I need to level a flagstone patio for a spa...

I am going to place a new spa on a section of my flagstone(over slab) patio. Though the flagstone is, for the most part flat, it isn't quite level, and there are slight variations in the height of the stones. SO, what I need to do is basically create some sort of pad, or deck the will be level and flat and support the weight of a spa.
My first inclination was to frame out and level a form, fill the gaps between the form and the flagstone and pour a slab on top of the flagstone. I had the idea as well to lay down a heavy duty tarp and place the form on top of it, pour the slab and then trim it down after the slab has cured.( to prevent the concrete from ruining the flagstone if I ever want to remove the slab).
What thickness slab would be sufficient in this application? ( spa weights in at 4000-4500lbs, 91"x84")
Any other ideas on how to accomplish this?
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GotBonus wrote:

Are you sure you want to ruin a section of a stone patio? What if you decide to sell it, or get a different size spa that no longer covers the same area, etc?
One idea, you could find some cushy type material, like stick on insulation or hopefully tougher and more durable, maybe 3/8 of an inch or so thick. Fasten that in strips maybe 4" apart across the bottom of the spa and then just sit it on the patio.
Or get some pressure treated 1 1/2 x 1 1/2 or similar type wood that's made for deck railings, etc. Build a frame out of this to go under the spa, using a plane as needed to get it to sit level. Then use a few minimal dabs of adhesive to secure it to the patio in a couple spots. Then you can get some friends to help sit the spa on it. Or else mark exactly where the frame sits on the patio, then remove it and fasten it to the underside of the spa. The position the spa on the patio to the marks.
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That is why i stated this idea....from my original post.
"I had the idea as well to lay down a heavy duty tarp and place the form on top of it, pour the slab and then trim it down after the slab has cured.( to prevent the concrete from ruining the flagstone if I ever want to remove the slab)."

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GotBonus wrote:

We used sand. TB
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What did you use to confine the sand if anything? Was yours on a hard surface? My concerb with this is that eventually the weight of the spa will slowly settle through the sand to the hard( unlevel) "pad" of flagstone.
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wrote:

There is only one correct path for installation of your spa. Follow the instructions that come with spa. Don't even think of varying from what they specify.
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