Three checks set the maximum span
A beam is only as good as the worst of three checks: bending strength, shear near the supports, and deflection (sag). The span you can reach is whichever of the three runs out first — for most floors that is deflection, not strength.
- Bending. The beam must not overstress in the middle, where the moment peaks. Moment grows with the square of the span.
- Shear. Checked near the supports, where the vertical force is highest. Matters most on short, heavily loaded beams.
- Deflection. The sag must stay under a limit (L/360 or L/240) so ceilings and floors do not crack or feel bouncy.
Species and grade drive the strength
| Species | Bending Fb | Stiffness E | Use |
|---|---|---|---|
| Douglas Fir-Larch | ~1200 psi (#2) | 1.9–2.0M psi | Most common for beams |
| Southern Pine | high | 1.8–1.9M psi | High-strength, marine |
| Hem-Fir | medium | ~1.3M psi | Roofs, medium loads |
| Spruce-Pine-Fir | ~900–1000 psi | 1.2–1.3M psi | Light residential loads |
Grade steps down the values: Select Structural is highest, then #1, #2 (most homes), then #3. Higher grade means fewer knots and defects, so more bending strength and stiffness. A wet service condition (moisture over ~12%) knocks another 3 to 20% off.
Deflection limits by finish
| Limit | Applies to | 20-ft span allows |
|---|---|---|
| L/360 | Floors, plaster ceilings (live load) | 0.67 in sag |
| L/240 | Floors, total load | 1.0 in sag |
| L/180 | Roofs with no ceiling below | 1.33 in sag |
Floor load is usually 40 psf live plus 10 to 20 psf dead. Beyond about 16 feet, glulam or LVL is normally cheaper and stiffer than stacking multiple sawn boards.
Common questions
What size beam do I need for a 16-foot span?
It depends on the load and species. For a typical floor load of 60 psf in Douglas Fir-Larch #2, a 16-foot span usually needs a built-up 3-ply 2x12 or a glulam around 1.75x14. Lighter loads may allow a 2x14. Always confirm against a span table or an engineer for your local code.
What do L/360 and L/240 mean?
They are deflection limits, the maximum a beam may sag. L/360 is the span divided by 360, used for floors and plaster ceilings so finishes do not crack. L/240 is looser and allowed for total load on some floors. For a 20-foot span, L/360 is 0.67 inches and L/240 is 1 inch.
Is a wood beam limited by strength or by sag?
On most residential floors the deflection limit controls, not strength. Wood has plenty of bending strength in reserve, but its stiffness is limited, so a beam can be strong enough yet still sag too much. That is why you always check both bending and deflection.
Can I swap Spruce-Pine-Fir for Douglas Fir-Larch?
Yes, but you will usually need a deeper beam. SPF has a lower bending strength (about 900 to 1000 psi versus 1200 for DF-L #2), so for the same span and load it often has to go one depth larger, such as a 2x14 in place of a 2x12.
