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Mutton

Calculating drain diameter

Does anyone know of a good formula or website with advice regarding diameter of a drain pipe. I mean the sort you lay in the ground, not the sort from gutters. It will be taking run-off from the house roof plus from the slope above the house and the patio. We were thinking it needed to be fairly generous so that it doesn't block easily with any crud washed into it or silt build up.
stumbling goat

Building regs used to say underground drainage will need to be 150mm, usually in a bed of pea shingle gravel, with a minimum fall over a distance and if to a soakaway, then that soakaway has to be a mimimum distance from any footings or buildings. I think it is 3 meters, hmm, maybe even 5.

Check building regs, for although they may not be enforceable for what you are doing, if you came to sell compliance would be good.

Please check though, I have not had to lay a drain for some time or dig a soakaway.

sg
Ty Gwyn

Underground soil pipes are 4in,as are normal drain gulleys.

Strange thing is,a smaller diameter pipe layed at the correct fall,has less tendency to silt up,because water flows through under greater pressure,keeping pipe clean.
onemanband

Most houses function fine on 4inch pipe
150mm/6 inch is only used on public sewers and large commercial.
Chudley construction handbook says 4inch will serve upto 20 dwellings. So unless you have a large mansion and several acres to drain 4 inch will be sufficient.
Also 4inch pipe and fittings are more readily available than 6inch and far cheaper.
If worried about silt/crud fit traps/ interceptors at bottom of downpipes and where surface water enters system.
Crud doesn't cause blockages - but badly laid drains will.
Tavascarow

With regards to the fall IIRC the optimum is 1 in 50.
& I agree with the others 4" pipe will be fine.
Mutton

Thank you. That's all very helpful.
Mutton

Building regs used to say underground drainage will need to be 150mm, usually in a bed of pea shingle gravel,

sg


By the way, does anyone know why the bed of pea shingle gravel? Also, is it necessary if there will be no vehicles driving over the drain pipe?
Jamanda

I'd bump this when Behemoth is in the house. It's right up his street.
Behemoth

Can't add much to what's been said.
Behemoth

Apart from the peas shingle is to ensure the pipe is well bedded and doesn't flex when under load (full of water), moving joints/deformation etc. I think.
onemanband

Building regs used to say underground drainage will need to be 150mm, usually in a bed of pea shingle gravel,

sg


By the way, does anyone know why the bed of pea shingle gravel? Also, is it necessary if there will be no vehicles driving over the drain pipe?

Several reasons;
Any ground movement/settlement will be taken up by the shingle so pipe will be less likely to move
shingle fills around the pipe easily and fully so reduces chances of settlement later on - and also makes backfilling easier.
shingle will also protect pipes from damage in stoney/rubbley ground
And it also makes pipes easier to find and repair

If pipe is going to be driven over - it should have a 6inch concrete capping (ontop of shingle) to prevent damage by point loads
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