Formula used
The calculator converts deck area loads into a beam line load. It applies selected material adjustments and section properties. It then checks bending, shear, deflection, bearing, and cantilever limits.
| Check | Formula | Purpose |
|---|---|---|
| Line load | w = q × tributary width | Converts area loading into beam loading. |
| Maximum moment | M = wL² ÷ 8 + PL ÷ 4 | Estimates simple-span bending demand. |
| Maximum shear | V = wL ÷ 2 + P ÷ 2 | Estimates end-support shear demand. |
| Uniform deflection | Δ = 5wL⁴ ÷ 384EI | Checks serviceability under distributed loading. |
| Point-load deflection | Δ = PL³ ÷ 48EI | Adds midspan concentrated-load movement. |
| Bending stress | fb = M ÷ S | Compares demand with adjusted bending capacity. |
| Shear stress | fv = 1.5V ÷ bd | Compares demand with adjusted shear capacity. |
| Bearing length | Lb = R ÷ bFc⊥ | Estimates support contact length. |
How to use this calculator
- Select whether to size, check, compare, or solve for span.
- Choose imperial or metric units before entering values.
- Enter deck geometry, post spacing, and overhang dimensions.
- Select lumber species, grade, beam size, and ply count.
- Add dead, live, snow, special, line, and point loads.
- Choose environmental and treatment adjustment factors carefully.
- Submit the form and review every utilisation percentage.
- Confirm the result with local code tables or an engineer.
Example data
| Input | Example value | Reason |
|---|---|---|
| Joist span | 12 ft | Typical ledger-to-beam distance. |
| Beam span | 8 ft | Distance between supporting posts. |
| Dead load | 10 psf | Decking, joists, and permanent materials. |
| Live load | 40 psf | People, furniture, and movable items. |
| Species | Southern Pine No. 2 | Common treated deck framing option. |
| Deflection limit | L/360 | Serviceability comparison value. |
Frequently asked questions
What is tributary width?
Tributary width is the deck width assigned to one beam. It converts surface loading into line loading. Interior beams may receive joists from both sides.
Does a wider beam always carry more?
Extra plies increase section width and bending capacity. Greater depth usually improves stiffness more efficiently. Connections between plies must also transfer loads.
Why does deflection govern many beams?
A beam can resist stress yet feel flexible. Deflection limits improve comfort and surface performance. Deeper members reduce movement substantially.
Can this calculator size a hot-tub deck?
It can include an additional area or point load. Hot tubs create large concentrated and dynamic demands. Professional design is strongly recommended.
How is a beam overhang checked?
The calculator estimates cantilever moment and shear. It also limits overhang relative to span. Local code rules may be more restrictive.
Are pressure-treated values automatically exact?
No, treatment processes and incising can reduce capacity. Product stamps and supplier data should be verified. The adjustment here is only preliminary.
Can I use engineered wood outdoors?
Only products approved for the exposure should be used. Moisture protection and connection details remain critical. Follow the manufacturer’s evaluation report.
Does the footing estimate include frost depth?
No, it estimates area from vertical soil bearing. Frost depth and uplift require separate checks. Local permit requirements control footing depth.
Why is continuous-beam behaviour simplified?
True continuous analysis depends on support stiffness and span arrangement. This tool uses conservative simplified reactions. Complex layouts need structural analysis.
Is the result suitable for permit drawings?
Not by itself. Permit authorities may require approved span tables or sealed calculations. Verify every result before construction begins.