Deck Load Capacity Calculator

Estimate residential deck joist spans (40 PSF Live + 10 PSF Dead). Check if your framing meets simplified code estimates.

Framing Details

Distance between beam/ledger supports.

Assessment

Enter framing details to check safety.

Understanding Deck Loads

Decks are subjected to two main types of forces that dictate their structural design:

1. Live Load (40 PSF)

This is the weight of "variable" things: people, furniture, movement, and snow (if applicable). Standard code requires decks to support at least 40 lbs per square foot (PSF). This means a 10x10 deck must safely hold 4,000 lbs of active weight.

2. Dead Load (10 PSF)

This is the weight of the structure itself—the joists, decking boards, and railings. The standard allowance is 10 PSF. If using heavy composite decking or stone pavers, this load increases significantly.

Critical Failure Points

  • Ledger Board: Where the deck attaches to the house. It carries 50% of the deck's load. It must be bolted or lagged; nails withdraw over time.
  • Post-to-Beam: Simply resting a beam on top of a post (with a connector) is safer than bolting it to the side of the post, where the bolts support the entire shear load.
  • Rot: Pressure-treated wood is required for all framing. "Ground contact" ratings are needed for posts.

5 Fun Facts About Deck Safety

Hot Tubs

Water weighs 8.3 lbs/gallon. A 6-person hot tub can weigh 5,000 lbs—exceeding 100 PSF. A standard deck will collapse without major reinforcement.

Overload

Cantilevers

Joists can hang past the beam (cantilever) by a ratio of roughly 1:4. If the backspan is 8ft, you can safely overhang ~2ft. It hides the posts!

Design

The "Z" Flashing

Ledger rot is the #1 killer of decks. "Z-flashing" (metal drip cap) behind siding and over the ledger is critical to keep water out.

Waterproofing

Galvanic Corrosion

Pressure-treated wood contains copper. Standard steel screws will dissolve rapidly (galvanic reaction). You must use Hot-Dipped Galvanized or Stainless Steel.

Chemistry

Tension Ties

New code requires "tension ties" to bolt deck joists directly to the house floor joists, preventing the deck from tearing away during earthquakes or overload.

Code

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