Also consider the length of the ducting and fans you are using.My turn to thread hijack. Looking at the Y piece you can buy for ducting, could I vent my bathroom and kitchen extractor through the same roof outlet? We currently have a crap kitchen hood that just filters rather than extracts, and going out through the wall is not feasible as the boiler is in the way. Could I go up through the ceiling and join it to the bathroom outlet? There'd obviously be backdraught shutters on both, so should stop the kitchen venting into the bathroom.