Guide · 7 min read
How to read a span table
A span table looks like a price list and behaves like a form. Every number in the grid is conditional on a set of choices printed around it - the member, the load case, the deflection limit, the species group, the grade and the spacing - and pulling a figure out without carrying those conditions along is the single most common way people end up with framing that does not match the table they think they used.
This guide walks through the anatomy of a span table, reads one cell end to end, and lists the misreadings worth checking for. Every example comes from the IRC 2018 tables reproduced verbatim elsewhere on this site. It is a preliminary-sizing reference, not a structural design - the code edition adopted where you build is set by your jurisdiction, so confirm anything you intend to construct with your local building official and a licensed design professional.
What "span" means before you read anything
The span in a span table is the clear span: the horizontal distance between the faces of the supports, not the overall length of the board and not the centre-to-centre distance between walls. A joist cut at 16 feet that bears three inches on a plate at each end has a clear span shorter than 16 feet, and the table is being asked about the clear distance.
For rafters there is a second wrinkle. The rafter tables publish the horizontal projection - the run measured flat, not the sloped length of the member you cut. On a pitched roof the rafter you cut is longer than the number in the table. Comparing your cut length against a published rafter span will make a perfectly adequate rafter look undersized.
The tables also assume repetitive members: three or more parallel members at the stated spacing, sharing load through the sheathing. A single isolated beam carrying the same nominal size is a different problem with a different answer.
The five keys printed around the grid
Before you look at a cell, you have to place yourself in the table. Five things identify which grid you should be reading and which row and column within it.
If you cannot answer all five, you are not ready to read a number. The most frequent failure is answering four of them and guessing the fifth - usually the grade, because the lumber is already stacked on site and nobody looked at the stamp.
- Member and load case. Floor joist, ceiling joist or rafter, and for which occupancy or snow load. This site publishes four: floor joists at 40 psf live load, floor joists at 30 psf live load, ceiling joists at 20 psf, and rafters at 30 psf ground snow.
- Live and dead load. Printed in pounds per square foot in the table heading. A 40 psf live load table and a 30 psf live load table are separate tables, not two columns of the same one, and the dead load assumption matters just as much.
- Deflection limit. L/360 for the floor tables here, L/240 for the ceiling joist table, L/180 for the rafter table. It is a serviceability criterion baked into every cell.
- Species group and grade. The grade stamp on the lumber tells you both. Douglas Fir-Larch, Hem-Fir, Spruce-Pine-Fir and Southern Pine each get their own block, and within it Select Structural, No. 1, No. 2 and No. 3 each get a row.
- Spacing. The on-center distance, given here as 12 in, 16 in, 19.2 in or 24 in. Wider spacing means each member carries more, so the published span shortens.
Reading one cell end to end
Take an ordinary case: a living-room floor framed with Spruce-Pine-Fir No. 2 2x10s at 16 in on center. Working through the five keys puts you in the 40 psf live load floor joist table, in the Spruce-Pine-Fir block, on the No. 2 row, in the 16 in column, at the 2x10 size - and the cell reads 15 ft 5 in.
That is the complete answer, and it is only an answer to that exact question. Change the species to Southern Pine, or the grade to No. 3, or the spacing to 24 in, and you are reading a different cell. Every per-combination page on this site exists to make that explicit: each one names all five keys in its heading, shows the whole grade column, and cites the source table underneath.
Spans on this site are written in a feet-and-inches form, so 15 ft 5 in is fifteen feet five inches, not fifteen point five feet. Printed code tables usually use a similar notation with a hyphen or a foot mark. Reading it as a decimal will overstate a span by up to eleven inches, which is more than enough to matter.
| What the table asks | This example |
|---|---|
| Member and load case | Floor joist, 40 psf live load |
| Dead load | 10 psf |
| Deflection limit | L/360 |
| Species group | Spruce-Pine-Fir |
| Grade | No. 2 |
| Nominal size | 2x10 |
| Spacing | 16 in on center |
| Published maximum clear span | 15 ft 5 in |
Blank cells, notes and footnotes
Not every cell holds a number. A blank in a published span table is a deliberate statement, and it does not mean the combination is unlimited or unavailable - it means the source printed a note there instead of a figure. In the rafter table reproduced here, some Select Structural, No. 1 and No. 2 cells are blank because the source gives a note rather than a span; in the ceiling joist table, a blank marks a span beyond the maximum the table shows.
Because the meaning differs from table to table, a blank always sends you back to the source rather than to an assumption. The table pages on this site render blanks as a dash and state what the source does at that cell in the grade note above the grid.
Footnotes carry real conditions too - bearing lengths, the assumption of dry service, restrictions on which grades are covered. They are not decoration, and the reason this site cites the exact publication, edition and table number on every page is so you can go and read them.
The misreadings worth checking for
Most span-table errors are not arithmetic. They are reading the right number from the wrong place, and they share a small number of shapes.
- Using overall board length instead of clear span between supports.
- Using the sloped length of a rafter instead of its horizontal projection.
- Reading a Select Structural row and buying No. 2 lumber, or reading No. 2 and receiving No. 3.
- Reading the sleeping-area table for a living area, or the other way round; the two differ throughout.
- Reading a ceiling joist span and framing a floor with it. Attic storage and a floored, occupied room are different load cases.
- Reading a spacing column that does not match what actually got laid out on site.
- Treating a table span as covering a point load, a bearing wall above, an opening, a cantilever or a notched member. None of those are in the table.
- Applying an indoor floor table to a deck. Deck framing is a wet-service condition sized from a different guide entirely.
- Using a span from one code edition while the jurisdiction has adopted another. Values do change between editions.
Editions, jurisdictions and what to do next
A span table belongs to a specific edition of a specific document. The values on this site are transcribed from the IRC 2018 tables and the American Wood Council's DCA6 deck guide, with the publisher, edition and table number cited on every page that shows a value. Later editions exist, and adoption varies by state, province and municipality - the edition that governs your project is the one your jurisdiction has adopted, which may be neither the newest nor the one reproduced here.
In practice that makes the workflow simple. Use the tables and tools here to arrive at a candidate size, spacing and species-grade combination quickly, then confirm that candidate against the code edition in force with your building department before ordering material. The span lookup tool returns a cell with its source cited; the size finder answers the reverse question of what size reaches a clear span you already have.
And keep the disclaimer in mind rather than treating it as boilerplate. Allowable spans depend on species, grade, spacing, live and dead load and the deflection limit together, and published tables cover ordinary repetitive framing only. Anything unusual - a point load, an opening, a long cantilever, a member doing double duty - is a design question for a licensed professional.
Frequently asked questions
Is the span in a span table the length of the board?
No. It is the clear span between the faces of the supports. A joist is cut longer than its clear span because it has to bear at each end, and a rafter is cut longer still because the table gives the horizontal projection rather than the sloped length. Measuring the board and comparing it to the table understates what you have.
What does a blank cell in a span table mean?
It means the source printed a note there instead of a figure, and what the note says depends on the table. In the ceiling joist table reproduced here a blank marks a span beyond the maximum shown; in the rafter table some cells are blank because the source gives a note for that size. Always go back to the cited source table rather than assuming.
Which table do I use for a bedroom floor?
Sleeping areas are covered by the 30 psf live load floor joist table, while living rooms, kitchens and halls use the 40 psf table. The 30 psf table publishes longer spans for the same lumber, so using it for a living area would overstate what the framing is allowed to do.
Can I use a span table if I do not know the grade of my lumber?
No. Species group and grade select the row, and the grade stamp is the only evidence of which row applies. Unstamped, reclaimed or mixed lumber has no published design values, and a building official will not accept a span table as justification for framing you cannot identify.
Do span table values change between code editions?
Yes, and the differences are real. Some species and grade rows changed materially between editions of the IRC, so a value quoted from one edition is not interchangeable with another. This site states the edition it transcribed on every page; the edition that governs your build is whichever one your jurisdiction has adopted.