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You can also use the plastic spa pads. I have had my spa on one since the summer of 2003 and through 3 winters and 3 summers it hasn't moved a bit. At about $300 its much cheaper that concrete and at least so far just as good.
Spiman: How are you getting your pea gravel 8" off the ground? Does any have any thoughts on whether a concrete pad should be level or sloped for drainage?