Need Help  Installing Hot Tub on Elevated Screened Porch Floor!!!
I have an elevated screened porch and the floor is supported by 2" x 12"'s on 16" centers. The sides are framed with three 2" x 12"s. They are close to 18' spans, unsupported in the center other than small pieces of 2" x 12"'s between then spans just to prevent twisting.
The whole porch is 18' x 18' and the perimter is supported by 10' long 8" x 8" posts (white pine I believe) about 8' apart that sit on an underlying concrete patio. I want to put a hot tub in the elevated screened porch, either in the center or in a corner, but believe I need to shore it up. The hot tub is 88" by 88" and will be approx. 4,000 lbs with water and people. I do not want to add additional posts under the porch as that will make the underlying patio unusable. I've gotten differing contractor opinions on this. One guy thought simply jacking up the underside of the porch and slipping a 1/4" thick by 6" wide by 9' long steel plate on top of the two posts and under where the 2' by 12" spans are connected with metal brackets to the perpindicular framing (to prevent the metal connecting brackets from collapsing) would be sufficient. Any help? 
Hot tub on patio
Hi!. You say the tub is 88"x88". This will hold 4500 litres or 9,900 lbs. plus 500900 lbs for 6 people. Your tub weight is therefore more like 10,000 11,000 lbs. Your structure won't take it without serious reinforcement, and then only if the tub is placed strictly in corners and heavily reinforced. If you put in the middle, your whole timber patio will collapse on first filling the tub. I would strongly recommend you get a structural engineer to calculate the full loads + safety margin and also check your existing conditions. Your concrete patio may also crack or fail. Trying different rule of thumb patches and adding bits and pieces without careful structural design is asking for disaster. Imagine stacking four cars on top of each other in the middle of your patio, and that's the sort of weight we're talking about. Cheers!, from OZ. :whistling2:

JoJo is correct, though he is a bit off on his numbers. You are going to need some major reinforcement.
Assuming your hot tub is like this one, it will weigh 550 lbs empty. With a capacity of 450 gallons, full of water it will weigh 4,300 lbs. NOW you add people. Using revised 2002 CDC numbers, and assuming you are sitting 4 males and 4 females, you are adding 1,420 lbs to your 4,300 lbs you already had. That is a total of 5,720 lbs. Now, equally important to how much weight you are adding is how much weight are you adding in terms of PSF. The surface area of your patio that your hot tub occupies is 54 sqft. This translates to 106 pounds per square foot on just the portion of the joists that the tub occupies, or about 18 PPSF for the overall structure (assuming there is no one else on the patio that is not in the tub, and that the patio is vacant of any other furniture or things that weight something ;) Taking the greater of the two numbers, the maximum span that 2x12s on 16" can handle is 12'. You stated that your span is 18'. This also does not take into consideration (a) the height of the columns holding up your patio, (b) the spacing of your columns, (c) what is being used to affix the joists to the perimeter 2x12s that are trippled up, etc. Also, if you do not plan properly, the result is not just something that is bowing, or aesthetically displeasing; you are putting the lives of the people above and below the patio on the line. Bottom line: I am ALL ABOUT DIY; but at least get a structural engineer to tell you how it should be built to support the load you want to add. So while JoJo's numbers were off, his advice and analogy is sound. Just my $0.02. 
This was how I arrived at the figures. I need to use metrics, then convert back to imperial measurements. one side of tub is 88" = 2235 mm. Square this and you get a surface area of 4995225 sq mm I know sides of the tub taper, so multiply area by 800 mm depth to roughly average out. Then we get a voulme of about 4 cubic metres or 4000 litres. 4000 litres of water weighs (multiply by 2.2) 8,800 lbs. The figure is closer to the higher end than the lower figure quoted. Also you need to allow for accidents, where the spa tub is filled by accident to its full capacity if a hose is left running. the figure I first quoted isn't unrealistic. You are right to point out the load is not insignificant, and can result in serious injury or death. Also, we don't know if the timber patio is rotted or the fixings have rusted, the wood is split or some other weakness is present. Structural Engineer a must!

This is the spa:
http://maaxcollectionhottubs.com/index.php/2010/11/471/ It weighs 4,085 lbs full. Only my wide and i will be using it, but assume worse case will be 2 women (at 110 lbs each) and 2 guys (at 200 lbs each). So we have up to 620 lbs in people and 4,085 lbs in hot tub and water and we have a total of ~4,705 lbs. There will likley be some minor amount of furniture in the porch, but not by the hot tub which will be in a corner. I had another contractor here earlier and he recommended:  Doing what the first contractor rcommended: Adding a 1/4" x 6" x 9' steel plate between the top of the two 8"x8" posts and the underside of the porch that they support along the perimter where there are metal brackets that affix the 2x12's to the permrimetr frame (to prevent the brackets from puuling out and failing); and  Doubling up the 2"x12"s on the side of the porch where the hot tub will be. He advised that that would be sufficient. However, he is not a structurral engineer, he is a custom home builder. I don't even know where to start to find a structural engineer. 
I forgot to add, that this is definitely considered a point load rather than a hundred people at once standing all over the patio (who would weigh approximately the same as the tub). Point loads as any engineer would tell you are far more dangerous and difficult to contend with. With a point load, the existing beams cannot distribute this load over the whole patio area and the safe floor loading over the whole area doesn't apply. If you do have a large party and the tub is in use as well, your combined load could easily exceed the original structure's design load limits. I cannot stress enough the danger that can result.
In Australia, we had a case where a wedding party of 80 or so people were having the time of their lives till the bride & groom made their departure. Then all the well wishers rushed onto the large balcony to bid them good buy, and you guessed it, down came the whole balcony, injuring many people, including the grooms motherinlaw, who was innocently standing underneath. 
I am not a structural engineer, I am an architect, so I have no interest whether you get an engineer or not. What I forsee is that if your patio fails and injures someone, you could be sued, hence my recommendation to get peace of mind. Anyway, it shouldn't cost an arm and a leg, and you could get several quotes from SE's to do computations for you and which you can hand to a builder. I would suggest you find one in the business directory, or the engineer's institute could recommend one, or look up firms on the net.
Also I would be amazzed a builder would undertake this work and take it upon himself to do his own structural computations. Doing the job right always costs a little more. cheers, Joe 
Quickest way to find one is to google your state name, "structural engineer", and your state's initials. Such as "Virginia structural engineer VA". There are also national chains, like Criterium Engineers, that either do this sort of thing out of local franchise offices, or can quickly direct you to a good resource.

Hi All again, I'm still very troubled with the figurers. I looked up the lead given by dutchswan and the manufacturer's figures, and guess what, they're way out. They state their 7'10" square tub weighs 560 lbs, and contains 450 US gallons, and fully filled total weight is 4303 lbs, but so does their bigger tub which is 115% bigger in volume. Rubbish no 1. By my calculations, the smaller tub in fact, contains about 3000 litres of water fully filled, or about 6,600 lbs, plus dead weight of 560lbs = 7160 lbs and the bigger tub, (8'2") square = 8234 lbs, so the manufacturer's figures are "one size and weight fits all". They're way, way out.
How come no one has picked up on this serious miscalculation by the manufacturer? 3000 litres divided by 3.785 = 792 US gallons, not the quoted 450 gallons quoted by the manufacturer. I'm sure a gallon of water in the Us weighs the same as a US gallon weighs in France, Australia or Timbuctoo. If designers take these loads stated by manufacturers as gospel, we have been under designing for God knows how many years, with serious consequences. I would like someone to prove me wrong, and if so, I'll eat humble pie. 
JoJo: This is not an open tub...it is heavily contoured with raised sitting platforms so it reduces the water capacity and weight.

The structure under the tub should project all the way to the ground onto footing below the frost line.

Thanks for all the feedback guys.
I have a local structural engineer coming by today...pulled his name off the Structural Engineers Association of Georgia website. 
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