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Calculate floor, ceiling, and roof joist spacing with maximum allowable spans. Determine pass/fail for your span and spacing combination, calculate number of joists needed, and estimate total lumber requirements.
Distance the joist will span
Floor: 40, Roof: 20-30
Typical: 10-15 psf
For calculating joists per sq ft
Max span for 16" OC: 11.8 ft
Number of Joists
10
Board Feet Needed
108.75 BF
Total Load (psf)
50 psf
Spans are in feet and based on standard lumber grades. Spans may vary by local building code and load conditions.
| Joist Size | 12" OC SPF | 12" OC Doug Fir | 16" OC SPF | 16" OC Doug Fir | 24" OC SPF | 24" OC Doug Fir |
|---|---|---|---|---|---|---|
| 2x6 | 10.6 | 11.5 | 8.9 | 9.6 | 6.8 | 7.4 |
| 2x8 | 14 | 15.2 | 11.8 | 12.8 | 9.1 | 9.9 |
| 2x10 | 18 | 19.6 | 15.2 | 16.6 | 11.8 | 12.8 |
| 2x12 | 22 | 24 | 18.6 | 20.2 | 14.3 | 15.6 |
| 2x14 | 25.6 | 27.9 | 21.7 | 23.6 | 16.7 | 18.2 |
| Joist Size | 12" OC SPF | 12" OC Doug Fir | 16" OC SPF | 16" OC Doug Fir | 24" OC SPF | 24" OC Doug Fir |
|---|---|---|---|---|---|---|
| 2x6 | 14.2 | 15.4 | 11.9 | 12.9 | 9.1 | 9.9 |
| 2x8 | 18.8 | 20.4 | 15.8 | 17.1 | 12.2 | 13.2 |
| 2x10 | 24 | 26 | 20.2 | 21.9 | 15.6 | 16.9 |
| 2x12 | 29.5 | 32 | 24.8 | 27 | 19.1 | 20.7 |
| 2x14 | 34.3 | 37.3 | 28.9 | 31.4 | 22.3 | 24.2 |
| Joist Size | 12" OC SPF | 12" OC Doug Fir | 16" OC SPF | 16" OC Doug Fir | 24" OC SPF | 24" OC Doug Fir |
|---|---|---|---|---|---|---|
| 2x6 | 11.8 | 12.8 | 9.9 | 10.8 | 7.6 | 8.3 |
| 2x8 | 15.6 | 16.9 | 13.1 | 14.2 | 10.1 | 11 |
| 2x10 | 19.9 | 21.6 | 16.8 | 18.3 | 13 | 14.1 |
| 2x12 | 24.3 | 26.5 | 20.5 | 22.3 | 15.8 | 17.2 |
| 2x14 | 28.3 | 30.8 | 23.9 | 26 | 18.4 | 20 |
Floor Joists
Live Load: 40 psf (residential)
Dead Load: 10-15 psf (self-weight)
Ceiling Joists
Live Load: 20 psf
Dead Load: 5-10 psf
Roof Joists
Live Load: 20-30 psf (snow)
Dead Load: 10-15 psf
Solid Blocking
Required for spans over 8 feet to prevent twisting and control deflection.
Cross Bridging
1x3 or 1x4 diagonal pieces between joists spaced 8 feet apart.
Installation
Install blocking at mid-span and between every joist for maximum support.
16" on center (OC) is the most common residential spacing. It provides a good balance between material cost and structural performance. 12" OC is used for heavier loads, while 24" OC is used for lighter loads or with larger joist sizes.
Use the span tables provided. Find your joist size, type, and spacing combination. The maximum span depends on the wood species, grade, and the loads the joist must support (live load and dead load).
On center spacing is measured from the center of one joist to the center of the next joist. For example, 16" OC means the distance from the center line of one joist to the center line of the adjacent joist is 16 inches.
Yes, sistering (installing a new joist beside an existing one and nailing them together) can increase span capacity. However, the connection must be properly nailed or bolted according to building code requirements.
Floor joists must support heavier live loads (40 psf for residential), while ceiling joists only support the ceiling material (20 psf). This means ceiling joists can typically span farther with the same size lumber.
Larger joist sizes allow greater spans. For example, a 2x12 can span almost twice as far as a 2x6 at the same spacing. The span increases with the depth of the joist (the vertical dimension).
Yes, different wood species and grades have different strength ratings. Douglas Fir and Southern Pine are stronger and allow longer spans than Spruce-Pine-Fir. Always check local building codes for approved wood species.
Dead load is the permanent weight of the structure itself (floor, joists, ceiling materials). Live load is temporary weight (people, furniture, snow). Total load is the sum of both, and joists must be sized to handle the combined load safely.
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