Joining sink waste to shower waste system

[stevext]

Hi All

After a bit of advice from those in the know.
(1)I'm extending my bathroom and re-fitting it to include a shower cubicle and moving the basin,can i attach the 32mm waste pipe for the basin into the 40mm waste for the shower so only a single waste pipe will go into the rainwater hopper on the outside wall?
(2)
My house is an older style property and has a combined drainage system into which the bath/basin/sink all dishcharged into the drainage via the guttering downpipe hoppers,is it still alright to connect via this method?

many thanks for the advice

Steve.h

[marrtin]

Theoretically, each should have it's own waste, but it is often found that wastes are combined.

The trouble with combining a basin and shower is the difference in heights and flow rate. When the plug is pulled in the basin water will almost certainly rise up into the shower tray and when the last drains away, then sucks the water from the shower trap allowing odours to enter the cubicle.

The problem can be overcome by clever plumbing and by using anti syphon traps, but unless you absolutely have to - don't.