Bathroom: waste+joists=problem.

[mattrgee]

Hi all

I'm in the process of renovating the main bathroom and hit a snag with the bath waste.

Before I go on, the pictures show a grey 110mm pipe running next to a joist. This is not a soil pipe, but the extractor fan for the kitchen below the bathroom.

I've attached a diagram showing the layout of the new bathroom. Essentially, the issue is the distance of the bath waste pipe. If it was a straight line to the soil stack it would be exactly 3 metres (the max distance according the my plumbing book). However, utilising the existing notches in the joists brings it to 4 metres. The waste pipe is 40mm btw.

I could reduce the run by adding new notches to the joists but I'm reluctant to do this as the bathroom appears to have undergone previous alterations leaving a number of big notches in a number of joists. At the very least I'm tempted to add steel plates to the joists around any notches that aren't being used as baths are rather heavy when full!

Any bright ideas on how to run the bath waste within the 3m limit?
Would a 4 metre waste run cause issues?
Should the joists be strengthened (noggins, steel plates etc) based on what the pictures show?

 

[John D v2.0]

Firstly those notches are way too big anyway, but if you did want to use, you can use 50mm pipe on longer runs. So 40mm through the notches by the bathbat it's a free standing bath, then up to 50 for the long run to the stack. Make sure you get adequate fall on all the runs.

 

[muggles]

I'd certainly be looking to strengthen those joists. Maximum notch depth is 10% of the depth of the joist. 22mm plywood glued and screwed to either side using Gorilla glue would achieve the best results.

Can you not run the waste above ground to get yourself into the correct joist run, then upgrade to 2" as it runs under the floor?

 

[dilalio]

Can you not run the waste above ground to get yourself into the correct joist run, then upgrade to 2" as it runs under the floor?

Might just be Op’s Drawing @muggles but it looks like a freestanding modern bath and most of those have absolutely no clearance beneath them for waste and trap... had to recently dig a pit in a tiled and screeded floor to get one in

 

[mattrgee]

Might just be Op’s Drawing @muggles but it looks like a freestanding modern bath and most of those have absolutely no clearance beneath them for wade and trap... had to recently dig a pit in a tiled and screeded floor to get one in

Yes, flush to the floor free standing bath.

Firstly those notches are way too big anyway, but if you did want to use, you can use 50mm pipe on longer runs. So 40mm through the notches by the bathbat it's a free standing bath, then up to 50 for the long run to the stack. Make sure you get adequate fall on all the runs.

In terms of using 50mm, does it need to be 50mm all the way from the bath to the soil stack? Not sure I can access the full run to install 50mm the full length.

 

[John D v2.0]

In terms of using 50mm, does it need to be 50mm all the way from the bath to the soil stack? Not sure I can access the full run to install 50mm the full length.

No, not at all, you can start in 40. as long as you increase the pipe size as soon as possible, and definitely don't decrease it again once it's increased as that's a prime point for blockages.
Our bathroom has a pair of 50mm stack dropping from it which combine together, one has two showers from short 40mm runs and one has a double wash basin from a longer (slightly too long officially) 32mm run. Everything works fine and you can only get it to pull the trap slightly by filling the sink both fully and draining at the same time.
Once it gets to the 50mm pipe it has plenty of space for air pressure to equalise.
If you have a pinch point that wouldn't happen.

 

[mattrgee]

Update:

I've figured out where the waste pipes join the soil stack and can now access the joints (see pic). In theory, I could run 50mm for the entire run of bath waste. However, the existing soil pipe boss is the previous size: 40mm.

Could 50mm waste be stepped down to 40mm just before it entered the soil stack? If so, would this cause problems?

Do 50mm strap on bosses exist? Screwfix don't seem to sell them and even then, I'm not sure if there's room to fit a 3rd (see pic)

Thanks again.

 

[John D v2.0]

Could 50mm waste be stepped down to 40mm just before it entered the soil stack? If so, would this cause problems?

No, and yes, pinch points can cause Syphonage or even waste build up. You might be able to add an aav further upstream but I wouldn't know if it would be approved. You could try and see what happens if it won't be checked by building control.

Do 50mm strap on bosses exist? Screwfix don't seem to sell them and even then, I'm not sure if there's room to fit a 3rd (see pic)

https://www.screwfix.com/p/floplast-sp319bl-strap-boss-black/82795
Plus one of these
https://www.screwfix.com/p/floplast-rubber-boss-adaptor-black-50mm/53507