How much weight can my deck hold? Hot tubs, planters and outdoor kitchens

What a deck is designed to carry

The residential code gives a design live load for decks, the weight of people and movable things, as a figure in pounds per square foot spread evenly across the surface. It is set for a crowd standing on the deck, not for a heavy object sitting in one place. The joists, beams, posts and footings are sized from that figure. A deck built exactly to code has reserve capacity for a party, and not much more.

Concentrated loads are different. A hot tub puts a few thousand pounds on a footprint of a few dozen square feet. The joists under it carry far more than their share, and the beam and the two or three posts under that corner carry nearly all of it. The footings under those posts then have to push that load into the soil without settling.

How heavy these things really are

Water weighs a little over eight pounds a gallon. A typical six-person hot tub holds several hundred gallons, so the water alone is often more than three thousand pounds before the shell, the equipment and the people get in. Spread over the tub's footprint, that is commonly two to three times the live load a deck is designed for, and all of it in one place.

Planters are sneakier. Wet soil can weigh roughly a hundred pounds per cubic foot. A large square planter two feet deep can hold well over a thousand pounds of soil and water, sitting on a few square feet of decking and, often, right at the edge over the rim joist. Outdoor kitchens combine stone or concrete counters, a built-in grill, masonry and sometimes a refrigerator, all in a line along one side of the deck. These are typical ranges, not calculations for your deck, and the point is only that they are far outside what the framing was sized for.

• Hot tub: water, shell, equipment and bathers, concentrated
• Planters: wet soil near the deck edge, often several at once
• Outdoor kitchen: counters, masonry and appliances along one side
• Pizza oven or fire feature: masonry mass plus heat clearances

Why this is a structural engineer's call

Whether a particular deck can carry a particular hot tub depends on joist size and spacing, span, beam size, post spacing, footing size and soil, the condition of the wood, and where on the deck the tub sits. That is a calculation, not a judgment by eye, and it is exactly what a licensed structural engineer does. Levi designs and builds decks and does the carpentry that reinforces them, and he brings in an engineer for the load analysis whenever a hot tub, a heavy planter arrangement or an outdoor kitchen is part of the plan. Carlsbad will typically expect engineered details on the permit for a hot tub on an elevated deck as well.

On an existing deck the engineer also needs to know what is actually there, so part of the process is opening up enough of the framing to confirm sizes, connections and the condition of the wood. A deck with twenty years of coastal weather on it may not have the capacity its original drawings suggest.

What reinforcing usually looks like

For a hot tub, the common approach is to build a dedicated support under the tub's footprint: additional joists or doubled joists at closer spacing, an added beam, extra posts, and new footings sized for the concentrated load, sometimes a small slab on grade under the tub with the deck framed around it so the deck carries none of the water weight. On a ground-level deck the slab approach is often the cleanest. On an elevated deck the framing is beefed up and tied into the structure the engineer specifies.

For planters and outdoor kitchens, the fix is similar at smaller scale: blocking and added joists under the load, a beam or posts under a counter run, and attention to the rim joist when heavy objects sit at the edge. Drainage matters too. Planters that drain onto the deck and hot tubs that splash keep the wood wet, so the design should get that water off the surface and away from the framing.

Planning it from the start

If a hot tub or outdoor kitchen is a possibility for a deck you have not built yet, say so during design. Reinforcing a plan on paper costs a little extra lumber and concrete. Reinforcing a finished deck means opening it up. Levi plans decks in Carlsbad and the surrounding North County communities with the eventual uses in mind, including where the tub's equipment will go, where the electrical conduit will run for a licensed electrician to connect, and how you will get to the tub's access panel without lifting deck boards.

Related questions

Sources and further reading