Enter Deck and Material Details
Example Data Table
| Span (ft) | Tributary width (ft) | Dead load (psf) | Live load (psf) | Material | Deflection limit | Suggested beam |
|---|---|---|---|---|---|---|
| 10 | 6 | 10 | 40 | SPF #2 (typical) | L/360 | 2-2x10 (typical outcome) |
Formula Used
How to Use This Calculator
- Measure the clear span between beam supports in feet.
- Estimate tributary width carried by the beam in feet.
- Enter dead and live loads in psf for your deck use.
- Select a material preset, or override Fb, E, and Fv.
- Pick a deflection limit such as L/360 for comfort.
- Press Calculate to see stresses, deflection, and a suggested built-up size.
Professional Notes
Load assumptions for garden decks
Beam sizing begins with credible surface loads. Dead load covers joists, decking, fasteners, rails, and any permanent planters or pavers. Live load covers people and movable furniture. Many residential decks use 10 psf dead and 40 psf live, but heavy finishes, storage, or rooftop gardens can raise both. Use conservative values that match intended use and local requirements safely.
Tributary width and line load
The beam carries only the floor area that tributes to it. Tributary width is commonly half the joist span to each side of the beam, measured perpendicular to the beam. The calculator converts area load to line load using w = (DL + LL) × tributary width. Because w drives moment, shear, and deflection, confirm framing geometry and include cantilevered joist effects when they exist.
Bending check and required section modulus
For a simple span under uniform load, maximum bending moment occurs at midspan: M = wL²/8. Allowable bending stress Fb depends on species and grade; real designs also apply code adjustment factors. The tool computes required section modulus and scans common 2x depths and built-up plies. It reports the smallest option that keeps bending stress at or below Fb for quick comparison during layout planning.
Deflection control for comfort
Strength alone is not enough; excessive sag feels bouncy and can damage finishes. The calculator checks deflection with Δ = 5wL⁴/(384EI) and compares it to L/ratio. Typical ratios are L/240 for basic utility and L/360 or tighter for better comfort. Since deflection grows rapidly with span, reducing span or adding plies often improves serviceability more than small load changes.
Practical detailing and verification
Use the suggested beam as a starting point, then confirm bearing length at posts, lateral bracing, moisture exposure, notches, holes, and connection hardware. Built-up beams need proper fastening so plies share load, and posts should align under reactions. Always verify against local building requirements and a qualified professional before construction, especially for elevated decks or unusual garden loads.
FAQs
1) What does the suggested built-up beam mean?
It is the smallest combination of common 2x plies that meets bending, shear, and deflection checks for your inputs. It assumes the plies act together through proper fastening and bearing.
2) How do I estimate tributary width correctly?
Measure the joist span perpendicular to the beam. Use half the joist span on each side that frames into the beam. Add any additional area whose joists bear on the beam.
3) Which deflection limit should I choose?
Use L/240 for utility decks and L/360 for better comfort. If you have brittle finishes, large planters, or sensitive connections, choose a stricter limit such as L/480.
4) Can I use this for cantilevers or point loads?
This version assumes a simple span with a uniform load. Cantilevers, concentrated loads, and irregular support conditions can increase moment and deflection. Model those cases separately or consult an engineer.
5) Why do my results show no size found?
Your span and loading may exceed the search range of up to four plies of 2x12. Reduce span, lower tributary width, use stronger material, increase plies, or consider engineered beams.
6) Do the material presets match local codes?
Presets are typical values for quick screening only. Actual allowable stresses vary by grade stamp, moisture, treatment, duration factors, and local requirements. Use manufacturer data or code tables for final sizing.