Waste pipe problem

[Jilly]

Hello, the plumber has just more or less finished first fix and I noticed the bath waste pipe hasn't got a fall. In my usual fashion I am now in the rainforest doubting his work (perhaps unfairly). He has agreed to correct this, but I can't help thinking this black waste pipe looks very flimsy and seems to have no provision for differential movement when it will be out of site under the block and beam of the extension. Is there some way of double skinning them? If any of these joints leak it will be an almighty problem to fix. Am currently trying to upload a photo

[Jilly]

 

[markc]

I am assuming he has solvent welded them in which case they won’t leak, I would be more concerned with the smaller pipes becoming blocked and no way to get to them