Every structural member is sized for two kinds of weight. Dead load is the permanent, built-in weight of the structure itself — framing, sheathing, drywall, flooring, roofing and fixed equipment. Live load is everything that moves or changes: people, furniture, appliances, stored boxes, and on decks, crowds. Both are expressed in pounds per square foot (psf) of floor or roof area.
The IRC sets minimum live loads in Table R301.5: 40 psf for living areas and decks, 30 psf for sleeping rooms, 20 psf for attics with limited storage and 10 psf for attics without storage. Span tables for joists and rafters typically assume a 10 psf dead load for floors, with an alternate column for heavier finishes such as tile. Roofs add snow load based on the local ground snow, which in northern states can be far larger than the live load.
Loads decide member size and spacing. Designing a floor for 40 psf instead of 30 psf can shorten the allowable span of a 2x10 by a foot or more. Concentrated loads such as hot tubs, gun safes and large aquariums are not covered by the uniform psf values and need their own design.