You must follow the manufacturers install instructions. Some will say to lay a bed of Structolite while others will say to use nothing. I have never installed one that did not require a bed of mortar or Structolite

Ballenger's answer is "spot on" as some manufacturers require a bedding ssubstrate and some don't. In fact, americast will actually void its warranty on any tub that is set in structolite, so there you go! Check with the tub installation instructions or get us a name brand and we will see what we can determine, OK?

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Mark