I have just discovered that the house I am close to buying has a soakaway.
As far as I know it’s a rainwater soakaway.
As I understand it this is just a means of dispersing surface water.
I have seen somewhere in the documents that one is supposed to remove the soakaway crates and remake the whole structure every 10-15 years.
From the few videos I’ve seen online it seems to be a set of interlocking plastic crates of perhaps 1m3 volume, in an underground pit, and wrapped in a plastic veil that filters small particles - all of this covered over with a layer of soil (or in my case astroturf).
Does anyone know anything about rainwater soakaways?
And has anyone had a good or bad experience with a soakaway?
any soakaway is only as good as the ground its in, ie a nice sandy gravely soil will be good and as long as there is a silt trap upstream of the crates that you can get to then they should last a very long time. We use them all the time for highway drainage and whilst they are not as maintainable as traditional perforated concrete chambers we have not had to replace that many unless they get swamped with silt.
I had a soakaway for the drainpipes in a previous house. A tree root grew up it, eventually blocking it and a fair amount of digging was required. The insurance covered it, though…
Our soakaway was under spec’ed for the property and resulted in standing water on the driveway when it rained. We claimed under new build cover and it was replaced with something the size of a mini car, shudder to think of the cost but all covered by nbc. One oddity as a result, the neighbours had a possible flood risk flag on the survey when they bought their house next door even though we are in a high area.
Hi Jimdog - yes clay is particularly rubbish at draining. We have quite a bit of it in norfolk with some areas of nice gravels from the glaciers in the ice age at the north of the county. A silt trap only neds to be a sump with a high level in and an out with a lid on it so you can get in and clear silt from the sump.
“The soakaway must be a minimum of 5m away from your building, 5m from a road and 2.5m away from any boundary. Soakaways should be avoided on the higher side of a retaining wall where there is likely to be a risk of de-stabilising the wall and/or flooding the neighbouring property on the lower side.”
The soakaway in this recently-built detached house is about 3 metres from the house.
The top of the soakaway is about 2 metres below the ground level of the house - and the soakaway is on the higher side of a retaining wall.
Below the retaining wall is a series of back gardens of other houses built at the same time by the same building company that run down a fairly steep incline below the house I’m about to buy.
The main outlet for this “Sustainable Urban Drainage System” appears to be the immediate neighbour behind, which is the highest of these few back gardens, which is about 3 metres below the top of the soakaway.
Also, the current owner has unfortunately removed the original soil/turf that was on top of the soakaway and replaced it with Astroturf.
Yes, the soakaway is above the back gardens to the rear, which cascade down a fairly steep slope.
The houses and the soakaway are about 7 years old.
My guess from having seen it and stood on it (before I knew it was a soakaway) is that surface water will flow into it through the astroturf and down into the soil under and around the soakaway and must also flow down into the various back gardens on the slope behind the house.
So does it take roof water , if so there will be a pipe into it . Either way it’s not good having it higher than the neighbours gardens… I’d ask for some survey’s before going further.
What type of professional would you get to conduct such a survey?
In my experience, a normal surveyor who makes Homebuyers Reports will just state vague possibilities and caveat everything to the hilt, so typically nothing will be learned.
I’d rather someone local with experience of building soakaways and drainage systems in these soils could give me practical advice as to how they believe it succeeds in draining surface water for that particular plot during an on-site inspection with me.
Or with a local drainage engineer, if such a thing exists?
Might be worth a chat with a local builder then? Without the ability to test soil infiltration rates it’s very difficult to say if it’s working well. Talk to the neighbours who’s gardens are below ti see if they have suffered in any storm events.
Yes, thank you so much for all your help in thinking this through.
I have viewed and visited many houses and gardens in my town over the last 15 years.
I can think of some roads where I’ve witnessed severely flooded gardens - so it’s a real problem in this town as elsewhere - in some new properties as well as some old ones.
When I viewed this property (a couple of months ago, during the rainy season) there was no sign of flooding in the garden and no visible sign of flooding in the gardens below to the rear.
That’s a good sign, but obviously just anecdotal, and not conclusive at all.
the capacity of the soakaway to take-up water, as if the feed piping and trap isn’t set right, you can find the water doesn’t flow away quickly and will look for other ways to flow, obviously especially in heavier rain.
mine, set in clay, was re-done c.4Y’s ago, as the previous on was at best amateur (when researched it seemed someone had run a gutter downpipe as the feed).
what the contingency protection is for over-flow from the trap if the system is full, especially relevant if the soakaway isn’t that deep and/or the angle of the pipe-run is shallow and/or the ground doesn’t absorb water e.g. heavy clay.
From a practical perspective, I wouldn’t put a fixed ground covering over a soakaway, as it will need attentions…and they are never big enough IME, especially given the way weather patterns have changed.
Beware builders also, as I’ve had renderers and others where, in cleaning up, they started to sweep towards the drain – some urgent and uncomplimentary language was required by me.
I would ensure you have a modern trap, not just a U/S-bend type – and make sure you clean it out on a regular basis. Put a leaf/moss catcher in the feeding gutter and also a cover around the base of the downpipe, to avoid leaf mulch etc getting access to the drain.
If you have clay soil, does that hamper the diffusion of water out of the soakaway? Does it disperse more slowly?
Why is running a gutter downpipe as a feed into a soakaway a bad thing? Is the downpipe extend underground on the way to the soakaway, or does the downpipe just feed water across the surface of your garden towards the soakaway?
By ‘fixed ground covering’, do you mean that the soakaway is covered over with Earth and perhaps also layers of stones or gravel, with perhaps plants grown on top?
What is the alternative to a fixed ground covering?
If you are concerned Jim, I’d suggest you find a local civil engineer that specialises in stormwater design to assess it properly for you. A well designed, built and maintained one should be fine. But, otherwise, maybe not.