Live Load vs Dead Load Explained (With a Real Deck Example)

In short: dead load is the weight of the structure itself, which is always there. Live load is the weight of people, furniture and anything else that comes and goes. For a residential deck, the IRC design values are 40 psf live load and 10 psf dead load, so every square foot of deck is designed for 50 lb, more where snow is heavy.

Dead load: the weight that never leaves

Dead load is everything built in: decking, joists, beams, railings, fasteners. Wood decks are light. A typical deck of 2x joists and wood or composite boards weighs well under 10 pounds per square foot (psf), which is why the IRC deck tables assume 10 psf.

Heavy finishes change that. Stone or tile on a mortar bed can add 15 to 25 psf on their own, and the standard span tables no longer apply.

Live load: people, furniture and parties

Live load is the weight that moves around. Codes do not try to count actual guests. They use a design value that covers a crowded deck: 40 psf for decks and balconies under IRC Table R301.5. That is about one adult standing on every 4 square feet, a party-level crowd.

Load	Typical value for a residential deck	Where it comes from
Dead load	10 psf	Assumed in IRC deck tables (Section R507)
Live load	40 psf	IRC Table R301.5
Snow load	Varies by location; IRC deck tables offer 50, 60 and 70 psf columns	Local ground snow load
Hot tub	Often 100 psf or more over its footprint	Not covered by the prescriptive tables: needs a design

How the load travels down

Every pound on a deck follows the same path to the ground. If any link is too weak, that is where the deck fails.

Half of each joist's load goes to the house ledger, half to the beam. The beam passes it to the posts, the posts to the footings, and the footings spread it into the soil.

Worked example: a 12 x 16 ft attached deck

Joists span 12 ft from the house to a beam, at 16 inches on center. The beam is held by posts 8 ft apart.

1. Total design load = 40 + 10 = 50 psf.
2. Each joist carries a strip 16" (1.33 ft) wide: 50 × 1.33 × 12 ft = 800 lb. Half (400 lb) goes to the ledger, half to the beam.
3. The beam carries half the joist span: 50 psf × 6 ft = 300 lb per foot of beam.
4. Each middle post holds 8 ft of beam: 300 × 8 = 2,400 lb.
5. Each footing must spread 2,400 lb into the soil. On average soil rated at 1,500 psf, that needs 1.6 sq ft, about an 18" round footing.

Try it with your own numbers in the deck footing size calculator, and check the joists with the deck joist span calculator.

Engineer's note: the weak link in most failed decks is not the joists, it is the connection to the house. The ledger in this example carries about 400 lb from every joist, which is why codes are so specific about ledger bolts and flashing. Also, 40 psf is an average over the whole deck. A hot tub, a big planter wall or an outdoor kitchen puts a concentrated load in one spot, and the standard tables do not cover that.

Common questions

Is 40 psf enough for a deck party?

Yes, for normal use. 40 psf over a 12 x 16 ft deck is 7,680 lb, roughly 40 to 50 adults. Building codes also include safety factors in the wood design values.

Is snow added to the 40 psf live load?

No. The deck is designed for the larger of the two, live load or snow load, not both at once. In heavy snow areas, the snow value controls.

Can I put a hot tub on my existing deck?

Not without checking. A filled 400-gallon tub with people in it weighs around 5,000 lb, often over 100 psf on its footprint. That needs extra framing and footings designed for it, or a separate concrete pad.

Written and checked by: Engr. Abdul Latif, civil and structural engineer. Load values follow IRC Table R301.5 and Section R507. For guidance only; your local code and building official have the final say.