How a deck holds weight

IRC R301.5 sets the minimum design live load at 40 psf (pounds per square foot) for residential decks. A 16×20 (320 sqft) deck designed to code holds 320 × 40 = 12,800 lb in distributed live load — enough for ~85 adults standing shoulder-to-shoulder, or a fully-loaded hot tub (5,000-6,500 lb) plus furniture and guests. The deck also carries its own ~10 psf dead load (the lumber, hardware, decking) on top of that.

Dead load = weight of the structure itself (lumber, fasteners, decking, railing). It's constant. Roughly 10-15 psf for a typical deck. Live load = weight added by occupants, furniture, snow, hot tubs, BBQs — anything that comes and goes. IRC R301.5 minimum is 40 psf live for residential. Snow regions add a snow-load factor on top (typically 30-70 psf depending on county). Hot tubs use a concentrated-load formula because their weight is small footprint, large mass.

Because the beam is the load concentrator. Every joist on the deck transfers its full tributary load (the area each joist supports) onto the beam at one point. A single 2×10 beam doesn't have enough section modulus to carry that concentration; doubling 2×10s (or going to 2×12 doubled) raises the moment capacity by ~2.2×. IRC R507.5 prescribes doubled or tripled beam options based on tributary area and span.

For a joist between two beams (or one beam + one ledger): tributary width = half the joist span on each side. Example: joists span 14 ft from ledger to beam, so each joist's tributary is 7 ft from the ledger toward the beam + 7 ft from the beam toward the ledger = 14 ft of joist length × 16″ O.C. spacing = 18.7 sqft per joist. Each joist carries ~18.7 × 40 psf = 750 lb of distributed live load.

At the soil. Footings transfer load via direct bearing — concrete in compression pushes against the soil below it. Soil bearing capacity (presumptive, per IRC Table R401.4.1): clay 1,500 psf, sand 2,000 psf, gravel 3,000 psf. A 12″ diameter sonotube footing has 0.785 sqft of bearing area, so on average clay it carries 0.785 × 1,500 = 1,178 lb safely. Heavier loads need bigger footings or better soil.

Joists fail first — they're the lightest structural elements in the load path. Visible signs before catastrophic failure: noticeable bounce, sagging boards mid-span, popping fasteners. Beam failure (less common) comes with cracking sounds at the post connection. Ledger pull-away from the house is the most catastrophic failure mode — it accounts for 90%+ of deck collapses in publishing data, and gives almost no warning. If you can see daylight between your ledger and the siding, do not use the deck.