Minimum fall required for foul pipe?

Ruggers · 11 Apr 2021

What's the minimum fall for a foul pipe underground over a long distance like 70m? The 110mm pipe.

I know 1 in 40 is ideal. Also, is there a minimum depth the inspection chamber bottom should be from when it exits the property under building regs?

Notch7 · 11 Apr 2021

Ruggers said:
What's the minimum fall for a foul pipe underground over a long distance like 70m? The 110mm pipe.

I know 1 in 40 is ideal. Also, is there a minimum depth the inspection chamber bottom should be from when it exits the property under building regs?

it depends

"A foul drain taking the discharge from a minimum of one WC can be laid at 1:80 (12.5mm per metre)."

charliegolf · 11 Apr 2021

Out of interest... Is it true that (other than vertical downpipes) a sewer fall can be too steep, and counter-productive? Or did I dream that?

Notch7 · 11 Apr 2021

charliegolf said:
Out of interest... Is it true that (other than vertical downpipes) a sewer fall can be too steep, and counter-productive? Or did I dream that?

That is true, the liquid travels too fast for the err......

A back drop manhole solve that.

Ruggers · 11 Apr 2021

I need to connect to a chamber in the road 73m away for a 4 bedroom house with 3 toilets and 2 bathrooms. 1 in 40 isn't achievable because the chamber isn't 1825mm deep.

^woody^ · 11 Apr 2021

charliegolf said:
Out of interest... Is it true that (other than vertical downpipes) a sewer fall can be too steep, and counter-productive? Or did I dream that?

Its known as the "self cleansing velocity"

A rule of thumb is 4" pipe 1:40 and 6" 1:60

Hugh Jaleak · 13 Apr 2021

Too shallow a fall and the solids can be left behind, whilst the water seeps away. Too steep a fall, and the water rushes off, leaving the solids behind. As per Woody, the correct fall is required to achieve a Self Cleansing Velocity where the water will take the solids along with it.

What sort of fall can you achieve over that distance? If going with a shallower gradient it needs to be constant, and ideally a straight run. If you cant get a suitable fall from the property to the nearest gravity sewer, the only option, (aside from a Septic Tank/Treatment Plant), is to pump it.

Ruggers · 16 Apr 2021

@Hugh Jaleak What do you mean by constant? Apart from one 90 degree to turn around the property after 8m, the rest should almost be a straight line apart from a slight offset possibly 15 degrees. But there will have to be rodding/inspection points at the recommended distances which I need to find out.

I believe lower gradients can benefit from being laid onto a dry ready mix of sand & cement to prevent any future sagging and then surrounded with pea gravel. I think 1:100 is achievable but don't want any less and would really like to edge it towards 1:100.

As the upstairs pipe exits the rear of the property into a chamber, i could pass the pipe under the block and beam floor and out to the front in a straight line down the drive to the road but i'm not overly keen on pipes in places you can never access them again. It would save 9/10m.

Hugh Jaleak · 16 Apr 2021

By 'Constant' I mean it must maintain the gradient over it's entire length, no dips, rises or flat spots, or you'll soon have a problem.

Used to be a maximum of 100 metres between Manholes, not sure if that's still the case. Know of plenty of runs that are considerably more mind you!

I've never bedded on anything other than peagravel, (or concrete when a shallow run under a building.) Going under a block and beam floor will be fine, provided the pipe has been laid correctly, virtually every new house is built using this method of flooring now I think. Check with BCO to see what they'll allow.

Ruggers · 18 Apr 2021

Since my last post i read 22m between chambers from this site but others mention 100 as you say.
https://www.jdpipes.co.uk/knowledge/inspection-chambers/when-is-an-inspection-chamber-required.html

Does pea gravel offer good support to prevent any sagging despite it being whats mainly used?

Hugh Jaleak · 18 Apr 2021

As I said above, never known pipes to be laid on anything other than peagravel, and I've seen everything from 100mm house drains to 1200mm storm sewers go in the ground, even box culverts are sat on a gravel bed. Just the size of the stones gets bigger for the bigger pipework.

This is what you need to read, Drainage and waste disposal: Approved Document H - GOV.UK (www.gov.uk) Appreciate they do state shorter distances between access points, (i.e. Chambers), but if you're putting in a straight run, in the event of a blockage, any decent jetter should carry around 80metres of hose, so can get at the issue from one end or the other. Your BCO will have the final say though, speak to them and agree the spec before doing too much work!

Ruggers · 19 Apr 2021

Thanks for your advice. I'll have a read through it.

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Minimum fall required for foul pipe?

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