How deep do my drains have to be?

[jess marley]

In the main, floor levels inside are the same those outside here. If I reduce the levels outside by 8-10 inches or so, do I then have to re-lay the drains by the house so that they are some standard depth below the surface? The bit I can see under the man-hole goes down to about 18 inches, so to reduce ground levels above this would surely have all wastes plummeting past only a smelly 10 inches from my perfumed toes at this point? Is this o.k?

Happy New Year.

 

[middi]

I wouldn't bother worrying about regulations on existing drains, just worry that if they are too shallow they are likely to crack under heavy traffic or even freeze, also make sure that they are sound.

Of course regulations apply and ideally suited to new builds, usually on older properties and with the passage of time the damned things are buried about 10 feet deep on some instances and seldom a problem.

I am not sure about regs depth, but vaguely remember them being 18 inches deep.

I am sure someone here will clarify.

 

[Gervase]

Part H of Building Regs is wonderfully vague on the depth of rigid drains:
"The depth of cover will usually depend on the level of connections to the system, the gradients at which the pipes should be laid and the ground levels."
However the recommended bedding for rigid pipes allows for 100mm of aggregate below and 150mm above which, given the 100mm diameter of the pipe, gives you a trench at least 350mm deep at its shallowest end.

Flexible drains should be at least 0.9 metres below any road, and 0.6 metres below any field or garden, and no more than 10 metres deep (Yes, thats TEN metres!).

But, as middi says, I wouldn't worry. Just make sure the gradient is OK to allow for efficient drainage and avoid blockages.

 

[MdB]

But, as middi says, I wouldn't worry. Just make sure the gradient is OK to allow for efficient drainage and avoid blockages.

And what sort of gradient should that be? I will need to replace my drains at some stage due to old pitch fibre pipes and the current system is very shallow with a very small gradient. A better gradient would mean that the pit will need to be replaced and lowered which will cause problems due to the level of the water table (we are near a stream). I'm thinking I will need a completely different system but I'm not worrying too much at the moment whilst it works ok.

 

[jess marley]

Thanks all, looks like I have no excuses then. Bah humbug.

 

[Gervase]

And what sort of gradient should that be?

That depends on your peak flow rate.
Less than one litre a second (which is hardly anything) requires a gradient of at least 1:40 to cope with a maximim flow of 9 litres a second.
Greater than one litre a second (most domestic 100mm pipes) has a minimum gradient of 1:80.

 

[biffvernon]

In the flatter parts of Lincolnshire, ideal drain gradients lead below sea level.

 

[MdB]

That depends on your peak flow rate.
Less than one litre a second (which is hardly anything) requires a gradient of at least 1:40 to cope with a maximim flow of 9 litres a second.
Greater than one litre a second (most domestic 100mm pipes) has a minimum gradient of 1:80.

Hmm thought as much. I have an approx 40m run and I very much doubt there is a 0.5m drop over that length currently. Thanks for the reply, I'll have to think carefully about what to do when the time comes.