look forward to your reply thanks bill

Sorry Bill can you be a little clearer about what you are asking please.

I'm not sure what you're asking either - depth of topsoil isn't a calculation needed for laying turf - unless you're saying you need to buy in topsoil to cover something, don't know what (paving? subsoil? tarmac?) so you can lay turf, and topsoil spread over hard surfacing with turf on top won't grow a decent lawn anyway.