I am getting my bathroom floors tiled (used to have linoleum). My contractor used nails and screwtight screws to set the hardiback board to the subfloor, without using any thinset. He told me that in new construction/newer subfloors, he would apply thinset, but that my subfloor would not absorb thinset (my house is 22 years old). If he applied thinset, it would eventually dry and crack as we walked on our floors. Is this correct or am I being told a load of garbage?
imo it's a load. the reason you put thinset under hardi isn't for stickum it's to prevent poping/flex you can get without it. floors are often uneven and on top of that when you screw down hardi some pokes through the back giving you slight lifted up effect. the thinset gives the hardi a solid uniform surface to sit on that's free of air pockets and high/lows.