Enough slope?

MarcusMaximus

I have a question about the slope of the waste pipes in a bathroom refurb. Both the basin and the shower pipes running as far as the photo below shows (basin is the one in the middle and runs about 2 metres) are well over 18mm/metre. That 50 cm section of the shower waste goes from 42mm above the floor under the trap to 7mm on the far right, which I think is substantially more than the minimum, is that right? (I measured from the bottom of the pipe, not the centre. I know it should be the centre but I'm still measuring the difference in height either way I think.)

However the section that goes through the wall to the right which is 30-40cm long and goes into the stack after a sharp drop, is very hard to measure as I've no 'floor' to measure from. So I put a level on it inside the wall and this is how it looks:

Is there any way to tell from the position of the bubble what the slope is, even approximately? Does it look like enough?

However I was able to measure the difference in height from floor to pipe centre over the first 100mm run. It is straight all the way so this should give me some idea of the slope over the whole run. It goes from 30mm to 24mm approx. = 6mm difference. So over the course of a metre that would be 60mm. Should that be ok? Thanks.

hesker

If the slope is too much the higher flow rate can create a siphon effect that will empty your water trap. Not sure what the max drop is but you can probably google it

MarcusMaximus

Ok, yes I'll check that thanks. Shouldn't think it's too much as I had to move the existing pipe down to floor level to accommodate the low profile plumbing. (There was a bath there previously and I'm replacing it with a shower.) So I was only able to put what seemed to me to be a slight downwards angle on it through the wall, as described above.

MarcusMaximus

https://www.boards.ie/discussion/comment/123891781#Comment_123891781

A really great tip thanks. I looked at this more closely and the overall fall is about 72mm over a 920mm run, which is above the max 55mm/m and substantially more than what's recommended. I can't change the piping without major surgery so I'm going to replace the trap with an anti-siphon one if available, or else fit an AAV. Hopefully that should help reduce the chances of siphoning and other problems. 🤞

Edit: It turns out that the 55mm max is a conservative figure and that many manufacturers and other sources say 18-90mm is the acceptable range. So my 72mm fall over a short metre run should actually be ok according to that.

Can anyone confirm this as I would like to be sure before installing the tray and tiles etc?

hesker

As a suggestion could you have a more gentle slope first and then a more severe one later on.

MarcusMaximus

That would depend on how much further on the steeper slope should be. The original waste pipe is on the far right in the photo and goes through the wall at that point. It is also located under the other pipes and needs to stay that way to accommodate the shower tray. I don't think I can move that without huge disruption.

So the only real way I could achieve what you suggest would be to either orient that first 45° elbow downwards and then have a much less steep run to that pipe, or else bring the run down at a point as close to it as I can manage somewhere else in the first accessible section. Any good?

hesker

I don’t know really. I was just thinking out loud and trying to understand why there is no siphon issue with my kitchen sink. Once the pipe goes laterally out the wall there is a very steep fall to the gulley. That’s where my suggestion came from.

Maybe post in the plumbing and heating forum.

MarcusMaximus

Well I think you're probably right that the steeper slope might be better further away from the trap. It's just to get there I'd have to have an immediate drop at the first elbow and then a more gradual slope from there. However I think that initial drop could also be problematic.

I had put this thread on the Plumbing and heating forum initially but got no replies so I asked for it to be moved here! Ah well, thanks for your help anyway. 😊

MarcusMaximus

Here is the relevant page from the current building regulations showing minimum/maximum slope being 18-90 mm/m. The diagram only shows sink and bath but I presume it's the same for a shower.

Does this mean that the setup in the photo in my first post with 72 mm/m overall slope should be ok, or is there still a risk of siphonage?