Land drainage

28 Mar 2011
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United Kingdom
Hi All

Just looking for a bit of advice on land drainage.

The patio at the back of our property clearly has drainage issues. The land on which it lies is raised to a level of about three feet above the building itself and is accessed by steps up from the path around the house. Although I haven't yet lifted them to check, the flags appear to be laid directly onto clay without any sand base, and during the thawing of recent snows the surface became, for want of a better word, very "squelchy" underfoot.

My first thought is to dig up the patio, lay the yellow plastic drainage pipes which I've found in other parts of the garden and which are perforated on their upper edge, and slope them down to drain into a soakaway which I will create in the garden at the level of the building. I'll then lay a sand base over the pipes and patio area and replace the flagstones.

The patio is probably about 14'x12' at its widest point; would I need to lay several pipes and take them into one drain, or would one pipe in the middle of this area suffice? Or is there another way of successfully draining the patio area? Might digging out say, four inches of clay below the flags and filling the space with gravel, then topping it with sand make for good enough drainage to render the laying of pipes unnecessary?

I'm floundering a bit here, having never had drainage issues concerning any previous property that we've lived in.

Thanks in advance.

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There are two things to consider;

Surface water and ground water.

surface water is obvious and easy to deal with by way of falls on the paving and linear drains if needed connected to soakaways or gulleys.

Ground water is very different though, which is causing your issues?i

If its groundwater then you need to deal with it near the retaining wall.

Does the retaining wall have weep holes?
Thanks for your reply.

Of course, you're absolutely correct that one aspect of the solution would be to ensure that there is sufficient fall to shed surface water; and right now, there clearly isn't. In fact, without actually putting a spirit level on it, the patio appears to be absolutely flat, and surface water is draining away via the unpointed joints between the flagstones. Even so, this winter there has regularly been a large pool of standing water in the middle of the patio after heavy rain, which given the unpointed state of the patio indicates to me, as an amateur in these matters, that the ground beneath must be saturated.

There is a block retaining wall on one side of the patio where the steps lead down to the property, and no, it doesn't have weep holes. However, it occurs to me that my easiest option for draining the ground water would be through another side of the patio, which consists of a dry stone wall constructed of odd bricks and rocks, and which could accept some sort of outflow; so that's where I'm looking to take the water. Does that sound reasonable?
Bare in mind that once it is laid with a proper fall on it and the joints are pointed the surface will be impermeable so water will run off and therefore will not saturate the sub base.

I think your issues are caused by poor laying as well.

Once laid on a good sub base and mortar bed i don't think you'll have any issues unless you have a very steep hill behind you creating a significant water pressure in the ground water
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Thanks again.

As it happens, there is quite a steep grassed bank leading down to the patio from our boundary with the farmland next door. At the top of this bank and after several weeks of damp weather the surface has turned into a quagmire, so there may indeed be an issue with ground water.

However, I'll relay the patio with a fall, point the joints and see where that takes us.
I have a low lying clay infested garden in a very wet area so I can feel your pain about drainage issues.

The first thing I noticed is the mention of perforated pipes. You probably don't want these under your patio as these are usually intended as part of a soil drain.

As has been mentioned I think you need to first consider where the water is coming from and then the drainage from the patio. My patio has a bit of a problem with standing water and I simply drilled some holes at ground level in the wall that borders it and run some drain pipes through this into a lower level before. This hasn't cured it completely and I still get a bit of a puddle in the middle where its too low, but its not bad enough for me to consider lifting the patio.

In the rest of the garden I have concentrated on trying to divert much of the water before it gets to the lowest point using perforated point sunk into gravel trenches.
Thanks for that.

This is the largest garden we have ever had by far, and certainly the first one featuring clay and which lies on a hillside. So, lots of hitherto
unknowables to deal with and many a potential solution to try.

Some of those solutions already covered here will be tested when the weather gets a little better, and warmer.
Has anyone here ever explored the garden drainage option of using rainwater crates? If so I'd be pleased to hear about their experience, because from what I've read so far I think they might provide
a solution to my problem.
A soakaway whether formed with crates (attenuation cells) or a rubble filled pit will only be useful if the ground is permeable enough for anything to soak away.

More often than not on heavy clay it just forms an underground swimming pool.

Look on the paving expert website and there is details of a test pit you can do to check suitability.
Thanks again. I have to confess that having read up about the topic I dug a trial pit a couple of days ago, and we had rain for most of yesterday so I'll check this morning whether I have that underground swimming pool. If not, I might invest in a small crate.

However, I'll keep the clay which was removed in digging the pit in case I have to refill it and look for a Plan B.
Well, I kept the clay, and just as well. After 24 hours of hail and rain, the area where I had sunk the crate was worse than ever. I reckon the entire patio drains into that small area, and what happened was that the crate itself filled up, the water was unable to drain away because of the impermeable clay walls of the hole, and thus as more water poured into the hole the surrounding surface began to fill up. I can only assume that the existence of the hole in which I had put the crate prevented the majority of the water from taking its previous route, possibly via the adjacent turfed lawn.

The crate is now out of the ground and deposited behind the shed for re-use, I know not when, the clay is back in the hole and I think I'm now resigned to having a wet garden in winter.

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