Waste crossing joist

Good morning folks.

I'm remodeling my bathroom and moving a basin to a different wall. I thought that everything was working out as I'm removing a bath and putting in a walk in shower, and the fall was more than sufficient for the extra run.

I had planned to tee the basin into the shower waste and stick an anti vac trap in. Problem is that the waste has to cross 1 joist and would foul it if run naturally. I know that the correct way to deal with this would be to drop the lot and tee into the stack lower down, but in my case that would involve replacing a 3in cast stack (Combined drainage system) which is what I am not too keen on, and even then I am into issues wit fittings to connect a waste to 68mm downpipe.

My thoughts (Bodge?) are that I could drop a 32mm flexi waste round the joist which involves about 2in drop. I know that this efectively creates a second trap, but it is accessible and I would connect it with compression joints so it could be removed and cleared.

Would this work or am I missing any factors?

If you were to pass your waste under the joist, of course that would create another shallow trap, and protrude through the ceiling below if it was on a first floor, wouldn't be so much of a problem if it was on ground floor with a raised timber floor, however, in my opinion, if you were to drill a large hole to pass the 32 dia waste pipe through the one joist, drilled in the middle of joist, and as near to wall or foundation support for that just one joist, it is not going to weaken in strength to that extent that it should poses any danger, or undue concern, it would do so if it went through more joists, and all depending of course on the size of your joist, if worst comes to worst, one can bolster the joist with at least 3mm thick steel plates on either side and bolted to joist with nut and bolts, and drilling larger hole through it and the steel plates to pass your waste pipe through. Sure there may be other ways too.

Thanks Mike.

It is a suspended floor with generous crawl space below.

Drilling joist is an option as they are 250mm , and would have been my default, except in this case the joist runs around 5 or 6 inches out from the parallel wall and for some reason is doubled up which makes me a little uneasy drilling it, not to mention more difficult to drill should I take that route.

Any views on the other downsides to my suggestion. Would it significantly restrict flow? The entire horizontal run is only around 0.5m.

The other option I have been considering is popping up above floor level the near side of the joist and running the waste to it through the vanity unit which I am using anyway. This seemed even more of a bodge , and although probably a better from a fluid mechanics perspective, I'd be embarrassed if anyone else saw a waste sticking up 12-14in outfrom the wall.

In the case of a suspended floor, there is no unsightly protrusion if it were on a first floor so yes I can't see it being much of an issue as long as the pipe passing below the joist is curbed gently to stop trapping things, what sort of bend are you going to use, or are you going to form your own gentle bend using a heat gun , minding that you would have a fair bit of water trapped in a gentle bend. (or the trap that will be formed) and you also said about using compression joints so how easy would that be to get to in case of a blockage., a gentle bend could be cleared more easily by drain clearing spiral from the above through a T section joining bath waste and sink waste.

I intended to use one of these. It is a magic flex from toolstation. I have used these previously and they are much more robust than those flexy concertina'd shower things. They are also twinwall so less likely to create blockages.

I mentioned connecting with compression. These are solvent, but I thought that it was inevitable that there would be some deposit at a low point so access may be advisable.

Bending plastic is an interesting proposition. I'll have a go at this and let you know how I get on. Is there any difference in pushfit/solvent pipe when it comes to bending?

I have used heat gun with care to bend and form various angles on PVC pipes, of both push fit and solvent joints, including one where the WC pan coupling angle had to be altered a few degrees, and application of heat worked fine, care is needed when heating as material can become brittle and bubble up if heated rapidly, and deformation can occur, so stuffing a pipe with lots of packing (I used a wet rag stuffed up inside a pvc pipe, packed it real tight, and then applied heat and formed a gentle bend without causing it to deform, but I think your approach with flexible bend may work ok, only worried about stuff getting stuck in the ridges, with that length you will probably end up making it more like a U trap and hence it may be more vulnerable to blockages.

Mike

Bent a waste pipe applying heat and was pleased with the outcome, however I realised that there was a point on the joist where it was not doubled up and I managed to drill a hole in the centre and run along it to the correct point. Probably a better long-term job.

Thanks for your assistance anyhow.

James

Good on you, a much better solution in the long term just as things should be intended, and no need to worry about sediments depositing in the dipped part and no water trapping either.