Recommended Beam
2-ply 2x12
Total Load
4000 lbs
Req. Modulus
60.0 in³
Safety
1.05x
2-ply 2x12
4000 lbs
60.0
1.05x
| Size | S (in³) | Safety | OK? |
|---|---|---|---|
| 2-ply 2x12← rec. | 63.3 | 1.05x |
Required section modulus: 60.0 in³
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Result
Load per foot = 8 × 50 = 400 lbs/ft. Required S = (400 × 10² × 12) / (8 × 1000) = 60.0 in³. A 2-ply 2×12 has S = (3.0 × 11.25²) / 6 = 63.3 in³. Safety factor = 63.3 / 60.0 = 1.06x.
Inputs
Result
Load per foot = 10 × 50 = 500 lbs/ft. Required S = (500 × 16² × 12) / (8 × 2600) = 73.8 in³. A 2-ply LVL 1.75×11.875 has S = (3.5 × 11.875²) / 6 = 82.3 in³. Safety factor = 82.3 / 73.8 = 1.11x.
For a 10 ft span with 8 ft tributary width and 50 psf total load, a 2-ply 2×12 Douglas Fir beam (S=63.3 in³) handles the required section modulus of 60.0 in³ with a 1.06x safety factor.
| Span Length | 2-ply 2×10 (S) | 2-ply 2×12 (S) | Required S (8 ft trib, 50 psf, DF) |
|---|---|---|---|
| 8 ft | 42.7 in³ | 63.3 in³ | 38.4 in³ |
| 10 ft | 42.7 in³ | 63.3 in³ | 60.0 in³ |
| 12 ft | 42.7 in³ | 63.3 in³ | 86.4 in³ |
| 14 ft | 42.7 in³ | 63.3 in³ | 117.6 in³ |
Tributary width is the distance from the beam to the midpoint between adjacent supports on each side. If joists span 24 ft with a beam in the middle, the tributary width is 12 ft (half the joist span from each side).
Section modulus (S) measures a beam's resistance to bending. It depends on the beam's cross-section dimensions: S = (width × depth²) / 6. The required S must not exceed the beam's actual S for safe design.
| Beam Size | 1-ply S (in³) | 2-ply S (in³) | 3-ply S (in³) |
|---|---|---|---|
| 2×6 | 7.6 | 15.1 | 22.7 |
| 2×8 | 13.1 | 26.3 | 39.4 |
| 2×10 | 21.4 | 42.7 | 64.1 |
| 2×12 | 31.6 | 63.3 | 94.9 |
LVL (Laminated Veneer Lumber) has an Fb of 2,600 psi vs 875-1,100 psi for solid lumber. Use LVL for long spans (14+ ft), heavy loads, or when dimensional lumber sizes become impractical (3-ply 2×12+).
| Property | Dimensional Lumber (DF) | LVL |
|---|---|---|
| Fb (bending stress) | 1,000 psi | 2,600 psi |
| Ply width | 1.5" | 1.75" |
| Max practical span | 12–14 ft | 20+ ft |
| Cost per board foot | $1.50–$3.00 | $4.00–$7.00 |
| Consistency | Varies (knots, grain) | Uniform engineered |
A safety factor of 1.0 means the beam exactly meets the requirement. Most engineers recommend at least 1.25x for residential construction. A factor below 1.0 means the beam is undersized and will fail.
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Last Updated: May 2, 2026
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