Laying grass on concrete

[brianirons]

There was a similar question posted on this subject and I noted the answers with interest. However, I have a slightly different problem which I would grateful for any advice that anyone might be able to provide. I have a swimming pool in the garden and after many years braving the UK climate have finally decided to fill it in with rubble and then lay some turf over it. Part of it, however, I would like to retain as a water harvesting reservoir and this would need a concrete roof constructed over the top. Obviously, I can make the roof at whatever level I like below the ground level but am wondering what the minimum depth would need to be and what would it need to be filled with to ensure the grass would stay in good condition? I am assuming a sandwich combinnation of gravel, sand and top soil might ensure adequate drainage but if anyone has any experience of this I would welcome suggestions.

Many thanks.

[AlBags]

I would have thought 8-10" of hardcore, mot, and more importantly rammed down soil with turf on top would be ok for the grass .. much less might drought the turf.