Conor

Part N - min and Max falls

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Posted (edited)

Ok. I'm not a waste water person, and my ground worker is only used to road drainage.

 

We have our main sewer connection at the bottom of the garden, with the first inspection chamber from the house (courtyard) 12m upstream. 

 

Main sewer connection and manhole has been done. I'm aiming for a fall of 1 in 40 from the courtyard chamber to this one.

 

We've very little cover to work with under the slab area. Looks like most falls will be around 1 in 40 to 1 in 60. That has the crowns of the pipes pretty much at the surface of the compacted stone. We have the main foul stack coming out, along with a drain for shower and basin in bathroom, a drain for a utility, MVHR condensate drain, and finally a 5th drain for a possible future kitchen.

 

I'm reading Part N and can't quite figure out the min and Max falls.

 

For "branch discharge pipes" the range is 1 in 18 to 1 in 90. Is this what I should be sticking to for these under slab pipes?

 

What's the ideal fall for the main vented stack that's comes out?

 

Min fall for a combined sewer according to table 3.2 is 1 In. 40, which matches my design, so all good there?

 

Setting the courtyard MH at about 10am so no panic on the replies....

 

 

Edited by Conor

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1 in 40 to 1 in 60 is fine for underground. You can go to 1 in 80 if the flow is high enough but in my opinion best to avoid if possible.

 

There is no maximum gradient as far as I'm aware (there used to be, but someone, somewhere decided it was no longer necessary). It was thought too high a gradient would result in solids and liquids separating, but this is apparently not the case (not done any first hand research myself).

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Can go as low as 1:80 if needed - any way you can go in slightly steeper and use an 11 degree elbow to drop it to 1:80 run into the manhole..?

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Posted (edited)

Thanks guys. We're setting out the courtyard MH now, will see what the levels are like. I'd assume I'd be best sacrificing a bit of fall for the main run for the 12m length to the sewer? Combined system, so will also be carrying roof drainage so high flow.

 

As you can see, tricky enough job. If we get it wrong, there's no second chamce.IMG_20200630_101544.thumb.jpg.1e811e30e602747dd45f44f727e84bb4.jpg

Edited by Conor

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That's what I did for my longest run to the treatment tank, no problems so far.

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