Can our deck support the hot tub we ordered?
January 21, 2005 9:57 AM   Subscribe

We're taking delivery of a new hot tub today and I'm not wholly sure that our deck is strong enough to support it.
Is there a simple way to to check that the deck can withstand the weight? (MI)

The tub is by SofTub, holds 300 gallons of water and they say it weighs about 2700 pounds full but that its weight per square foot is less than 90 pounds/sq.ft.

The deck is about 15 years old, has no visible signs of rot, sagging or other easily found clues that it's gonna collapse. The deck is supported by 4x4's set into cement with 2x6 runners supporting the decking above.

Need more info? Ask and I'll provide as best I can.
posted by fenriq to Home & Garden (13 answers total)
I see you are on the West Coast - any chance the wood on your deck is cedar or redwood? (Asking b/c those woods are much stronger rot-resistant than normal pressure treated wood)
posted by mlis at 10:08 AM on January 21, 2005

How far apart are the runners? What are they made out of? What kind of planking is the decking made out of, and how long are the planks?
posted by SpecialK at 10:10 AM on January 21, 2005

The low tech solution is to have a lot of friends over, and get them to stand close together. Of course, this might not be the safest solution.
posted by bh at 10:15 AM on January 21, 2005

I could have sworn someone's asked this question before, but I can't find it.

The building code has span tables (length of span vs. load) for different species/sizes of wood members, so you may be able to check that.
posted by LionIndex at 10:17 AM on January 21, 2005

First, the most important piece of information we don't have is the distance covered by the span. That is, what is the distance from the nearest 4x4 under the deck on one side of the tub to the nearest 4x4 on the other side, assuming the 4x4s fully support all deck runners/joists.

For resources: There are a bunch of software packages that can help you do this. One that I've used is Beamchek. There is a demo version with the spans limited to between 10-11 feet and 15-16 feet. If your deck runners are between that length (or you're willing to round up for a ballpark figure) and you can make an educated guess about weight and other parameters, you can probably use the demo version.

There's an excellent book called Workshop Math that has all sorts of these calculations as well, a local library might have it. As previously mentioned, many books have span/deflection tables. Lastly, people on will sometimes do the calc for you if you post them all the relevant information.

Also, please don't forget the weight of the people in your calculations - it's not just water + tub, at least if you want to have any fun.
posted by true at 10:24 AM on January 21, 2005

Okay, an info update.

The deck is redwood, top planks are 2x6's and I believe most are 10 feet long.

I will have to get home and get under the deck to check the interspan distance but it seems to me that its about four to six feet.

There is a firm 4x4 almost directly underneath the proposed tub spot so I'm thinking it is probably okay but my wife's worried (even though I told her if we break the deck we get to build a new one).

And true, you are quite right. A hot tub without no bodies is no fun at all. Figure on another 200 to 600 pounds depending on how many folks come over to soak.

Thanks for the good thoughts already!
posted by fenriq at 10:31 AM on January 21, 2005

I could have sworn someone's asked this question before, but I can't find it.

Me too. And here it is.
posted by vacapinta at 10:52 AM on January 21, 2005

How far above ground is the deck? I could have sworn there was something in the building code that stated hot tubs had to rest on /in the ground--the one on which I worked was part of a deck--in this case, the section of the deck with the tub was about two or three feet off the ground, so a hole was dug under the deck, the fiberglas tub was placed in the hole, and then dirt and sand filled in around the tub.
posted by fandango_matt at 11:03 AM on January 21, 2005

It sounds a lot like our old deck in Minnesota, which held a regular hot tub (not SofTub) for years and years, until we moved and the new people tore it out (why? I have no clue).

Surprised the hot tub people didn't offer to come out to your house and check things out, or at the very least give you a checklist to go over, before they sold it to you.
posted by GaelFC at 11:50 AM on January 21, 2005

I thought most cities required an engineer to sign off on these things before they allowed them to happen. I suspect if you have a structural engineer come out for 30 minutes to measure and calculate, he or she could tell you definitely if it'd be ok. I suspect they'd charge a 100-200 bucks, tops, for the time.
posted by mathowie at 11:54 AM on January 21, 2005

Mathowie, I think, if its going to be built into a deck or needs a structure to support it then I would have to get a permit.

GaelFC, bought it a trade show but they did talk with me for a while about the footprint it was going to have (which is where the 90 pounds per square foot number above came from). Did you have any trouble with the deck feeling unstable or sagging?

I'm thinking that a call to my local fire department would be the simplest and least expensive solution.

fandango_matt, the spot on the deck where I'm thinking of putting it is only about a foot and a half above the ground at the tallest point.
posted by fenriq at 12:27 PM on January 21, 2005

fenriq, a structural engineer can tell you definitely if it is safe to put a tub on top of a deck. I'd do it, considering how much decks cost to repair and replace.
posted by mathowie at 1:24 PM on January 21, 2005

Quick update: I spoke with the local fire department, they referred me to the community development office who gave me some formulas to run.

Basically, if the proposed footprint is more than 40 pounds per square foot (what most decks are built to) then it will need reinforcing.

At just over 81 pounds a sq. ft. before bodies are added, I will definitely be getting the deck reinforced before getting the tub into action.

Thanks for all of the help. Now I've gotta figure out where to put the tub while the deck is toughened up.
posted by fenriq at 1:58 PM on January 21, 2005

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