How much weight can a 5 inch concrete slab hold per square foot?

I have a back porch that is a concrete slab 5 Inches thick. Its about 15 x 20. I want to put a hot tub that is 85 inches by 85 inches. The tub weights 700lbs empty. I can hold 400 gallons of water. It can hold 6 people but lets just include a max of 4 people at 200LBs each. How many pounds per foot can my 5 inch concrete slab hold without cracking?

7 Answers

  • Anonymous
    1 month ago

    People are made up of mainly water, so adding in a person just displaces the same volume of water.=no weight change(as they basically float).

    . The tub has a floor strip of wood (rather than 4 individual legs) so it can hold a ton.  The concrete under your car is maybe 4" and the car weighs a good ton or more on 4 tires.  No problem. So tub weighs with water 3500lb=1 3/4ton. The slab can hold 5 tons easy and you are nowhere near that.

      Rebar is normally put into the concrete while wet so it stops it from cracking but even without 5" will not do anything substantial.

  • 1 month ago

    You will be only loading the concrete slab to about 80 pounds per square foot. Less than a human foot exerts. Should be no problem.

  • Joe
    Lv 4
    1 month ago

    Have a professional engineer come in and test the slab and what is under the slab.  Call the local Building Inspection Office if you need some PE names.  With the two tons you stated, that 5" slab will crack without rebar and/or compacted base of 3/4 crushed. 

  • 1 month ago

    Is there rebar in the slab?

    Is it on the ground, or above the ground?

    What is under the slab?

    The mix used for the concrete also matters.

    In short, while the slab is PROBABLY sufficient, you need an engineer ON SITE to determine the actual strength of the slab.

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  • 1 month ago

    assuming your slab is on the ground -- do you know what is underneath it?  dirt?  crushed rock? sand?  how thick?  is the slab reinforced?  to what specs -- residential?  [and in your area, those are?] -- check with local building permits office to see if a permit was pulled when the slab was poured .. permit should include the design specs ...  -- grampa

  • Anonymous
    1 month ago

    What is underneath the concrete slab?  Is it on the ground or are there beams and columns underneath it that go to ground.  If so, what is the spacing between the support columns.   Even so, the design cannot be checked since the reinforcing in the slab is not known.  Your 400 gallons weighs about 3,500 lbs.+ concrete 4000+700+800 for a total of 9000 lbs.   That's too heavy without some support columns directly underneath the slab and hot tub.

  • T C
    Lv 7
    1 month ago

    So max weight would be close to 5500 lbs.

    You are well within range...even if it's medium strength concrete.

    My hot springs 7 person tub sat on a 4" 12x12 slab for years without a problem.

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