Hot-Tub on an Exterior Deck 3

[KarlT]

Does anybody have any experience in designing a deck attached to an existing house, which needs to support a hot-tub? I have asked the arch. for footprint and weight when the hot-tub is full. Are there any specific requirements for supporting that kind of load? (I'm thinking I may need to use double wood joists depending on the load)

 

[LPPE]

I'd imagine hot tubs vary in depth from manufacturer to manufacturer, but 2 feet of water will put a load of 124.8 psf. You'll find you'll need quite a stout deck.

 

[IRstuff]

We have an 8-ft hot tub in our deck, but it's actually being supported by a concrete pad.

TTFN

 

[CAOranger]

Yes, let it sit on a concrete pad.

If you need to use wood, it is possible. You need to check shear stress and make sure that the hangers are adequate.

 

[bylar]

Yes I have.
Always the tub manufacture has provided the weight of the tub full of water and with the weight of the number of people the tub will accomodate. They usually use 200# per person. The deck indeed has to be stout. Remember along with the tub weight there is usually several people around the tub.

Get the weight from the manufacture or decline the job.

 

[swBausch]

If the tub is 8 foot square, the heaviest ones (Cal Spa) are just under 1500 pounds, and no more than 500 gallons.

The SouthernPine site has span tables for 150 lbs/ft joists:

http://newstore.southernpine.com/images/ref202.pdf

 

[mkberry]

If the deck is too high to support the tub with a slab, analyze the deck framing with the Hot Tub in the worst places possible for each member. You never know if they might decide to position the tub differently than they indicated to you. This will involve several load cases. For a simple deck with joists supported by the house on one end and a beam and columns on the other: Look at the bending and deflection of the joists and beam with the load in the middle. Check the shear, connections, columns and footings with the load near the supports. Check the wall of the house you are connecting to and verify the effect on any wall openings below the deck too. Seismic or wind may require some bracing too.

 

[SimpleStructures]

My wife wanted to put a pool on my elevated wood deck so I checked the deck for what it would support. It will only support 6 inches of water before it, theoretically, is unsafe.

The critical element to check is probably the joist support to the house. Install some Simpson hangers if you don't have them already. Either way, if it's a typical residential deck, it will not support the weight of a hot tub without some retrofit.

 

[KarlT]

Thank you all for the very helpful comments. I did get in contact with a local hot-tub supplier and it seems that they typcially put the tub on a slab on grade and build the deck around the tub to suit. These units are self contained and they are filled and drained with a garden hose, so therefore there are no potential problems with piping, etc. from shifting of the tub. If it did move around too much, the only problem would be the water level would tell you how crooked the tub actually is. (Or how crooked they built it!)

Incidentally, the standard tubs around here are 7'x7'x37" high, with 345 gallons of water and 5-6 persons. Self weight of tub is around 500-600 lbs and total weight is a bit over 4000 pounds - way too much for a typcial wood deck. (Especially when you throw in the reductions for wet service, pressure treated, etc)

 

[Focht3]

Hmmm,

It depends on the builder, i.e. how well it is framed. I have one on a wood deck, and it's performing fine. I'll be replacing the wood in a few years (since it was built in 1996), but it shows no signs of distress. (It's about 6 feet off the ground - on dolomitic limestone. I can get close to the supports without much trouble.) The deck is in better shape than the spa, incidentally -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[CSEllc]

I have done them both ways....supported by the framing and on a slab. It really was a function of the size (and therefore the loading) on the tub.

If you do support it on the deck...using galvanized joist hangers (or stainless steel) is critical since the tub will have some chlorine in the water and will corrode the steel for the hangers.

 

[dicksewerrat]

Did anyone read about the deck collapse in Chicago last year? 40 people on a second story deck. A few died. put in a safety factor that you can sleep with. People do not do what is prudent all the time.

 

[dbuzz]

There have been many collapses of timber decks and balconies in Australia over the years. The problem is often that the decks/balconies aren't "engineered" but are installed by owner-builders using poor materials, such as Oregon pine and softwood timbers, and sub-standard methods of construction.

The decks and balconies that fail do so during an overload event, such as a large crowd at a house party. In your case one can just imagine, after a few drinks, a group deciding to see how many people they can fit into the hot tub!

The Royal Australian Institute of Architects have initiated a "Balcony Week" to pulicise the potential problems with these structures and to encourage owners to complete annual checks (

http://www.medialaunch.com.au/257/).

Be conservative in you design/check!

 

[CSEllc]

Regarding deck collapses....There have been several more than the one in Chicago.

The problem, generally, has not been the member sizes (floor joists etc) but rather the connections between the legder and the main house and lateral stabilirty of the columns.

Generally, contractors will only nail the ledeger board to the main house rim. This is INADEQAUTE. The nails corrode, "loosen up", begin to pull-out etc. The ledger board must be bolted to the rim board with, preferably hot-dipped galvanized, thru-bolts or lag bolts (or lag screws if you prefer that nomenclature). Many local building departments realizing the problems have required contractor have decks "engineered" depending on the size. Many are also requiring the ledger be bolted if not requiring the deck "engineered".

Several years ago (maybe 5-6 years), a magazine (I think it was This Old House) published an artilcle on this topic and had several details in the article. I no longer have the article, but I used those details and incorporated them into my practice with some modifications.

 

[Joecandraw]

Good question i just got a hot tube myself. A 8 foot hot tub will weight about 1500 to 2000 pounds. So yes you need to support it. But to design a deck for it you need to know how high the deck will be off the ground. Two main factor: how are the post attacked to the ground and how is the deck braced. The area under the deck needs to be supported with
4 post,one at each corner of the deck. If the deck is very high it will need cross bracing, the bracing should be thru bolted with 1/4 bolts. The bracing style needs to be X pattern on all four sides most often. The girders need to be strong in the tub area. I would use a 2 2x12 to make a beam for the girders and use 2x10 joist at 12 inches on center all attached with metal brackets. Because decks shack. And hot tubes are heavey. As far as the post size i can not size them i do not know the heigth of the deck or were the tub will set on the deck.