I converted a standard tub area with a tiled shower stall. The floor of the shower stall is tile. Under the floor tile the guys that did the work put cement board and then laid the tile directly on the cementboard. The grout keeps coming out and some of the tiles are loose. We pried them out, replaced them and regrouted but the problem continues. I do not want to rip out the entire floor of the shower. Is there a way to regrout, and then seal that floor so that moisture will not get through it? Is there some product that will work to keep the grout from breaking up? Thanks

Who ever did the original floor job cut a lot of corners. Your cement board has a tiny bit of movement and flexing so the grout will always come out and no amount of caulk will help. If you want it done right you need to rip it out down to the wood floor. Check the web for sites that explain shower pan installation or redi-made shower floors. Try this one for a start Repairing Leaking Shower Pan (http://www.thetiledoctor.com/repair/leakypan.cfm)