I just installed an acrylic tub in an alcove in a bathroom. I used 5/8 green board (mold resistand drywall) in the entire bathroom. Instead of doing what I should have and pushed the tub to one side while furring out the other wall, I center the tub in the alcove and the 5/8 fit perfectly behind the tub flange on each end. Now the tub flange sticks out 1/4" which presents a problem when tiling.
My quick fix idea is to install 1/4" Hardibacker over the top of the 5/8 green board bringing the hardi flush to the flange and and then tile over that to within 1/8 of the tub deck.
The Question .... If I install it over the drywall can I just use long screws and attach it per the normal screwing schedule or do I need to put some sort of mastic behind it to help support?
Hi, that tub flange needs to be screwed to something solid... the studs + any furring strips you need for a tight fit. The backer, whatever you choose, needs to go over the flange, not behind it. Is this just a tub? A tub and surround? A two piece remodeline tub/shower combo?
You also want to consider things like, any shower valves or other fixtures that might need to line up with your finish walls... for instance, if you've centered the tub in the alcove, you may now find out that a mixing valve, if you have one, will no longer line up with the new wall...