I am presently installing bathroom tile. I have tongue and groove boards, then 3/4 in particle board. I want to install the backerboard over the particle board without removing it. I plan on using thin set under the backerboard. The flooring is solid and level and walls and cabinets have been installed over the particle board. Therefore removal of the particle board is going to be a problem. I also plan on using 1 5/8 screws to get through the particle board and into the wood floor below. Does anyone see a real problem with this plan? Also, what are the advantages of using a latex additive (modified thinset) instead of water for under the backer board? Will this keep the particle board from soaking up the moisture from the thinset? I plan on caulking around all outside edges for movement and waterproofing. Especially around the tub. When the particle board was installed (20 years ago) they put tar paper under it to keep moisture from the floor. I thought about putting down a layer of this before applying thinset and backer board. Any thoughts?