Bathroom Sink and Shower Waste Pipe Question

I apologise in advance if my teminology is not spot on, but I want to connect a new 32mm waste pipe for a new bathroom sink into my existing 40mm shower waste pipe.

The location of the new sink would mean I would have to drill through joists to reach the soil pipe, which I want to avoid if possible. If I connected it to the existing shower waste pipe it would mean I could lay it with the run of joists.

I am planning on using a 40mm to 32mm reducer connected to a 40mm equal tee.

Does this sound ok to anyone? Or is it likely to create problems having two waste pipes being connected into each other?

Any help would be appreciated and thanks in advance.

The obvious risk is that the basin waste water will come out of the shower waste. All depends on slopes and distances. Nearer the T is to the soil pipe, the better.

As Kev has just replied elsewhere:

kevplumb said:

one of these

Click to expand...

They prevent backflow.

You shouldn't really be cutting 32mm out of your joists... will weaken the floor... You should also cut joists in the right place, not anywhere, but within limits between 25% and 40% (I think!!!) of the entire length of the joists span. Seen BIG holes cut in joists and the whole floor is dangerously weak.