Underground waste minimum depth ?

[gasjunkie]

Hi team,

Garden room project requires a new drain running to my rear most chamber.
The problem is the bottom of this chamber is 0.45m so it's quite shallow.
The garden room is higher so there should be enough fall but the new pipe won't be all that deep.

A bit of googling and I'm getting mixed answers...
I'd add it's running under the lawn so no vehicles just a lawn chair in finer weather.

Thanks in advance.

.

 

[Ric2013]

You are right in that the gold standard for Building Control will be Approved Document H and, while it suggests a minimum cover of 0.6m for either plastic or clay pipe laid in a field, it then suggests that if this level of cover cannot be achieved, 'the pipes should, where necessary, be protected from damage' and goes on to suggest a method, which leaves a lot to interpretation.

My feeling is that in a garden rather than a field where heavy mechanical agricultural machinary is unlikely, it is not 'necessary' to protect the pipes from damage. It will ultimately be your Building Control Officer calling the shots and as plumbers we tend to leave these decisions to Building Control, but my inclination would be to try to see if he or she will come to the decision that "Supersleve" clay (more resistant than plastic, but easy to use as it has push-fit plastic joints) with some granular fill around (or even over it to increase the bedding factor to beyond the class B method that the depths are based on) and the fact that Supersleve claims to often exceed the BS EN 295 that the depths are based on (thus increasing the bedding factor further - see the Supersleve manufacturer's installation guide online) is already belt and braces in this situation. Failing that, you can start to look at reinforced concrete covers or whatever, but it just strikes me as frankly ridiculous in a garden.

 

[Timmy D]

You are right in that the gold standard for Building Control will be Approved Document H and, while it suggests a minimum cover of 0.6m for either plastic or clay pipe laid in a field, it then suggests that if this level of cover cannot be achieved, 'the pipes should, where necessary, be protected from damage' and goes on to suggest a method, which leaves a lot to interpretation.

My feeling is that in a garden rather than a field where heavy mechanical agricultural machinary is unlikely, it is not 'necessary' to protect the pipes from damage. It will ultimately be your Building Control Officer calling the shots and as plumbers we tend to leave these decisions to Building Control, but my inclination would be to try to see if he or she will come to the decision that "Supersleve" clay (more resistant than plastic, but easy to use as it has push-fit plastic joints) with some granular fill around (or even over it to increase the bedding factor to beyond the class B method that the depths are based on) and the fact that Supersleve claims to often exceed the BS EN 295 that the depths are based on (thus increasing the bedding factor further - see the Supersleve manufacturer's installation guide online) is already belt and braces in this situation. Failing that, you can start to look at reinforced concrete covers or whatever, but it just strikes me as frankly ridiculous in a garden.

Supersleve, 150mm filler, then concrete paving slabs. Should leave you about 150mm of backfill for grass. Not ideal but about the best option I can see.

Edit: messed up the formatting, it’s staying.

 

[Ric2013]

Supersleve, 150mm filler, then concrete paving slabs. Should leave you about 150mm of backfill for grass. Not ideal but about the best option I can see.

Would you agree he's get away with less, but your option will probably be what BCO will want to see, or am I being cavalier?

 

[Timmy D]

Would you agree he's get away with less, but your option will probably be what BCO will want to see, or am I being cavalier?

It’s a garden, not heavy traffic, so would get away with less than the .6m minimum.

However if BCO are involved, I’m sure the slabs will be a must.

Near in mind, it’s not a huge addition, it’s not a poured slab, just precast concrete slabs.

You want to ensure you have more than minimum fall, as it will be well above the frost line.

 

[gasjunkie]

ok thanks guys, appreciated and makes sense.

I know the ideal is 25mm, I can't get near that - how little fall is practical?

 

[Ric2013]

ok thanks guys, appreciated and makes sense.

I know the ideal is 25mm, I can't get near that - how little fall is practical?

Studies have shown you could have a fall of 1:1000 without any statistically significant increase in the incidence of blockages, but with a much reduced flow - but if you look at page 16 of Approved Document H (Google it) table 6, you're looking at 1:40 with no WC or 1:80 with at least 1 WC (12 to 25 mm/m) as a 'recommended minimum'. If flow is not an issue and you only have a handwash basin or whatever, a lesser fall may be practical, but the problem might be getting the BCO to see it that way.

Really, the answer is either you need to comply with Approved Document H (methods shown in approved documents are already approved - hence the name), or you probably need something like a building services engineer to write the technical specification. You are ALLOWED to deviate from the approved documents, but the onus is then on you to convince the BCO that what you are proposing is a valid alternative way to comply with the Building Regulations.

 

[gasjunkie]

ok thanks Ric2013...

The bit that confuses me is that 1:40 is a lesser gradient because I have. WC ?

So I have one WC and basin at the end of my 17m drain addition then it meets the house and multiple bathrooms along it's total 36m length.

If I run a completely new drain for the whole lot, what is ideal fall pls?

A 110mm foul drain taking the discharge of less than 1 l/s should be laid at a 1:40 (25mm per metre) fall. A foul drain taking the discharge from a minimum of one WC can be laid at 1:80 (12.5mm per metre).

 

[Ric2013]

ok thanks Ric2013...

The bit that confuses me is that 1:40 is a lesser gradient because I have. WC ?

Yes, that is what ADH says, anyway.

 

[Chuck]

The bit that confuses me is that 1:40 is a lesser gradient because I have. WC ?

1:40 is a steeper gradient than 1:80. When the WC is flushed the relatively rapid high-volume discharge 'sweeps' down the drain, which helps clear it so a lower gradient Is required.

 

[Ric2013]

Correction. What chuck says is correct.

 

[gasjunkie]

ok thanks for clarifying guys...

So I have one WC and basin at the end of my 17m drain addition

Then it meets the house; multiple bathrooms along another 17m original section.

Total 36m length, five loos in total.

If I run a completely new drain for the whole lot, what is ideal fall pls?

25mm per m or 12.5mm per m