Joist Span Table
Span is the distance between supports, never the board length.
Compiled by DIY Repair Guide · reviewed 2026-09-05 ·how we research these
A span table answers one question: how far can this joist reach between supports before it is no longer safe. The tables are published by the American Wood Council and adopted into the building codes, and they are the reference an inspector will use.
Two things trip people up. <strong>Span is measured clear between bearing points</strong> rather than the length of the board you bought. A 16-foot joist resting on walls at both ends has a span slightly under 16 feet. And a joist that satisfies the table can still produce a floor that bounces noticeably, because the deflection limit code uses is about safety and finish cracking, not comfort.
How to Read a Span Table
Three inputs determine the answer: the size of the joist, the spacing between joists, and the species and grade of the lumber. Change any one and the allowable span changes.
Spacing is measured on center, from the center of one joist to the center of the next rather than the gap between them. 16 inches on center is the residential default. 12 inches on center buys you more span, and 24 inches on center gives you less.
Floor Joists. Residential Living Areas
These figures are for a 40 psf live load with 10 psf dead load, the standard for living areas, at a deflection limit of L/360. Southern Pine No. 2, a common structural grade.
- Sleeping rooms use a lower 30 psf live load, which allows slightly longer spans. Most builders use the 40 psf figure throughout for simplicity.
- Species matters considerably. Douglas Fir-Larch is comparable to Southern Pine. Spruce-Pine-Fir and Hem-Fir are weaker and give shorter spans, often by a foot or more at 2×10.
- Grade matters too. No. 1 gives more span than No. 2. Stud and Utility grades are not intended for joists at all.
| Joist size | 12" o.c. | 16" o.c. | 19.2" o.c. | 24" o.c. |
|---|---|---|---|---|
| 2×6 | 10' 9" | 9' 9" | 9' 2" | 8' 6" |
| 2×8 | 14' 2" | 12' 10" | 12' 1" | 11' 0" |
| 2×10 | 18' 0" | 16' 1" | 15' 2" | 13' 5" |
| 2×12 | 21' 9" | 18' 10" | 17' 2" | 15' 4" |
Deck Joists Are Different
Decks carry a higher live load, 40 psf live plus 10 psf dead is standard, but the exposure and the requirement to use treated lumber change the practical numbers, and many jurisdictions apply stricter deck provisions after a run of collapse incidents.
Deck spans are typically shorter than interior floor spans for the same lumber, and the guard, ledger and footing details matter far more than the joist span for safety. The ledger connection to the house is the single most common failure point in deck collapses, ahead of the joists themselves.
| Joist size | 12" o.c. | 16" o.c. | 24" o.c. |
|---|---|---|---|
| 2×6 | 9' 11" | 9' 0" | 7' 7" |
| 2×8 | 13' 1" | 11' 10" | 9' 8" |
| 2×10 | 16' 2" | 14' 0" | 11' 5" |
| 2×12 | 18' 0" | 16' 6" | 13' 6" |
Code Minimum Versus a Floor That Feels Solid
The deflection limit in the tables is L/360. Over a 16-foot span, that permits about half an inch of sag under full design load. That is enough to protect the structure and to stop plaster and tile cracking. It is not enough to stop a floor feeling springy when someone walks across it.
If bounce matters to you, and it does under tile, stone, or in a room with a piano, design to L/480 or better. In practice that means going up a joist size, tightening spacing from 16 to 12 inches on center, or adding a mid-span beam. Any of the three costs far less at framing stage than living with it afterwards.
A floor that meets code and still bounces has not been built wrong. It has been built to a limit that was never about comfort. If you are specifying a floor rather than inspecting one, ask for L/480.
What Voids the Table
Span tables assume a simple, unmodified joist carrying a uniform load. Several common situations break that assumption.
- Notches and holes. Codes restrict where and how much you may cut. Broadly: no notches in the middle third of the span, holes no larger than one third of the joist depth and kept at least two inches from the edges. A plumber who has notched the bottom edge of a joist at mid-span has substantially weakened it.
- Point loads. A wall, a beam bearing down mid-span, a bath full of water, or a kitchen island puts a concentrated load the table does not account for.
- Cantilevers. Joists extending past a support have their own rules and are not covered by simple span tables.
- Wet or exposed conditions. Lumber used where it will get damp requires treated material and reduced values.
- Engineered lumber. I-joists and laminated veneer lumber (LVL) do not use dimensional lumber tables at all. Use the manufacturer's published spans, which are usually considerably longer.
Never cut, notch or drill an existing joist to route a pipe or duct without checking the limits first, and never do it to an engineered I-joist without the manufacturer's hole chart. Cutting the top or bottom flange of an I-joist can destroy its capacity entirely.
Working Out What You Have
To use the tables on an existing floor, you need three measurements from underneath.
- Actual joist dimensions. Remember nominal sizes are not real sizes. A 2×10 measures about 1½ by 9¼ inches. Measure rather than assume.
- Spacing on center. Measure from the center of one joist to the center of the next, over several joists, since spacing is not always consistent in older houses.
- Clear span. The distance between the inside faces of the supports the joist bears on, including any mid-span beam or bearing wall, which is easy to overlook and dramatically changes the answer.
Common questions
How far can a 2x10 floor joist span?
Roughly 16 feet 1 inch at 16 inches on center in Southern Pine No. 2, under a 40 psf live load at L/360. Weaker species such as Spruce-Pine-Fir give noticeably less, so confirm the species stamp before relying on a figure.
Is span the length of the joist?
No. Span is the clear distance between the supports the joist bears on. A board resting on walls at both ends has a span slightly shorter than its overall length, and a mid-span beam divides one long joist into two much shorter spans.
What does 16 inches on center mean?
The distance from the center of one joist to the center of the next is 16 inches, measured center to center rather than across the gap. It is the standard residential spacing, and it is why a tape measure carries red marks every 16 inches.
Can I drill a hole through a floor joist?
Within limits. Generally the hole must be no more than one third of the joist depth, kept at least two inches from the top and bottom edges, and away from notches. Engineered I-joists have their own hole charts and cutting a flange can destroy the joist.
Why does my floor bounce if it meets code?
Because the code deflection limit of L/360 is about structural safety and finish cracking, not comfort. A floor can be entirely compliant and still feel springy. Designing to L/480 or better is what removes the bounce.