Could these tiles be stacked to create a thicker base to fill a shower floor and make it level with an outside deck to make this handicap accessible for a wheelchair to roll from outside the shower into the shower? Or would it be possible to build a window frame type structure under the tiles to create the height needed? Would that reduce the strength of the tiles?

You can stack out Rugged Grip-Loc Tiles on top of each other, but we would recommend placing a piece of plywood between the two floors to keep the stability of the floor intact. The tiles are made from durable polypropylene plastic and will not crack under the weight of traffic on the floor whether it is from feet or a wheelchair. These tiles are very easy to install as they are interlocking and can get removed and replaced if damaged. These tiles are also very slip resistant and are resistant to mold and mildew growth.