Minimum depth for foul drainage?

[Alan Ambrose]

Hi,

 

I am trying to work out the foul water falls which I think are quite tight. I know the invert level of the destination and the drainage distance and the ground level at the building, but what is the minimum depth of the pipe near the foundation?

 

I've seen references to 300mm to top of pipe and less when concreted over. Does that sound right? Also, is there a sensible minimum depth given you would prefer for it all not to freeze up?

 

TIA, Alan  

[PeterW]

22 minutes ago, Alan Ambrose said:

I've seen references to 300mm to top of pipe and less when concreted over. Does that sound right? Also, is there a sensible minimum depth given you would prefer for it all not to freeze up?

150mm from top of pipe but must be encased in concrete. Unlikely to freeze with a decent fall (ie 1:40)

[Alan Ambrose]

Ah thanks Peter,

 

Alan

[MattD]

Hi, just reviving this thread in order to understand the other half of the question (i.e. what is the actual minimum depth of foul drainage, where there are no nearby buildings) – @Alan Ambrose mentioned having seen references to 300mm to the top of the pipe – presumably that means without any kind of covering. The Building Regs (H1) seem a little unclear on this – §2.44 says:

 

Quote

Where pipes have less than the minimum recommended cover in Table 8, 9 or 10, the pipes should, where necessary, be protected from damage by a reinforced concrete cover <...>

 

Table 10 gives the "Limits of cover" of a thermoplastic pipe as being 0.6-7m. So that would indicate that 600mm (not 300mm) cover is required above all foul drainage. However, I'm unsure what is meant by "where necessary" above – does this mean that it is necessary to protect sections of the pipe where the minimum cover is not met, or should "where necessary" be interpreted as meaning there is flexibility in this, depending on the whims of the Building Inspector? I also note that §2.44 uses the word 'recommended cover', whereas Table 10 says 'Limits of cover'. The former suggests room for interpretation, whereas the latter does not.

 

In short, I'm trying to work out if it is always necessary to bury foul drainage by ≥600mm, where no reinforced concrete cover is used.

[joe90]

My drains were laid to a minimum of 300mm and passed by BC. My understanding was if car traffic was going over then re Inforced concrete over was required.

Edited October 4, 2023 by joe90

[MattD]

7 minutes ago, joe90 said:

My drains were laid to a minimum of 300mm and passed by BC.

Thanks Joe; was this encased in concrete? Everything I'm reading (other than on this forum, e.g. Building Regs and this guide ) points to 600mm being the minimum cover where concrete isn't involved... Would definitely appreciate if someone can clarify where this discrepancy comes from as 300mm would be a lot easier to achieve!

[joe90]

Just now, MattD said:

Thanks Joe; was this encased in concrete?

Sorry edited my post about concrete over. As BC is not consistent I would suggest you confirm with your own BC officer.

[kandgmitchell]

I'd argue it's all about protecting the pipe from damage. A minimum depth of 600mm in fields is there to prevent farm machinery from affecting the drain. However, a drain laid under a garden path outside the back door of a house is much less likely to come to harm. So arguably Table 10 in Part H wouldn't apply to my second scenario. The 300mm figure for drains I think arises from Diagram 10 which shows the bedding arrangements. I would have thought that was perfectly acceptable in residential gardens with no car loading.

 

As for freezing, well all the external gulleys would have their traps frozen before the drain becomes a problem and most foul sewers are quite warm from hot water, washing machines, dishwashers etc discharging into them. I've usually found upon lifting a cover, that drains smell of washing powder rather than what most people think they ought of......

[MattD]

Thanks all. Spoke to BC who confirmed that the 600mm mentioned in the regs is just guidance and 300mm ought to be sufficient when away from buildings and vehicle traffic. In other words anything less than 600mm is taken on a case-by-case basis, but in an uncomplicated case, 300mm is fine.

[Mr Punter]

Don't go deeper than you need to.  I have had 150mm cover on drains near to the building.  Would need a bit thicker with RC capping if under a driveway.