a 1/8 inch gap gives a pitch of 48.125 inches. Every panel pushes the next joint another 1/8 inch past the 48-inch module the framing was laid out on. After four panels the joint is half an inch out. After eight, it is a full inch out — and a joist face is 1-1/2 inches wide, so the edge only has 3/4 of an inch either side of center before it is hanging over air. The eighth joint misses the joist.

That is what the "SIZED FOR SPACING" line in the stamp is for. Take 1/8 inch off the sheet, add the 1/8 gap back, and the pitch is exactly 48.000 inches. It never drifts, however long the run. The standards allow the room to do it: plywood may be up to 1/16 under its stated size and never over, OSB is plus or minus 1/16, and APA's guide describes square-edge rated panels as typically 48 by 96 with a plus zero, minus 1/8 tolerance.

The one to watch when you are ordering is tongue-and-groove Sturd-I-Floor. It is 48 inches wide on the pallet, but once the tongue is home it covers 47-1/2. Count sheets on 47-1/2 or you will be one short.

What the code allows, and what people build

Read the third table honestly and it says something uncomfortable: a 24/16 panel — 7/16 of an inch — is rated as a subfloor over joists at 16 inches on center. That is code-compliant.

Nobody builds it. Real floors go down at 19/32 to 23/32, and for good reason: those code numbers are limits, not targets, and the allowable live load at maximum span is the point where the assembly stops passing, not the point where it feels solid. Put anything rigid on top — tile, stone, large-format anything — and the finish is far less forgiving than the code, because deflection that a carpet ignores will crack a grout joint.

Where a table gives you a minimum, the minimum is the last panel that works, not the right one.

Where this stops

Three limits worth saying out loud.

The span table assumes panels continuous over two or more spans, strength axis across the supports, with a 10 psf dead load. A single sheet bridging one bay is outside it, and so is a heavy tile-and-mortar floor build-up.

The thickness bands are what the standards allow, not what your supplier stocks. A mill running consistently near the top of a category is as compliant as one running near the bottom.

And panel moisture matters more than the fractions do. Panels are shipped at up to 18 percent moisture content and the standards measure thickness at 9 percent. A sheet that has spent a wet week on site is thicker and wider than the day it left the mill, which is the same reason the 1/8 inch gap exists in the first place.