Size of a soakaway

[Steve]

Anyone have any idea of a formula to use constructing a soakaway?

Googling around gave:

Vol(m3) = Area to drain(m2) × (rainfall rate(assumed 50mm/hr)/3000)

Which in the example gave 60m2 needing 1m3 size, but I only need about 4m2 which gives 7cm3, which sounds too small.

any ideas?

Steve

[mark]

7cm is about a spoonful so maybe a bit on the small side. The 50mm times the roof area is correct. 4m² *50mm = 0.2m³

This is for the volume of the void, so it depends on what you are back filling the soakaway with, how big the hole should be.

mark

[Calvin Sambrook]

Tell me you're joking, right? A soakaway for 4m2? It's not going to produce enough water to worry about.

Anyway if you really want to dig a hole try an upturned (clay) flowerpot. Remember a soakaway is only a buffer to hold the water until it can, guess what... soak away so you're not going to need to hold very much.

[Martin Bonner]

specifically that to size a soakaway properly, you need to know how porous your soil is. Someone living on sand/gravel can just connect a downpipe into the ground, someone living on heavy clay is going to need a major piece of engineering to support the required hole.

[Phil L]

Anything less than 10m2 doesn't need a soakaway, just dig a small hole, stack bricks in pairs (with holes in) about 5 high, so ten bricks will do it.

Cover with a few shovelfuls of gravel and forget about it.

[Steve]

Thanks all, I'll throw something simple together.

Its actually more of a precaution, I'm seeing some damp on a party wall (below dpc below floor level for next doors garage), the downpipe from the conservatory is right next to the point where it seems to come from, and I cannot see the bottom of the downpipe so I'm improving dranage just in case. Presently it flows about 5M away from the downpipe but just terminates into soil.

Steve

[Donwill]

Years ago I remember checking the ground for a soakaway for a septic tank, it was a bit clayee so the procedure was to dig a 1 metre cube hole in the ground, fill it with water and check how long it took to drain away, on that basis it was decided how large the soakaway had to be, it was a very long time ago and I cannot remember any more detail or where the procedure information came from but I would imagine that there may be a similar procedure for surface water soakaways, albeit a larger volume of water to get rid of.

Cheers Don

[RobertL]

So the soakaway needs to hold the water from the roof when 50mm of rain falls in one downpour.

Robert

[chunkyoldcortina]

Do you really mean 4m^2, as in 2m x 2m?

[mark]

So the soakaway needs to hold the water from the roof when 50mm of rain falls in one downpour.

Yes, but 50mm is quite extreme.

mark

[PCPaul]

When I built our conservatory I needed a soakaway for the 5mx3m roof. Without knowing the calculations, and knowing that we wanted a 12" gravel border all round it, I improvised.

The 5m border of the conservatory furthest from the house was dug out to about 24" deep, 12" wide all the way along (done while we had a local by- the-hour mini digger in to do the foundation pads for the steel suspended floor).

Then I lined it with landscaping fabric, 20" of cheap coarse gravel/small stones, another layer of landscaping fabric then 4" of decorative gravel. The two downspouts from the conservatory roof just go onto the top.

I was worried it might silt up over time but it's been there about 10 years now with no problems at all.