Laying a raised lawn above a concrete slab

Question

I hope you are well.

I've scoured the internet for a good answer to this, but can't find anything definitive and keep reading contrary articles and recommendations.

I have a rolled gravel patio adjacent to a swimming pool. With a little two year old running around, I wanted to create a lawn for him.

Unfortunately, 10 cm below the gravel is a slab of concrete in which much of the pipework for the pool is buried (I believe). I've lifted about 4 tiles in various places to test and I keep coming up against the concrete each time. The good news is, I can raise the area by building a step down down to the pool.

I can probably build a 30 cm high retaining wall and fill with it with fresh soil, giving me around 25 cm of depth for the roots, accounting for the length of the grass having it roughly flush with the edge of wall. Do you think this would work? Should I be worried about anything? Should I put the soil directly onto the concrete or lay anything as a base layer?

I've read in some places that 20 cm should be adequate, but have seen others complaining about lawn failing to establish and dies off in the summer with a similar situation.

The garden is South West facing and gets a lot of sunshine. The property is in the French Alps and so gets a fair amount of snow in the winter in case either of this factors into the matter.

Thanks for any help or suggestions.

Answer 1

Yes, well as can be expected, thanks, hope you are well, too. Probably a lot of the advice you have received will be good in detail, the issue is just establishing a zig-zag solution path through that detail. 25 cm is adequate for grass roots, and maybe more than enough depending on species of grass, but in order to get through periods of dry weather the grassy plot depends on being able to pull moisture up from even deeper layers by capillary action. So, once established you will need to watch for signs of the surface drying out and give additional water as required.

You can test for concrete at a specified depth in regular soil by pushing a rod down until it meets resistance other than stones.

Soil can go down on the concrete, but a layer of something might be helpful if you anticipate needing to remove the soil at some point in the future, say to perform maintenance on the pipes in the concrete. Check with the pool installer to see if there is a downside. The layer will help get the last remnants of soil out, if that is important.

Talk to your local garden centre to get a feel for the grass seed available; note its drought resistant properties and talk to neighbours (garden club, municipal parks department?) about their experiments with different grasses that work in your area.

Since the plot is intended for the enjoyment of your child, bear in mind what the surface will be like if and when it does go dry. Avoid the use of fine particle clays which will hold moisture quite well but as and when they dry the surface goes hard as the concrete you are trying to cover and will be very hard to re-wet. A sandy loam type of soil is preferred - the roots can go deep and even when it does dry out the loose surface will be softer, more like a sandbox. The downside to a looser surface is that next to a pool, if the location is windy dry soil particles will more easily blow into the pool.