Drainage design — right approach?

[PeterW]

Pull the toggle and wait for the water to rise and float out of the skylight ..??

 

[Mr Punter]

4 hours ago, Dreadnaught said:

 

So could I just lay a drain at 1:8 over 20 m to achieve my 2.5 m drop, no chambers needed? Presumably not.

  

(Or 1:13 for the foul drain?)

 

You should have a chamber near the boundary of your property.  Make the drains up to that point as shallow as possible as they will be cheaper and safer to lay and easier to access and rod if ever needed (which they probably won't be).  Lay the bit from your chamber to the sewer connection as steep as is needed.

[Tyke2]

On ‎04‎/‎09‎/‎2018 at 17:30, Dreadnaught said:

 

So could I just lay a drain at 1:8 over 20 m to achieve my 2.5 m drop, no chambers needed? Presumably not.

 

(Or 1:13 for the foul drain?)

For domestic drainage the design is very easy.

 

For SW there is not a max gradient really. But for FW 1:10 is considered the max gradient to avoid the "splat" factor at changes of direction.

best practice is to have 300 mm of cover to the pipe at the head of the run i.e the top manhole. so with a 100mm pipe it would be 400m to invert. Use a proprietary plastic manhole. use a gradient of 1:60 minimum to get a self cleansing velocity (1:40 preferable) gradient over the distance  to the outfall drain (public sewer) . if the new pipe is then above the outfall you can use a backdrop as someone has stated before. or build a manhole at the higher depth just before the outfall manhole at the higher level and have the last short length at a steep gradient up t0 1:10.

 

Hope this helps.

[Jeremy Harris]

As I understand it, the old "rule of thumb" that you must not have too steep a gradient on a foul drain has now been superseded, as it's been proven that there is no maximum gradient for foul drains, and I can't find out where the idea that there should be ever came from.  Certainly it always used to be the case that people believed that there were problems with foul drains that were too steep a gradient, and many years ago I remember being told that this was because the liquid content would travel faster than the solids, leaving them behind.  Apparently that isn't true, so now there's no need for back drops, or any of the other ways to get around having too steep a gradient on a foul drain, and you can have one as steep as you like.

 

I suspect that a part of this is because we now use very smooth plastic foul drains, with very few joints and what joints there are being very smooth.  I can't find proof of this, but the regs now allow you to have a foul drain as steep as you like, which makes life a fair bit easier in some cases.

[epsilonGreedy]

16 hours ago, JSHarris said:

As I understand it, the old "rule of thumb" that you must not have too steep a gradient on a foul drain has now been superseded...

 

I can't find proof of this, but the regs now allow you to have a foul drain as steep as you like, which makes life a fair bit easier in some cases.

 

 

This is odd. A building standard accepted as received wisdom for years has been over turned and yet no reference can can found.

[Jeremy Harris]

5 minutes ago, epsilonGreedy said:

 

This is odd. A building standard accepted as received wisdom for years has been over turned and yet no reference can can found.

 

There's no reference in Approved Document Part H to a maximum gradient for foul drains any more: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/442889/BR_PDF_AD_H_2015.pdf

 

Read section H1 Section 2 and you'll find that now it only stipulates the flattest allowable gradient, with no mention of a steepest allowable gradient.  It's been like that for a long time, but folk memory lingers on...

[epsilonGreedy]

1 minute ago, JSHarris said:

 

There's no reference in Approved Document Part H to a maximum gradient for foul drains any more: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/442889/BR_PDF_AD_H_2015.pdf

 

Read section H1 Section 2 and you'll find that now it only stipulates the flattest allowable gradient, with no mention of a steepest allowable gradient.  It's been like that for a long time, but folk memory lingers on...

 

 

This is a bit Orwellian.

 

For years the industry chanted, "Shallow foul drain good, steep fall bad".

 

Then one day we wake up to "Shallow is good and err because there is no mention of steep it must be ok".

[PeterW]

Just checked my copies of Building Construction Handbook. 

 

The 1998 version says 1:40 to 1:80 for 4” / 110mm pipes for domestic and lateral drains and 1:60 to 1:100 for 6” / 150mm laterals and “Minor Sewers”. 

 

2009 version says minimum 1:80 for smaller and 1:100 for larger and no maximum. 

 

So looks like somewhere in that 11 year period it changed ..! Wasn’t there a 2002 BRegs update ..?

 

Edited to add I’ve been right back to 1991 and the same flow table is in each version with no maximum and even shows the graph for 1:10 gradient flow rate.... 

[Jeremy Harris]

7 minutes ago, PeterW said:

Just checked my copies of Building Construction Handbook. 

 

The 1998 version says 1:40 to 1:80 for 4” / 110mm pipes for domestic and lateral drains and 1:60 to 1:100 for 6” / 150mm laterals and “Minor Sewers”. 

 

2009 version says minimum 1:80 for smaller and 1:100 for larger and no maximum. 

 

So looks like somewhere in that 11 year period it changed ..! Wasn’t there a 2002 BRegs update ..?

 

I'll admit it missed me.  Until a couple of years ago I still thought there was a max gradient, believing the reason for this being the oft-quoted one that liquids travel faster than solids, so too steep a gradient would leave the solids behind.  Someone pointed me at the current version of Part H where it's clear that the old maximum gradient condition had been removed, so you can now have foul drains as steep as you like.  Very useful for some with deep invert levels at the sewer connection, or on steeply sloping sites, I suspect, as it may remove the need for an intermediate back drop chamber.

[Ed Davies]

Scottish regs are vastly less detailed. There's a bit more in the technical handbook but the main bit is just:

 

A drainage system outside a dwelling, should be constructed and installed in accordance 
with the recommendations in BS EN 12056-1: 2000, BS EN 752: 2008 and BS EN 1610: 1998.

I suspect you'd need to look at the change history of those to find the source of the change which is then reflected in Part H.

[PeterW]

8 minutes ago, JSHarris said:

Very useful for some with deep invert levels at the sewer connection, or on steeply sloping sites, I suspect, as it may remove the need for an intermediate back drop chamber.

 

Just saved me a £1500 man entry chamber and construction costs ..!! Will take it to the 1200mm max and leave it there ...