Load
The image above depicts an evenly distributed deck load. Building codes require decks to be designed to carry a uniformly distributed load over the entire deck. If evenly distributed, half of the load on an attached deck can be carried by the deck-to-house connection, and the other half can carried by the posts.
Dead loads and live loads are assumed to be uniformly distributed across the entire deck surface for attached and free-standing decks. But in reality, the live load is usually near the outer railing edge of the deck and balcony. That's where people gather.
The image above depicts a typical deck load distribution. People tend to gather near the railings of a deck, and so more load is likely carried by the posts.
40 psf Live Load
The International Residential Code (IRC provides minimum required design loads for the various loads a deck structure may receive. The IRC provides design tables for decks up to a 10 psf (pounds per square foot) dead load and 40 psf live load. Most decks are designed and built for loads of 40 to 60 pounds per square foot. The 2021 IRC includes snow loads of 50, 60, and 70 psf. Refer to 2021 IRC R301.2.
A 40 psf live load is equivalent to a 200 lb person standing in every 2.5 ft × 2 ft area on the deck. That's a lot of people standing near each other on a deck.
Stairways also must support this load, but in addition, must support a concentrated load.
Hot Tubs
Hot tubs are common on decks, but they provide a significant concentrated dead load. Hot tubs filled with water and people are heavy and can weigh a couple of tons. Hot tubs require framing that can support more than 100 pounds per square foot. Engineered or alternative designs for supporting a hot tub may be necessary but may be handled with an over-design in the joist and beam sizing, spacing, or spans.
Information used with permission of www.nachi.org