I've recently removed the old, rotten decking in my back yard and was hoping to lay some turf down. However, below the decking appears to be at least 8" or more of rubble and hardcore and bits and pieces. I've been looking online at depths needed for top soil etc, and about drainage. There's plenty of drainage, but the gaps are going to mean that top soil on its own is just going to wash away when it gets wet.
I've attached an image, which hopefully will show up, and demonstrate the situation.
If I add a layer of gravel and sharp sand, then lay 6" of topsoil before adding turf, will that be sufficient to stop the soil washing away so my lawn would stay green?