New lawn over hardcore

We’ve just had a concrete slab taken up from our garden with a plan to replace part of it with paving and part with a new lawn sown from seed. The contractors have lifted the concrete, and laid a 10cm deep layer of crushed concrete hardcore as part of the base structure for the paving, but have laid this over the entire area rather than just the part we want paved. Our plan had been to prepare the ground for the lawn by bringing in 15cm of top soil, but with the crushed concrete hardcore there is only space for about 8cm of soil if we keep the level as we want.

The contractors are telling us that this will be fine, that the hardcore will act as a good base layer for the lawn stop the soil settling too much, and that should drain without any problems. From what I’ve read I have some serious doubts, but am not an expert and thought I’d ask for your thoughts. How bad is the lawn likely to be in only 8cm of soil. I’ve seen lots of write ups about irregular coloured lumpy patches where rubble has been buried under a lawn, but we do at least have a uniform surface. Will it just be a bit drought intolerant, or is it likely to be a real issue? Is the problem fixable by adding a few more cm of soil (we can probably get to 10, but not 15), or is the only option to have all the hardcore dug out?

Thanks for your advice.

 

We had a small path about 6 feet long at the top of the garden I broke it all up into small pieces but left it in place. This was only a temporary fix at the time so I put about 10cm of soil over it followed by the turf. Now most of the time you would never know it was there but when we get a dry spell it’s the first bit to loose it’s green colour. If I was you I would remove it from the area or at least some of it so you can get more soil down. If you could dig it in before you put soil on top it would be the best of both worlds.

 

Welcome to the forum @ivanol You will need to remove the crushed concrete before thinking about lawn. I suggest you tell the contractors to do it as they have not done as you asked, and are trying to fob you off.
I have a 'hard standing' area here on my site using crushed concrete. When it`s very wet there is a green haze over it, but otherwise it looks awful.

 

I’d also get them back in as they haven’t done what you’ve asked.

 

Come the first drought and the grass above the hardcore will die.

 

Thank you everyone. This is pretty much what I thought, but good to have confirmation from you all. Will get the stuff taken out.

 

Just to add a follow up on this - we had the company back in to dig things out, and put in a decent depth of topsoil. This took until very late May, which is when we then seeded, right at the start of the driest summer in years! Despite this, admittedly with a lot of watering, the lawn has established beautifully, so thanks to all for your advice.

 

It's great to hear that @ivanol and thanks for letting us know. Why not stick around for a while and join in with our chatter...

 

Thanks Tetters. Must admit I logged in to ask another question, and realised I hadn't updated you all on the outcome of the last one! I updated you, and then found a thread where I could actually contribute. Still haven't got round to asking my next question, so will certainly stay for chatter for a while!

 

Interesting question. Bye the bye, Welcome to the forum.

Perhaps some folk are of the understanding that grass, or for most of us, a lawn must be based upon nothing but soil, of whatever substance or quality. When you see grass growing happily on slate/tiled roofs etc makes you wonder. In similar circumstances. Perhaps you have seen arial photos of historical sites of bygone years, often even in favourable climes, an outline can be seen. Baiscally grass will grow anywhere. For a lawn, the thickness of the turf plus another 3-4 inches of soil will survice.

In practice the main reason grass "appears" to die in dry periods is due to the fact that the rubble, even solid concrete has most times accelerated the drainage of water. Come rainfall and soon the green sward will brighten up your garden again.

 

The bit of grass that used to grow over a base of crushed concrete on my patch does have a green haze over it when the rain comes, but it is by no means ever as healthy looking as the rest Mike, and after this drought I`m wondering if it will ever look green -ish again.

 

I'm resurecting an old thread, but thought it would be good to update as these things take time.

A year down the line we're very glad we went to the hassle of getting the hardcore dug out. Generally the lawn looks great, but the areas at the very edge where the soil depth was a bit lower have all gone brown in the recent hot weather. I suspect the whole lawn would have been brown and unhappy if we hadn't got it sorted at the beginning.