This is probably one of those "suck it and see" or "how long's a piece of string" questions. I have to remove a bath and install a stone shower tray, in its place, on a concrete floor. I'd like to keep it as low as possible. What happens about the waste and waste pipe? I'll use a shower trap and once it's in, it's in. Not happy about difficult access but I'll just have to live with it. I need to take the waste pipe through an external wall where it'll discharge into a gully (the one the bath currently discharges into). I don't think I can use the hole the existing bath waste pipe goes through as it'll be too high for the shower tray. If I make a channel in the screed am I likely to be creating problems for myself where the new waste pipe penetrates the external wall? I don't want to risk introducing damp problems.