Hot tubs, pergolas, and point loads: when a deck needs to be designed twice.

Distributed load vs. point load — the distinction that matters

When code and span tables talk about deck live load, they mean weight spread over the whole surface — a party, patio furniture, snow in some climates. The joists share it. Deflection is gradual. Every member carries a fraction.

A point load is different: a heavy object with a small footprint — a hot tub shell, a grill island, a pergola column — concentrates force on one joist, one beam, one post, one footing. The tributary area shrinks to almost nothing. A joist sized for 40 PSF across its length can be overwhelmed by a single concentrated load sitting on mid-span, where bending stress is already highest. The symptom isn't always dramatic: a soft spot underfoot, a rail that won't stay square, a post base that keeps working loose. Sometimes it is dramatic.

The hot tub math, honestly

A mid-size acrylic hot tub empty weighs 500 to 800 pounds. Fill it with water — about 8.3 pounds per gallon — and add four adults, and you're commonly at 5,500 to 6,500 pounds total. That load sits on a cabinet footprint often smaller than 8×8 feet, sometimes on four plastic skirting pads that align with only two joists underneath.

Run that against a typical deck: 2×8 joists at 16 inches on center, spanning 12 feet, might be perfectly adequate for a 40 PSF design load. Put half a hot tub's weight on one joist bay and you've asked that single joist to carry several times the load it was selected for — in bending, in shear at the hanger, and in the post below it. The footing under that post was sized for a deck, not a small room full of water. This is why a hot tub deck is a second engineering pass, not a furniture placement decision.

What that pass looks like in practice: identify the tub's footprint and support path; double or triple joists under the shell; add a dedicated beam and posts directly under those joists; upsize footings to the new tributary load (see TB-102); keep access for service and drainage; and verify guard and stair geometry still work after the structure changes. Sometimes the right answer is a reinforced deck zone plus a separate equipment pad at grade — less romantic, structurally honest.

Pergolas, kitchens, and the loads you don't feel

Not every point load is vertical. A pergola bolted to a deck rail or ledger adds dead load — the timber itself — plus wind uplift, which tries to peel the assembly off the deck in a storm. A 12×16 cedar pergola can add hundreds of pounds of permanent load and thousands of pounds of transient uplift to connections that were detailed for a guardrail. The failure mode isn't the pergola flying away; it's the rim joist rolling, the ledger screws shearing, or the rail posts levering out — same physics as TB-105, different trigger.

Outdoor kitchens and built-in seating follow the vertical path: stone or concrete countertops run 25 to 40 pounds per square foot before you put anything on them. A pizza oven, a built-in grill, a fire feature — each one needs a place in the load path, not just a pretty corner on the rendering. We either design the deck for these from the start or isolate them on their own footings, which is often the cleaner answer for anything masonry.

When the deck should say no

Height makes everything worse. An elevated deck — six, eight, ten feet in the air — multiplies the consequences of deflection and connection failure. A hot tub on a second-story walkout is rarely the right structural answer; a ground-level reinforced pad with a path to the house often is. Long cantilevers, existing undersized framing, and decks built before modern connector hardware are all reasons we'll recommend against a heavy point load rather than "make it work" with sistered joists and hope.

Saying no isn't a sales tactic. It's the same instinct that drives freestanding instead of ledger (TB-101) and full-depth footings instead of pier blocks (TB-102): the structure should know what it's carrying before you stand on it.