I am creating an office in a VERY old [about 60 yrs old] garage. I am also on a very tight budget (I am a student). The garage floor is in somewhat bad shape (pitted, rough, etc). My plan was to use 1/2" Durock ( cement board, backerboard)on top of the concrete slab, with the smooth side up (doesnt really matter ... ). I am most certainly NOT going to tile over the Durock (as even the slightest flex will pop tiles). The purpose was to "quickly and cheaply" get a fairly flat, smooth subfloor. Then I am either going to put tongue-and-groove plywood down, with office-style carpet on top, or a faux wooden floor.

My question is, what is the best way to secure the Durock to the concrete slab? I am aware that Durock is NOT meant to be secured to concrete. However, I do not have the budget to fix the existing slab (using self-leveling concrete, etc). I thought PL-400 would be strong enough to secure it, but one test piece has proven otherwise. Then I figured using flat head tapcons (or similar), but that may also not be enough. Again, as I am not putting down tile, a minute amount of flex may be ok, but I want it secured as best as possible. Should I use thinset or mortar to secure the Durock down?

Any help is greatly appreciated.