In our small groundfloor bathroom I\\\'m planning to join bath and handbasin waste to the 110mm WC pipe which goes vertically thru the concrete subfloor to join the drain. I want to try and hide plumbing under the floor in 2 inches or so of celotex insulation topped with cement board and tiled. I have a double boss adaptor solvent fix type, which means I\\\'ll need to chip at least an inch out of subfloor around the plastic waste pipe to get it hidden and provide enough fall. Bath waste has a valve on it to stop siphoning, do i need one for the basin? the Basin is a tiny cloakroom one, goes into a bottle trap, straight down almost a metre , then turns nearly horizontal along the floor and round another right angle for about another meter and a half, into the waste. When its done it\\\'ll be hidden underneath an electric heating matt and some big tiles, Also want to put the hot and cold basin 15mm pipes under floor, then buried in wall, - do i need ducting or is all this asking for trouble? 
