wHEN REPLACING A TUB ,NOT A TUB SHOWER UNIT, IS ATTACHING THE FLANGES TO THE WALL AND PLACING ALEDGER BI\OARD ON BACK WALL SUFFICIENT OR DOES BOTTOM OF TUB REQUIRE SUPPORT?
A 2 x 4 stringer should be sufficient for the back edge. The steel tubs usually come with styrofoam on the bottom, don't remove this. It's basically for soundproofing. Unless your new tub has clips placed along the flange of the tub, then you screw it to the studs. If not, then trying to drill through porcelain and steel will be most difficult. If you want added support on the bottom mix some sand mix dry and set the tub in that.