Doing a bathroom with bath,separate shower cubicle and toilet/basin. shower waste runs under bath so was going to tee it in under there but do not want any syphoning of the trap when bath plug is lifted (down stream of shower tray. Trap for shower is standard so do I run it separate to 100mm soil stack out side or can I join into bath waste. Can't for the life of me remember how we did it on site. 
