Understanding Beam Span Factors
A wood beam's ability to span a distance depends on several factors: its species, its size, the number of boards laminated together (plies), and the load it must support. [16] A larger beam made of a stronger wood species can naturally span a greater distance.
The load on a beam is often determined by its "Tributary Width." This is half the distance to the next support on either side of the beam. [3] A larger tributary width means the beam is supporting a larger floor area and therefore can span a shorter distance.
Key Factors & How They Affect Span
| Factor | Effect on Span |
|---|---|
| Wood Species | Stronger species like Southern Yellow Pine can span longer distances than species like Douglas Fir or Cedar. [7, 13] |
| Beam Size (Depth) | Increasing the depth of a beam (e.g., from a 2x8 to a 2x12) dramatically increases its strength and allowable span. [16] |
| Number of Plies | Adding more plies (e.g., going from a 2-ply to a 3-ply beam) increases the beam's width and strength, allowing for a longer span. [4] |
| Tributary Width | Increasing the tributary width increases the load on the beam, which significantly *decreases* its maximum allowable span. [3, 14] |
How to Use the Calculator
- Select the species of lumber you plan to use.
- Choose the nominal size of the individual boards (e.g., 2x10).
- Select the number of plies (boards) that will be fastened together to make the built-up beam. [4]
- Choose the tributary width—the total width of the floor area that the beam will support.
- Click "Calculate Max Span" to see the result based on common building code load requirements (40 psf live load, 10 psf dead load). [8]
Frequently Asked Questions
What is a "built-up" beam?
A built-up beam is created by fastening multiple pieces of dimensional lumber (like 2x10s) together side-by-side. This creates a much stronger beam than a single piece of lumber of the same size. [4]
Is this calculator a substitute for a structural engineer?
No. This calculator provides a simplified estimate for initial planning based on standard assumptions. [1] For any load-bearing structure, especially with complex loads or long spans, you must consult a qualified structural engineer and adhere to your local building codes.