Example data table
| System | Span | Spacing | Size | Uniform load | Point load | Limit | Estimated total deflection | Outcome |
|---|---|---|---|---|---|---|---|---|
| Imperial | 10 ft | 16 in | 2x10 | 50 psf | 0 lb | L/360 | ≈ 0.31 in | PASS |
| Imperial | 12 ft | 16 in | 2x8 | 60 psf | 200 lb | L/360 | ≈ 0.79 in | FAIL |
| Metric | 3.0 m | 400 mm | 2x10 | 2.4 kPa | 0 kN | L/360 | ≈ 7.9 mm | PASS |
| Metric | 3.6 m | 400 mm | 2x8 | 2.9 kPa | 0.9 kN | L/360 | ≈ 20.1 mm | FAIL |
Formula used
δallow = L / (limit ratio)
How to use this calculator
- Choose your unit system and enter the span and spacing.
- Select an area load (surface load) or a direct line load.
- Optionally add a midspan point load for concentrated weight.
- Pick a size preset or enter custom joist dimensions.
- Choose a stiffness preset or enter a custom stiffness value.
- Select a deflection limit, then click Calculate.
- Review total deflection, allowable deflection, and pass/fail.
- Use Download CSV or Download PDF to export.
Why joist deflection matters outdoors
Garden decks, pergola walkways, and platform frames can feel strong yet still bounce. Deflection is a serviceability measure that affects comfort, joint movement, and surface finishes. Excess flex can loosen screws, open board gaps, and crack toppings. This calculator estimates midspan deflection so you can compare sizes, spacing, and materials before you build.
Key inputs that drive the calculation
Span is the biggest driver because uniform-load deflection increases with the fourth power of span. Joist depth also matters because the moment of inertia depends on depth cubed, so a small depth increase can noticeably stiffen the member. Material stiffness, represented by E, reduces deflection as E rises. Spacing changes the tributary width and therefore the load carried by each joist.
Converting surface loads to line loads
Many garden projects start with an area load that combines dead load and live load. The calculator converts area load to a per-joist line load using tributary width, based on joist spacing. If you already know the load assigned to one joist, choose line load instead. An optional point load can represent a planter, water feature component, or concentrated storage.
Understanding limits and utilization
The pass or fail check compares total deflection to an allowable value such as L/360. Utilization indicates how close the design is to the selected limit; 100% means the predicted deflection equals the allowable amount. If it fails, reduce span, reduce load, increase depth, tighten spacing, or select a stiffer material. The suggested maximum uniform load helps you back-calculate a target load for redesign without changing every input.
Practical guidance for garden construction
Use conservative loads where wet soil, snow, or frequent gatherings are possible. Long-term creep can increase deflection, especially in humid climates, so add margin when finishes must stay flat. Ensure bearing length, blocking, and fastening to reduce vibration and distribute concentrated weights. This tool assumes a simply supported joist; for cantilevers, continuous spans, notches, or unusual supports, confirm details with local guidance.
FAQs
1) What support condition does this tool assume?
It assumes a simply supported joist with a uniform load, plus an optional point load applied at midspan. The reported deflection is the maximum elastic value at midspan.
2) When should I use area load instead of line load?
Use area load when you know the surface loading on the deck or platform. Use line load when the load is already assigned to one joist, such as a continuous curb.
3) Why does span affect deflection so strongly?
For uniform loading, deflection scales with span to the fourth power. Increasing span a little can increase predicted deflection a lot, even if size and material stay unchanged.
4) What does an L/360 limit mean?
L/360 means the allowable deflection equals the span divided by 360. It is a common serviceability target for comfortable floors, though local requirements and finishes may differ.
5) Can it evaluate notched, drilled, or damaged joists?
No. Notches, holes, decay, and cracking reduce stiffness in ways this simplified model does not capture. Use manufacturer rules and professional review when members are altered.
6) How should I treat the stiffness presets?
They are typical values that vary by species, grade, and moisture. If you have a specified modulus from a datasheet or stamp, enter that value for closer results.