This is a drop in style fiberglass tub. The instructions say to lay a bed of mortar or plaster , then sit the tub in it. It has to have 100% support on the bottom and no weight on the lip where it sits on the framing. My question is this; Is there something besides good old mortar that perhaps is lighter to fill this bill? It seems with the gap i think I'm gonna have going by the instructions, that i may need two bags or better of mortar to be deep enough.
thanks,
steve