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
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?