Strengthening a floor

We are pulling out a Jacuzzi brand whirlpool and replacing it with a Sanijet. According to Jacuzzi, they require floor strength of about 55 lbs per sq feet. Surprisingly, Sanijet wants 100 lbs per sq feet. I am looking for ideas on strengthening the floor. I have access from underneath. I plan on adding an additional piece of sheet rock on top and screwing it to the joists. Sistering the joists would probably be difficult because a portion of the floor is cantilevered but I could put in a bunch of blocking between joists underneath the whirlpool if that would help. Any suggestions would be appreciated.
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I should have provided some additional info with my post:
The house is 10 years old and built by Toll Brothers. Generally speaking it is sloppily built but the floors were at least well spec'd out. For example, under the kitchen there are all double joists even though the span is not particularly long. According to the blueprint the double joists are "to support the cabinets". Thru out the house the joists are 10 inches wide, 16 inches apart.

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Art wrote:

Generally speaking (real general), you can cantilever up to 3x the joist size for normal loading.
You mention the joists are doubled in the kitchen, and are vague on the span. Are we to take it as you are putting the tub in the kitchen?
Anyways, try a span table for your loads. http://www.southernpine.com/spantables.shtml
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the most part the flooring system of joists seems to be well done. The kitchen wasn't double joist because of the span..... they were double joist because the designer wanted the kitchen floor extra strong. I don't know what is under the jacuzzi floor yet in the master bedroom. The span there is short and if I find double joists I'll stop worrying though I will put an extra layer of 3/4 inch plywood under the Jacuzzi no matter what.
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I have verified that the joists under the whirlpool tub are 2x10" 16 inches apart. Too bad.... I was hoping to find double joists.

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The 55 psf figure "looks low."
Water is about 64# per cubic foot. If your pool is 1' deep, the water load is 64 psf.
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Art wrote:

A cubic foot of water weighs about 62 pounds. A container 5'x5' with 3' of water would weigh about 4,650 pounds. The load per sq.ft. would be 186 pounds. If you weighed 186 pounds you put about the same load on the floor. IOW, I kinda suspect your current floor is more than adequate :)
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dadiOH wrote:

However, if you are really worried, some 4x4 posts under the joists where the tub is going would ease your mind.
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A more accurate approximation would be:
You an 25 of your closest friends huddled in that same 5x5 area.
Weight on a 1-sqaure foot area is carried by more than one floor joist. The subfloor spreads the load. Blocking will hel transfer load to adjacent joists as well but I suspect the OP will need more than just blocking. Normal residential construction (in my area) calls for 40 lbs/sf on the 1st floor and 30 lbs/sf on the second.
-Steve
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On a floor, I'd strongly suggest an additional layer of PLYWOOD, not sheet rock (aka wall board), at least 3/4" thick.
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My original post was made 3 am. That is the only explanation for why I typed sheetrock instead of plywood. Thanks for pointing it out.
wrote:

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Oh darn, now I have to rip out the sheetrock I just put in to strengthen my floor...
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Sheets of plywood in the floor are a good way to distribute the load as well.
In a room with just the spa, take the floor down to the bare sub floor, glue and screw a layer of plywood.
Plywood can be used as cross bracing from below, since you already know where the pipes and such need to be run.
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Blocking will help some but I think you still have essentially the whole load on at most half a dozen joists. Vertical support from below would be better. How free are you to interfere with the space below? You could put a 4X4 prop under each joist that runs under the whirlpool, or run a support beam across, perpendicular to (and beneath) the joists, with a couple of 4X4s or those adjustable steel jackposts. -- H
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wrote:

Below is a finished basement with a suspended ceiling so vertical support would be impossible. The only option would be to sister the joists. The good part is that the jacuzzi will be at the end of the joists which are short. The jacuzzi is partially located in a cantilevered portion of the floor. The cantilever and about 10 feet out from it is separately boxed out from the rest of the joists and joins the rest of the joists with a double joist parallel to the cantilevered wall.
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