my addition is built with a slab on grade- it will have 3/4" plywood then 3/4" hardwood in th rooms- this means the bathroom subfloor is 1-1/2" too low from finish floor height. i am wondering what the best way to make this distance up is? i will be putting heat mats in the floor also. should i use mortar to build up the subfloor a little or just a thick setting bed when i set the tiles? i think she wants the small octogon tiles so that would be tough........ thoughts?