Hi, I need help with a problem.I need to tile in a shower/bathroom and want to use hardibacker onto existing plaster board. Problem is, the plaster board appears to be made of hollow sections and has only one upright in the middle! Can I use gripfill and plug and screw the boards directly onto it? The wall concerned separates the bathroom from the kitchen. Conversion of this old victorian building to flats were done by John Wayne and co. in the 70's.