Replies

1. 
   CaseyF | Jul 20, 2007 06:07am | #1

   With a deck that close to the ground what I've done is frame a hole in the deck. Header it off with doubled joists and use joist hangers.

   Then dig yourself a footer and build yourself a form and pour the concrete right in (Technically this can be done before or after). This makes a pad to set the tub on. Make sure it's real nice and level. Also make sure you have good measurements on the size of the tub.

   This way you don't have to worry about the weight of the tub on the actual deck. And you can have the top of the hot tub at or near the deck height. If this is practical for your clients I think it looks nicer than a tub sitting on top.

   If this isn't what you are looking for I'm sure someone else will be around to better answer your question.

   Peace,

   Casey

   p.s. I'll try to find some pictures, it's been a few years.

   http://www.streets.org
2. 
   CaseyF | Jul 20, 2007 06:19am | #2

   It occured to me shortly after posting that I really only transferred your problem from the deck to the pad. When we had built ours it was really thick. Probably something like three feet or more.

   So I guess you might need someone to size that for you even if you do go that way. Sorry.

   Peace,

   Casey

   http://www.streets.org
3. 
   fingersandtoes | Jul 20, 2007 08:28am | #3

   I do work at a resort that has 16 hot tubs, all on decks with joists sized as per our codes for 40 lbs psf. The most deflection I have seen is about 3/4" over 10 ft. My major concern is that the ledger or joist hangers might fail in shear. The only structural problems I've seen had to do with rot, no doubt made worse by the tub.

   I would size and space your joists as usual and then pour a footing and 4x4 post at joist centred under the tub. Think carefully before deciding to sink the tub into the deck. They require access to the pumps and filters through panels which shouldn't be blocked.

   You might want to do a search, as this topic has come up and I recall there were a wide variety of opinions, most recommending a lot more beefier solution I have.

   
   

   ---

   Edited 7/20/2007 1:31 am ET by fingersandtoes

1. 
   Piffin | Jul 20, 2007 01:10pm | #4

   3/4" deflection in only ten feet iss a structural deficit that would trouble me seriously!Wood does not normally fail catastrophicly like steel will, but what you describe is wood in failure and it will continue to get worse until it gets bad. It is currently sending out warnings that it is severely overloaded. 

    

   Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

1. 
   fingersandtoes | Jul 21, 2007 05:03am | #8

   "3/4" deflection in only ten feet iss a structural deficit that would trouble me seriously!"

   I'm not happy about it either. Unfortunately it's not really my decision what they do about it. It has provided an interesting lab-like situation, where I can monitor the effects of the load on the deck. The wood deflected as soon as the tubs were filled and over the intervening 5 years it hasn't changed.

   They did have one deck collapse. The beam rotted and sheared off. Amazingly, the deck only dropped about a foot - the tub didn't even slide off. The whole thing just hung there off the joist hangers in the ledger. I emptied the tub, jacked up the deck, replaced the beam, and a way they went.

2. 
   GRCourter | Jul 21, 2007 02:49pm | #9

   40 psf for a hot tub!  I assume that they are quite small.  A 6' round hot tub covers 27 sq ft, at 40 psf this would mean (at a pound a pint) that the hot tub would hold less than 130 gals of water, and that is not taking into consideration the weight of the  tub or the bathers.  I installed a 5' round redwood hot tub and it holds 500 gals which is 4000#'s  or 2 tons.  The deck surronds the tub but the tub is on a slab.

4. 
   Matt | Jul 20, 2007 02:12pm | #5

   You are talking at least 80 PSF live load.  You really need an engineer for that.

1. 
   pixburd | Jul 20, 2007 03:59pm | #6

   Two feet of water on a deck is 125 LB/sq ft ... without people and structure. NO standard deck should support a hot tub.

1. 
   fingersandtoes | Jul 21, 2007 04:52am | #7

   Hot tubs give you much closer to The 80lbs/sq ft Matt mentioned. And again , its not the whole deck that has this load, but an area of about 36 sq ft. A 2 ft high post on a 2 ft pad directly below it can easily hold up what is really a 3000lbs point load on its own.

1. 
   GHR | Jul 21, 2007 05:59pm | #11

   fingersandtoes is more accurate in his estimate than the rest.I would suggest that one leave non-prescriptive designs to the engineers.

5. 
   jdbuilder | Jul 21, 2007 05:33pm | #10

   Steve, I've built many decks made to support a hot tub. What I do is basically just really overbuild the whole deck(assuming it's not much bigger than the spa). Example- On a 12x12 deck for a spa, I would have 9 piers, below frost, typ. 8" diam. with the bottom belled out as big as you can get it. These piers would be for three parallel beams(assuming the deck is detached from the house). Beams would be 6x6 post with 2x12 beam notched in on each side, and lagged together. Joists would be 2x12, 12" O.C. This way, your joists only span about four feet or so. Your beams would be spaced to allow for a cantilever of a foot or so on each end, and equal spacing in the middle.