Exceeding maximum fall for sink waste pipe?

[MJNewton]

I believe the recommended fall for 40mm waste pipes is 18-90mm per metre... I am struggling to stay within this though on a new kitchen sink installation in order to avoid some obstacles and fit everything in. It might end up being more like the equivalent of 120mm per metre - might this cause issues? The waste pipe is just under a metre long and connects into an internal soil stack via a strap boss adapter.

 

I could maybe put in a bit of a dog leg with two 45 degree bends to allow the pipe to be a bit flatter but that feels like more a bodge than having too steep a pipe...?

Edited October 9, 2020 by MJNewton

[PeterW]

No issues - carry on ..!

[MJNewton]

I think it was the risk of the water moving too fast that was my concern, resulting in gurgling. Or is that unlikely in what I am assuming would be regarded as relatively short run? I don't think I could squeeze in an AAV if so.

Edited October 9, 2020 by MJNewton

[PeterW]

Can always go past the last connection with a tee and then run the pipe up the back of the sink  with an AAV on it. 

[MJNewton]

Okay thanks; might experiment with positioning a bit and see what might fit if the need arises.

[Conor]

For such a short length, doesn't matter.

[Nickfromwales]

10 hours ago, Conor said:

For such a short length, doesn't matter.

1000%. 
Just plumb it in and worry yea not. If you’re super worried, run it in 50mm so you are guaranteed an air break over the descending fluids. 

[Conor]

1 hour ago, Nickfromwales said:

1000%. 
Just plumb it in and worry yea not. If you’re super worried, run it in 50mm so you are guaranteed an air break over the descending fluids. 

And I highly doubt the BC officer will get a measure and calculator out for all little branches in a build. My (external foul) drainage was signed off from about 20m away...

[MJNewton]

Thanks everyone. It's more for my own peace of mind rather than BR compliance. I've actually managed to run things now just within spec having moved things around a bit. 

[Adsibob]

I have a similar problem, but at the opposite end of the scale. I have quite a long 40mm sink waste pipe, about 1m then a 45 degree bend, then about 30cm then another 45 degree bend, then a long piece which about 2.6m long.

The 2.6m long piece looked rather straight and level so I took a spirit level to it and it dos have a fall, but not very much. Maybe 1:100. Is that enough? 

 

[Carrerahill]

On 09/10/2020 at 21:15, MJNewton said:

I believe the recommended fall for 40mm waste pipes is 18-90mm per metre... I am struggling to stay within this though on a new kitchen sink installation in order to avoid some obstacles and fit everything in. It might end up being more like the equivalent of 120mm per metre - might this cause issues? The waste pipe is just under a metre long and connects into an internal soil stack via a strap boss adapter.

 

I could maybe put in a bit of a dog leg with two 45 degree bends to allow the pipe to be a bit flatter but that feels like more a bodge than having too steep a pipe...?

You could always stick to standards then drop it on 45's more abruptly somewhere to stay within regs.

 

With sinks, any solids should be very small and lightweight, say coffee grounds, so I don't see too great a fall being a huge issue, but there does remain the chance that solids will be left behind by fast flowing water abandoning its solid friends. The benefit of vertical falls is that yes it increases waste speed for a moment, but the water/solid velocity remains within spec on the horizontal (diagonal), with everything moving faster on the verticals together before continuing at a solid clearing rate. 

[markc]

1 hour ago, Adsibob said:

I have a similar problem, but at the opposite end of the scale. I have quite a long 40mm sink waste pipe, about 1m then a 45 degree bend, then about 30cm then another 45 degree bend, then a long piece which about 2.6m long.

The 2.6m long piece looked rather straight and level so I took a spirit level to it and it dos have a fall, but not very much. Maybe 1:100. Is that enough? 

 

Sink wastes are mostly water with very few solids so shallow or steep slopes rarely cause a problem. It’s long run foul drains where falls are critical to keep the “logs” moving along

Edited January 13, 2022 by markc