My superintendent is in the process of installing one of these. Iy is 3 separate pieces. The problem I have is, the tiles have been leaking water for quite some time, and when one of the tiles was pulled off , there was all this old black crap that fell out from behind.He wants to put the tub surround directly over the old mess. Is this a health problem, as in, isn't this probably mold that should be removed first? and isn't the wood behind likely to be rotted out and damp?