Garden Drainage

Hi, I am in the process of getting my garden completely landscaped. The patio currently tilts toward the house but will now tilt towards the garden lawn. I was told that I don't need drainage as the lawn will drain the water (getting 15 cm of new topsoil installed over clay soil). The patio will be porcelain.

Can I please ask if this sound right, or should I be concerned about waterlogging?

 

Be concerned regarding the relative position of patio surface to house damp-proof course. Should be at least 3 bricks below. Also consider the relationship of the lawn to surrounding terrain. Does it drain now? A soak-away or french drain could be inserted in the lawn to take water away- (remember it only flows downhill !), if there is somewhere for it to go.

 

Our garden was a bit of a state so hard to tell if drainage issues would have been present. It is higher than both the neighbours gardens as well. Our neighbour who has a nice garden said that hers often gets waterlogged in the rain but I am not sure if that is because it may have not been prepared properly when they did it.

 

I personally would incorporate some grit and composted bark into the clay prior to adding topsoil.

 

It has never seemed right to me to just put topsoil directly onto clay especially if the clay is hard packed.
It's unnatural, I would think that the clay needs to be broken up to some extent.
Are we talking about solid clay or just a clay soil.

 

How much grit would be needed? Just sprinkle it all around or a proper layer of grit above the clay soil and under top soil?

 

I have 84 sq mt of lawn so am imagining a huge amount of grit would bee required?

 

It may be that 'your' rainfall drains in to her garden on its way to the sea. This is not a problem (to you!). I would dig a hole, say 300mm deep,and monitor any water level twice daily after every rainfall, to see how quickly it falls. Clay is less quickdraining than sand or chalk. If the hole remains full, this may demonstrate the 'watertable'; ie the level of natural water in the ground, which changes over a longer timescale.
The material you remove from the hole( it need only be 300mm square) will indicate by its colour the conditions prevailing; feel free to post photos of what you find.

 

When its heavy clay soil, I would recommend 50-50 but that tends to be when they are only going to incorporate sharp sand, to much fines in the sand that will bind with the clay. When you have very heavy clay soil then it generally means it's very fine material, no drainage. it's best to incorporate additives that are in various particle sizes. A bag of composed bark is bulky compared to grit but it has various particle sizes as it will have passed through a 0-15mm screen prior to bagging.

So for very heavy clay soil, I would recommend incorporating a mixture of different particle sizes. Sharp sand, grit and composted bark or soil conditioner. then you should not need the same quantity of material if just adding sharp sand.
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The building company who got the contract to build our whole estate about 12 years ago scrimped and saved everywhere they could. This included adding the thinnest layer of top soil that they could get away with (typically between 6-12") on all the gardens. And it's thick clay underneath, exactly the same as yours.

In summer the lawn looks amazing, even light rain isn't a problem. But prolonged periods of rain turn the lawn into a large patch of green bog, and don't even get me started in winter, ugghh!

We've had 8 years of it now and we are fast beginning to realise that the only option is for us to do away with the lawn completely and change it to India paving or something similar.

I'm not too sure about taking the top soil off and mixing loads of stuff in with clay, seems like a big messy job to me, and more importantly will it resolve the flooding/waterlogging problem in the event of prolonged winter mushy weather, I'm not sure about that.

 

It is a fact that most developments are performed by stripping off top soil, some of which is stockpiled on site; the remainder 'sold' away. They then dig footings and trample this material with heavy and repetitive running by plant while construction occurs. The site is then 'landscaped' with a veneer of topsoil and turfed. Given that much development is upon floodplain, drainage is problematic. Unfortunately the (minority) interest of gardeners is way down the list.