Deck Load Capacity Calculator

Understanding Deck Loads

This deck load capacity calculator helps you estimate whether your deck framing span is likely to be safe under common residential code assumptions. It is a practical tool for homeowners and DIY builders who want to check joist spans before adding furniture, grills, planters, or heavier features like an outdoor kitchen or hot tub. The tool compares your framing details against a conservative live and dead load model and shows whether your span is within a typical limit.

The idea is straightforward: every deck must support a mix of people and objects plus the weight of the structure itself. Loads are measured in pounds per square foot (PSF), which lets you compare different deck sizes on the same basis. Building codes also set deflection limits, which help keep a deck from bouncing or sagging too much under load. This calculator uses a common L/360 deflection limit and standard loads so you can get a quick sense of safety before planning changes.

How to use this calculator

1. Enter your joist size, spacing, and lumber type or grade.
2. Measure your actual joist span from support to support.
3. Review the default live and dead loads or adjust them for heavier finishes.
4. Click Calculate to see the estimated allowable span and your pass or warning status.

1. Live Load (40 PSF)

Live load is the weight of people, furniture, movement, and temporary items like snow. Most residential codes require decks to support at least 40 lbs per square foot (PSF). That means a 10x10 deck should safely handle 4,000 lbs of active weight. If you expect large gatherings or want to place concentrated loads in one area, it is wise to be conservative.

2. Dead Load (10 PSF)

Dead load is the weight of the deck structure itself: joists, decking, fasteners, railings, and built-in features. A typical allowance is 10 PSF, but heavy composite boards, stone pavers, or thick planters can push that higher. Adjusting dead load helps the calculator reflect real materials rather than defaults.

Critical Failure Points

• Ledger Board: Where the deck attaches to the house. It carries a large share of the deck load and must be properly flashed and bolted. Nails can pull out over time.
• Post-to-Beam: A beam sitting on top of a post with a connector is safer than a side-mounted beam, which places shear on the fasteners.
• Rot: Pressure-treated lumber is required for framing, and posts should be rated for ground contact to prevent decay.

Use this tool to explore scenarios like adding a hot tub, extending a deck span, or changing decking materials. If your deck is near the limit, or you are working with unusual loads or tall structures, consult local building codes and a qualified engineer. The calculator is a helpful starting point for understanding deck safety, not a substitute for professional design.