Laying turf over hardcore
I'm helping a friend out with his garden and he's recently had it pretty substantially relevelled with a mixture of topsoil, rubble and relatively large stones. Most of the lawn area is the original turf (which is actually going to be removed and relaid with new turf) but about 30m squared is the rubbley area. The ground is pretty compacted and I was wondering about the following:
How much topsoil should we add over the stoney ground?? I was thinking about 100mm but he wants to go for less (topsoil being quite expensive).
Should we lay any kind of sheeting over the stoney stuff (I'm concerned about it being quite porous)?
Any help would be greatly appreciated.