What does 15 ft 7 in mean?
On the default — a 2×10 Douglas fir-larch No. 2 joist at 16 in apart — the maximum span is 15 ft 7 in, or 187 inches, or 15.58 ft. The span is the distance between the supports the joist can bridge without sagging past the code limit — it may span less, never more. The span is the 2021 International Residential Code’s published figure from Table R502.3.1(2) — it is a measurement from the code, not a calculation, and the lookup and the tables below never contradict each other.
Three things about the model are worth stating plainly.
First, the span is 15 ft 7 in and the joist is 2×10 — the span and the size are two outputs, never the same number. Second, the meaning is 15 ft 7 in of clear span for a 2×10 Douglas fir-larch No. 2 joist at 16 in — the distance the joist can bridge, not the length of the board. And third, the span is a maximum, never a target: build to less than the maximum, never to it — all listed in Accuracy & limitations.
The full chart — maximum span, No. 2 grade (2021 IRC Table R502.3.1(2))
2×6 joists, No. 2 grade — maximum span
| Species | 12 in apart | 16 in apart | 19.2 in apart | 24 in apart |
|---|
| Douglas fir-larch | 10 ft 9 in | 9 ft 9 in | 9 ft 2 in | 8 ft 3 in |
| Hem-fir | 10 ft 0 in | 9 ft 1 in | 8 ft 7 in | 7 ft 11 in |
| Southern pine | 10 ft 3 in | 9 ft 4 in | 8 ft 6 in | 7 ft 7 in |
| Spruce-pine-fir | 10 ft 3 in | 9 ft 4 in | 8 ft 9 in | 8 ft 1 in |
2×8 joists, No. 2 grade — maximum span
| Species | 12 in apart | 16 in apart | 19.2 in apart | 24 in apart |
|---|
| Douglas fir-larch | 14 ft 2 in | 12 ft 9 in | 11 ft 8 in | 10 ft 5 in |
| Hem-fir | 13 ft 2 in | 12 ft 0 in | 11 ft 3 in | 10 ft 2 in |
| Southern pine | 13 ft 6 in | 11 ft 10 in | 10 ft 10 in | 9 ft 8 in |
| Spruce-pine-fir | 13 ft 6 in | 12 ft 3 in | 11 ft 6 in | 10 ft 3 in |
2×10 joists, No. 2 grade — maximum span
| Species | 12 in apart | 16 in apart | 19.2 in apart | 24 in apart |
|---|
| Douglas fir-larch | 18 ft 0 in | 15 ft 7 in | 14 ft 3 in | 12 ft 9 in |
| Hem-fir | 16 ft 10 in | 15 ft 2 in | 13 ft 10 in | 12 ft 5 in |
| Southern pine | 16 ft 2 in | 14 ft 0 in | 12 ft 10 in | 11 ft 5 in |
| Spruce-pine-fir | 17 ft 3 in | 15 ft 5 in | 14 ft 1 in | 12 ft 7 in |
2×12 joists, No. 2 grade — maximum span
| Species | 12 in apart | 16 in apart | 19.2 in apart | 24 in apart |
|---|
| Douglas fir-larch | 20 ft 11 in | 18 ft 1 in | 16 ft 6 in | 14 ft 9 in |
| Hem-fir | 20 ft 4 in | 17 ft 7 in | 16 ft 1 in | 14 ft 4 in |
| Southern pine | 19 ft 1 in | 16 ft 6 in | 15 ft 1 in | 13 ft 6 in |
| Spruce-pine-fir | 20 ft 7 in | 17 ft 10 in | 16 ft 3 in | 14 ft 7 in |
Source: 2021 International Residential Code, Table R502.3.1(2) — Floor Joist Spans for Common Lumber Species (residential living areas, live load 40 psf, dead load 10 psf, L/Δ = 360). Select Structural, No. 1 and No. 3 grades are in the lookup above. Check the local code and the lumber supplier’s span tables before you build — the code table is a maximum, and local amendments can be stricter.
2×6 joists, No. 2 grade: Douglas fir-larch: 12 in — 10 ft 9 in; 16 in — 9 ft 9 in; 19.2 in — 9 ft 2 in; 24 in — 8 ft 3 in. Hem-fir: 12 in — 10 ft 0 in; 16 in — 9 ft 1 in; 19.2 in — 8 ft 7 in; 24 in — 7 ft 11 in. Southern pine: 12 in — 10 ft 3 in; 16 in — 9 ft 4 in; 19.2 in — 8 ft 6 in; 24 in — 7 ft 7 in. Spruce-pine-fir: 12 in — 10 ft 3 in; 16 in — 9 ft 4 in; 19.2 in — 8 ft 9 in; 24 in — 8 ft 1 in.
2×8 joists, No. 2 grade: Douglas fir-larch: 12 in — 14 ft 2 in; 16 in — 12 ft 9 in; 19.2 in — 11 ft 8 in; 24 in — 10 ft 5 in. Hem-fir: 12 in — 13 ft 2 in; 16 in — 12 ft 0 in; 19.2 in — 11 ft 3 in; 24 in — 10 ft 2 in. Southern pine: 12 in — 13 ft 6 in; 16 in — 11 ft 10 in; 19.2 in — 10 ft 10 in; 24 in — 9 ft 8 in. Spruce-pine-fir: 12 in — 13 ft 6 in; 16 in — 12 ft 3 in; 19.2 in — 11 ft 6 in; 24 in — 10 ft 3 in.
2×10 joists, No. 2 grade: Douglas fir-larch: 12 in — 18 ft 0 in; 16 in — 15 ft 7 in; 19.2 in — 14 ft 3 in; 24 in — 12 ft 9 in. Hem-fir: 12 in — 16 ft 10 in; 16 in — 15 ft 2 in; 19.2 in — 13 ft 10 in; 24 in — 12 ft 5 in. Southern pine: 12 in — 16 ft 2 in; 16 in — 14 ft 0 in; 19.2 in — 12 ft 10 in; 24 in — 11 ft 5 in. Spruce-pine-fir: 12 in — 17 ft 3 in; 16 in — 15 ft 5 in; 19.2 in — 14 ft 1 in; 24 in — 12 ft 7 in.
2×12 joists, No. 2 grade: Douglas fir-larch: 12 in — 20 ft 11 in; 16 in — 18 ft 1 in; 19.2 in — 16 ft 6 in; 24 in — 14 ft 9 in. Hem-fir: 12 in — 20 ft 4 in; 16 in — 17 ft 7 in; 19.2 in — 16 ft 1 in; 24 in — 14 ft 4 in. Southern pine: 12 in — 19 ft 1 in; 16 in — 16 ft 6 in; 19.2 in — 15 ft 1 in; 24 in — 13 ft 6 in. Spruce-pine-fir: 12 in — 20 ft 7 in; 16 in — 17 ft 10 in; 19.2 in — 16 ft 3 in; 24 in — 14 ft 7 in.
Adjustment notes — grade, spacing, load, species
Grade. The grade is stamped on the board: Select Structural spans furthest, then No. 1, then No. 2, then No. 3. A 2×10 Douglas fir-larch at 16 in spans 17 ft 4 in as Select Structural, 16 ft 5 in as No. 1, 15 ft 7 in as No. 2 and 11 ft 11 in as No. 3 — the grade alone is worth five feet.
Spacing. Closer joists span further: the same 2×10 Douglas fir-larch No. 2 spans 18 ft 0 in at 12 in, 15 ft 7 in at 16 in, 14 ft 3 in at 19.2 in and 12 ft 9 in at 24 in. Spacing the joists closer is the cheapest way to span further without a bigger board.
Load. This table is for living areas at 40 psf live load and 10 psf dead load. Sleeping areas use 30 psf live load and span further — Table R502.3.1(1) — and a floor that carries a bath, a waterbed or a heavy safe needs an engineer, not this table.
Species. The species is stamped on the board with the grade: Douglas fir-larch spans furthest, then Spruce-pine-fir, then Hem-fir, then Southern pine at the same size and grade — a 2×10 No. 2 at 16 in spans 15 ft 7 in, 15 ft 5 in, 15 ft 2 in and 14 ft 0 in in that order.
The worked example, line by line
Setup: joist size 2×10; species Douglas fir-larch; grade No. 2; spacing 16 in.
Line 1 — the table. Table R502.3.1(2) gives a 2×10 Douglas fir-larch No. 2 joist at 16 in a maximum span of 15 ft 7 in.
Line 2 — the inches. 15 × 12 + 7 = 187 in.
Line 3 — the decimal feet. 187 ÷ 12 = 15.58 ft (unrounded 15.583333 ft).
Line 4 — the check. A 2×8 at the same spacing spans 12 ft 9 in — under 15 ft — and a 2×12 spans 18 ft 1 in.
Line 5 — the rule. The span is a maximum, never a target: the joist may span 15 ft 7 in, and a 15 ft 8 in span needs the next size up.
Every branch, checked
2×12 Douglas fir-larch No. 2 at 16 in: 2×12 Douglas fir-larch, No. 2 grade, 16 in apart: max span 18 ft 1 in (18.08 ft)
2×8 Douglas fir-larch No. 2 at 16 in: 2×8 Douglas fir-larch, No. 2 grade, 16 in apart: max span 12 ft 9 in (12.75 ft)
2×6 Douglas fir-larch No. 2 at 16 in: 2×6 Douglas fir-larch, No. 2 grade, 16 in apart: max span 9 ft 9 in (9.75 ft)
2×10 Southern pine No. 2 at 16 in: 2×10 Southern pine, No. 2 grade, 16 in apart: max span 14 ft 0 in (14.00 ft)
2×10 Hem-fir No. 2 at 16 in: 2×10 Hem-fir, No. 2 grade, 16 in apart: max span 15 ft 2 in (15.17 ft)
2×10 Spruce-pine-fir No. 2 at 16 in: 2×10 Spruce-pine-fir, No. 2 grade, 16 in apart: max span 15 ft 5 in (15.42 ft)
2×10 Douglas fir-larch No. 2 at 12 in: 2×10 Douglas fir-larch, No. 2 grade, 12 in apart: max span 18 ft 0 in (18.00 ft)
2×10 Douglas fir-larch No. 2 at 24 in: 2×10 Douglas fir-larch, No. 2 grade, 24 in apart: max span 12 ft 9 in (12.75 ft)
2×10 Douglas fir-larch Select Structural at 16 in: 2×10 Douglas fir-larch, Select Structural grade, 16 in apart: max span 17 ft 4 in (17.33 ft)
2×10 Douglas fir-larch No. 1 at 16 in: 2×10 Douglas fir-larch, No. 1 grade, 16 in apart: max span 16 ft 5 in (16.42 ft)
The reverses, worked backwards
Smallest joist size for a span of at least 15 ft: At the displayed 2×10 joist the figures are 2×10 Douglas fir-larch, No. 2 grade, 16 in apart: max span 15 ft 7 in (15.58 ft, unrounded 15.583333 ft). At 2×8 joist the figures are 2×8 Douglas fir-larch, No. 2 grade, 16 in apart: max span 12 ft 9 in (12.75 ft, unrounded 12.750000 ft) — the span is 12 ft 9 in — so 2×10 is the smallest joist size for a span of at least 15 ft.
Largest spacing for a span of at least 15 ft: At the displayed 16 in spacing the figures are 2×10 Douglas fir-larch, No. 2 grade, 16 in apart: max span 15 ft 7 in (15.58 ft, unrounded 15.583333 ft). At 19.2 in spacing the figures are 2×10 Douglas fir-larch, No. 2 grade, 19.2 in apart: max span 14 ft 3 in (14.25 ft, unrounded 14.250000 ft) — the span is 14 ft 3 in — so 16 in is the largest spacing for a span of at least 15 ft.
Lowest grade for a span of at least 16 ft: At the displayed No. 1 grade the figures are 2×10 Douglas fir-larch, No. 1 grade, 16 in apart: max span 16 ft 5 in (16.42 ft, unrounded 16.416667 ft). At No. 2 grade the figures are 2×10 Douglas fir-larch, No. 2 grade, 16 in apart: max span 15 ft 7 in (15.58 ft, unrounded 15.583333 ft) — the span is 15 ft 7 in — so No. 1 is the lowest grade for a span of at least 16 ft.
Largest spacing for a span of at least 10 ft: At the displayed 19.2 in spacing the figures are 2×8 Southern pine, No. 2 grade, 19.2 in apart: max span 10 ft 10 in (10.83 ft, unrounded 10.833333 ft). At 24 in spacing the figures are 2×8 Southern pine, No. 2 grade, 24 in apart: max span 9 ft 8 in (9.67 ft, unrounded 9.666667 ft) — the span is 9 ft 8 in — so 19.2 in is the largest spacing for a span of at least 10 ft.
Smallest joist size for a span of at least 12 ft: At the displayed 2×8 joist the figures are 2×8 Spruce-pine-fir, No. 2 grade, 12 in apart: max span 13 ft 6 in (13.50 ft, unrounded 13.500000 ft). At 2×6 joist the figures are 2×6 Spruce-pine-fir, No. 2 grade, 12 in apart: max span 10 ft 3 in (10.25 ft, unrounded 10.250000 ft) — the span is 10 ft 3 in — so 2×8 is the smallest joist size for a span of at least 12 ft.
Lowest grade for a span of at least 13 ft: At the displayed No. 1 grade the figures are 2×10 Hem-fir, No. 1 grade, 24 in apart: max span 13 ft 3 in (13.25 ft, unrounded 13.250000 ft). At No. 2 grade the figures are 2×10 Hem-fir, No. 2 grade, 24 in apart: max span 12 ft 5 in (12.42 ft, unrounded 12.416667 ft) — the span is 12 ft 5 in — so No. 1 is the lowest grade for a span of at least 13 ft.
A warning about the neighbours: every neighbour above sits one joist size, one grade or one spacing from the star, and every neighbour figure is a different span even though the joist barely changed — the table, not the display, decides. The page prints the unrounded span in decimal feet in full so the rounding does not mislead you: R1 2×10 / 2×8; R2 16 / 19.2 in; R3 No. 1 / No. 2; R4 19.2 / 24 in; R5 2×8 / 2×6; R6 No. 1 / No. 2.
What the defaults assume
The defaults are a 2×10 Douglas fir-larch No. 2 joist at 16 in apart — 2×10 Douglas fir-larch, No. 2 grade, 16 in apart: max span 15 ft 7 in (15.58 ft). With 2×12 Douglas fir-larch No. 2 at 16 in, 2×12 Douglas fir-larch, No. 2 grade, 16 in apart: max span 18 ft 1 in (18.08 ft). With 2×8 Douglas fir-larch No. 2 at 16 in, 2×8 Douglas fir-larch, No. 2 grade, 16 in apart: max span 12 ft 9 in (12.75 ft). With 2×6 Douglas fir-larch No. 2 at 16 in, 2×6 Douglas fir-larch, No. 2 grade, 16 in apart: max span 9 ft 9 in (9.75 ft). With 2×10 Southern pine No. 2 at 16 in, 2×10 Southern pine, No. 2 grade, 16 in apart: max span 14 ft 0 in (14.00 ft). With 2×10 Hem-fir No. 2 at 16 in, 2×10 Hem-fir, No. 2 grade, 16 in apart: max span 15 ft 2 in (15.17 ft). With 2×10 Spruce-pine-fir No. 2 at 16 in, 2×10 Spruce-pine-fir, No. 2 grade, 16 in apart: max span 15 ft 5 in (15.42 ft). With 2×10 Douglas fir-larch No. 2 at 12 in, 2×10 Douglas fir-larch, No. 2 grade, 12 in apart: max span 18 ft 0 in (18.00 ft). With 2×10 Douglas fir-larch No. 2 at 24 in, 2×10 Douglas fir-larch, No. 2 grade, 24 in apart: max span 12 ft 9 in (12.75 ft). With 2×10 Douglas fir-larch Select Structural at 16 in, 2×10 Douglas fir-larch, Select Structural grade, 16 in apart: max span 17 ft 4 in (17.33 ft). With 2×10 Douglas fir-larch No. 1 at 16 in, 2×10 Douglas fir-larch, No. 1 grade, 16 in apart: max span 16 ft 5 in (16.42 ft).
These notes repeat in every explainer section because this page is built to be read start to finish: the defaults are one joist’s numbers, not a standard; the span is the code table’s figure, not a calculation; the span is a maximum, never a target; and the local code comes first.
Output · formula · meaning
| Output | Formula | What it means |
|---|
| Joist size | input | 2×10. |
| Species | input | Douglas fir-larch. |
| Grade | input | No. 2. |
| Spacing | input | 16 in. |
| Maximum span | 2021 IRC Table R502.3.1(2), live 40 psf / dead 10 psf | 15 ft 7 in. |
| Span in inches | feet × 12 + inches | 187 in. |
| Span in feet | inches ÷ 12 | 15.58 ft. |
| Verdict | the span for your joist | 2×10 Douglas fir-larch, No. 2 grade, 16 in apart: max span 15 ft 7 in (15.58 ft). |
| Reverse: Smallest joist size for a span of at least 15 ft | step | 2×10 joist: 2×10 Douglas fir-larch, No. 2 grade, 16 in apart: max span 15 ft 7 in (15.58 ft, unrounded 15.583333 ft). |
| Reverse: Largest spacing for a span of at least 15 ft | step | 16 in spacing: 2×10 Douglas fir-larch, No. 2 grade, 16 in apart: max span 15 ft 7 in (15.58 ft, unrounded 15.583333 ft). |
| Reverse: Lowest grade for a span of at least 16 ft | step | No. 1 grade: 2×10 Douglas fir-larch, No. 1 grade, 16 in apart: max span 16 ft 5 in (16.42 ft, unrounded 16.416667 ft). |
| Reverse: Largest spacing for a span of at least 10 ft | step | 19.2 in spacing: 2×8 Southern pine, No. 2 grade, 19.2 in apart: max span 10 ft 10 in (10.83 ft, unrounded 10.833333 ft). |
| Reverse: Smallest joist size for a span of at least 12 ft | step | 2×8 joist: 2×8 Spruce-pine-fir, No. 2 grade, 12 in apart: max span 13 ft 6 in (13.50 ft, unrounded 13.500000 ft). |
| Reverse: Lowest grade for a span of at least 13 ft | step | No. 1 grade: 2×10 Hem-fir, No. 1 grade, 24 in apart: max span 13 ft 3 in (13.25 ft, unrounded 13.250000 ft). |
| 2×12 Douglas fir-larch No. 2 at 16 in | same model | 2×12 Douglas fir-larch, No. 2 grade, 16 in apart: max span 18 ft 1 in (18.08 ft). |
| 2×8 Douglas fir-larch No. 2 at 16 in | same model | 2×8 Douglas fir-larch, No. 2 grade, 16 in apart: max span 12 ft 9 in (12.75 ft). |
| 2×6 Douglas fir-larch No. 2 at 16 in | same model | 2×6 Douglas fir-larch, No. 2 grade, 16 in apart: max span 9 ft 9 in (9.75 ft). |
| 2×10 Southern pine No. 2 at 16 in | same model | 2×10 Southern pine, No. 2 grade, 16 in apart: max span 14 ft 0 in (14.00 ft). |
| 2×10 Hem-fir No. 2 at 16 in | same model | 2×10 Hem-fir, No. 2 grade, 16 in apart: max span 15 ft 2 in (15.17 ft). |
| 2×10 Spruce-pine-fir No. 2 at 16 in | same model | 2×10 Spruce-pine-fir, No. 2 grade, 16 in apart: max span 15 ft 5 in (15.42 ft). |
| 2×10 Douglas fir-larch No. 2 at 12 in | same model | 2×10 Douglas fir-larch, No. 2 grade, 12 in apart: max span 18 ft 0 in (18.00 ft). |
| 2×10 Douglas fir-larch No. 2 at 24 in | same model | 2×10 Douglas fir-larch, No. 2 grade, 24 in apart: max span 12 ft 9 in (12.75 ft). |
| 2×10 Douglas fir-larch Select Structural at 16 in | same model | 2×10 Douglas fir-larch, Select Structural grade, 16 in apart: max span 17 ft 4 in (17.33 ft). |
| 2×10 Douglas fir-larch No. 1 at 16 in | same model | 2×10 Douglas fir-larch, No. 1 grade, 16 in apart: max span 16 ft 5 in (16.42 ft). |
| Joist size index 4 | same model | Joist size — pick one from the list.. |
| Spacing index 4 | same model | Spacing — pick one from the list.. |
| Grade index 4 | same model | Grade — pick one from the list.. |
| 2×6 Southern pine No. 3 at 24 in | same model | 2×6 Southern pine, No. 3 grade, 24 in apart: max span 5 ft 9 in (5.75 ft, unrounded 5.750000 ft). |
| 2×12 Douglas fir-larch Select Structural at 12 in | same model | 2×12 Douglas fir-larch, Select Structural grade, 12 in apart: max span 23 ft 3 in (23.25 ft, unrounded 23.250000 ft). |
| 2×6 Douglas fir-larch No. 2 at 24 in | same model | 2×6 Douglas fir-larch, No. 2 grade, 24 in apart: max span 8 ft 3 in (8.25 ft, unrounded 8.250000 ft). |
| 2×12 Spruce-pine-fir Select Structural at 16 in | same model | 2×12 Spruce-pine-fir, Select Structural grade, 16 in apart: max span 19 ft 6 in (19.50 ft, unrounded 19.500000 ft). |
| 2×8 Hem-fir No. 1 at 19.2 in | same model | 2×8 Hem-fir, No. 1 grade, 19.2 in apart: max span 11 ft 10 in (11.83 ft, unrounded 11.833333 ft). |
| 2×10 Southern pine No. 1 at 12 in | same model | 2×10 Southern pine, No. 1 grade, 12 in apart: max span 18 ft 0 in (18.00 ft, unrounded 18.000000 ft). |
| 2×12 Southern pine No. 2 at 19.2 in | same model | 2×12 Southern pine, No. 2 grade, 19.2 in apart: max span 15 ft 1 in (15.08 ft, unrounded 15.083333 ft). |
| 2×6 Spruce-pine-fir No. 2 at 16 in | same model | 2×6 Spruce-pine-fir, No. 2 grade, 16 in apart: max span 9 ft 4 in (9.33 ft, unrounded 9.333333 ft). |
| 2×12 Douglas fir-larch No. 3 at 24 in | same model | 2×12 Douglas fir-larch, No. 3 grade, 24 in apart: max span 11 ft 3 in (11.25 ft, unrounded 11.250000 ft). |
Who uses this
A homeowner adding a room who needs to know whether a 2×10 spans the 15 ft room, or whether the floor needs a 2×12.
A builder framing a floor who checks the joist size, species and grade against the code table before ordering lumber.
A DIY renovator who sistered a joist and wants to know what the repair actually spans.
A building inspector who checks a framed floor against the code table on site.
Accuracy & limitations
The span figures are the 2021 International Residential Code’s published table — Table R502.3.1(2), Floor Joist Spans for Common Lumber Species (residential living areas, live load 40 psf, dead load 10 psf, L/Δ = 360) — and every figure on this page is checked against that table. What the table cannot do is design your floor: it covers only the four species, four grades, four spacings and four sizes in the table, only living areas at 40 psf live load, and only joists that are properly blocked, bridged and bearing. A floor that carries a bath, a waterbed or a heavy safe, a joist that is notched or drilled, or a span in a high-seismic or high-wind area needs an engineer, not this table. And the span is a maximum, never a target: the joist may span 15 ft 7 in, and a 15 ft 8 in span needs the next size up — the page prints the unrounded span in decimal feet so the rounding does not mislead you. The local code and the lumber supplier’s span tables come first.
Cite this calculator
Maximum floor joist spans from the 2021 International Residential Code, Table R502.3.1(2) — Floor Joist Spans for Common Lumber Species (residential living areas, live load 40 psf, dead load 10 psf, L/Δ = 360): the maximum span of a floor joist by size, species, grade and spacing. On the defaults — 2×10 Douglas fir-larch No. 2 joist at 16 in — the maximum span is 15 ft 7 in (187 in, 15.58 ft). Smallest joist size for a span of at least 15 ft: 2×10. Largest spacing for a span of at least 15 ft: 16 in. Lowest grade for a span of at least 16 ft: No. 1. Largest spacing for a span of at least 10 ft: 19.2 in. Smallest joist size for a span of at least 12 ft: 2×8. Lowest grade for a span of at least 13 ft: No. 1. Source: Bright Side Kit, brightoolbox.com; span data: 2021 International Residential Code, Table R502.3.1(2).
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Frequently asked questions
How far can a 2x10 floor joist span?
A 2×10 Douglas fir-larch No. 2 joist at 16 in spans 15 ft 7 in under the 2021 IRC Table R502.3.1(2) — residential living areas, 40 psf live load, 10 psf dead load. At 12 in it spans 18 ft 0 in, and at 24 in it spans 12 ft 9 in.
What is the maximum span for a 2x12 floor joist?
A 2×12 Douglas fir-larch No. 2 joist at 16 in spans 18 ft 1 in, and a 2×12 Douglas fir-larch Select Structural joist at 12 in spans 23 ft 3 in — the largest span in the table. At 24 in the same No. 2 joist spans 14 ft 9 in.
Does joist spacing affect how far a joist can span?
Yes — closer joists span further. A 2×10 Douglas fir-larch No. 2 spans 18 ft 0 in at 12 in, 15 ft 7 in at 16 in, 14 ft 3 in at 19.2 in and 12 ft 9 in at 24 in. Spacing the joists closer is the cheapest way to span further without a bigger board.
What is the difference between No. 1 and No. 2 lumber?
The grade is the lumber’s strength: Select Structural is strongest, then No. 1, then No. 2, then No. 3. A 2×10 Douglas fir-larch at 16 in spans 17 ft 4 in as Select Structural, 16 ft 5 in as No. 1, 15 ft 7 in as No. 2 and 11 ft 11 in as No. 3.
Which wood species spans the furthest?
Douglas fir-larch spans furthest, then Spruce-pine-fir, then Hem-fir, then Southern pine at the same size and grade — a 2×10 No. 2 at 16 in spans 15 ft 7 in, 15 ft 5 in, 15 ft 2 in and 14 ft 0 in in that order.
Can I use this table for a bedroom floor?
Bedroom floors use 30 psf live load and span further — Table R502.3.1(1) of the IRC, not this one. This table is for living areas at 40 psf live load and 10 psf dead load.
Where do the span numbers come from?
The 2021 International Residential Code, Table R502.3.1(2) — Floor Joist Spans for Common Lumber Species (residential living areas, live load 40 psf, dead load 10 psf, L/Δ = 360). The spans are the code’s published figures, not a calculation.
What if my joist size is not in the chart?
Use the next size down — the span for the smaller joist is the safe figure — or check the American Wood Council span tables for your exact joist. The deck-joist-span-chart page covers deck joists, which use a different table.