Deck Expansion with Hot Tub

by Scott
(Washington)

I love the site! I've never built a deck, but am working on plans to help my son-in-law build a deck-expansion onto their new house. The deck would span a steep slope and would be somewhat
odd-shaped.

Looking from the back of the house, it would be 20' out on the right, 8' wide and 10' out on the left 8' wide with 62' connecting the two sides. This would wrap the deck around the back of the house. He wants to put a hot tub on it or build a hot tub deck.

I am expecting the joists to be 16" OC and the beams to be 6' apart and parallel to the house with supporting 4x4 posts 6' apart with 4x4 45 degree supports (an updside down peace sign). Any better suggestions or ideas? Thanks!

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Apr 24, 2010
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Consider the weight on your deck
by: Editor - Rich Bergman

That sounds like a rather large deck. Typically decks are designed for a 50 psf load. That is 40 psf live load - the weight of people and things. And a 10 psf dead load - the weight of the structure. So if you imagine an area 10x10 (100 square feet), the deck would be designed to hold a maximum of 5000 lbs. That is quite a lot actually.

However a hot tub can change the math a bit. And I urge you to not take my comments as the gospel here. But they will be a fair guide for you to start with and then you must check with a local engineer who can do the math for you.

But lets take a typical hot tub that is a 6 or 7 person tub, 96"x96" and has 420 gallons of water - at 8.35 lbs per gallon that's 3507 lbs. This kind of tub is about 1000 lbs empty. Add 6 large people at an average of 200 lbs each - That's 1200 lbs.

Add that all up and we are now at 5700-6000 lbs over an area of 64 square feet. That works out to 94 psf. Almost twice as much weight as an average deck is designed to hold. So at least for the area where you intend to support your tub you are going to have to design for a heavier load. This means bigger joists and probably at 12" o.c. I would also recommend using 6x6 support posts for your beams in any case.

Here are a few numbers to consider. If you think your soil is fairly soft and can only hold 1500 psf and you add an extra 50% to to your tub area of the deck (6000x1.5=9000) you will be designing it to hold 9000 lbs. 9000 divided by 1500 psf means you need at least 6 support posts underneath that tub perimeter to spread the load lightly enough that the soil does not compress.

This would be like having two beams with three support posts under each beam under the tub area. All of this is to say I would recommend that you take your final drawings to a structural engineer and pay him the $100-$200 it takes to run the numbers and make sure you have it designed well enough. Sitting hot tubs on decks should be taken seriously and planned properly to ensure its safe for everyone.

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