I have a shower with a concrete pan, all angled correctly. It doesn't look like it leaks (the bathroom was half demolished when we bought the house). I want to put tile (probably pebbles on a sheet set in thinset) over the concrete. Is this possible? Any suggestions on this - should I score the concrete for better adhesion, seal it??? I am assuming I will be able to extend the waste by the 3/4 inch or so I will need. Thanks.