You don't need topsoil to grow a lawn. Most professional sports venues
have zero topsoil, the sod grows on a blend that is at least 90% sand.
Oriole Park at Camden Yards had to completely redo their field last year
because the organic matter had built up on its own from 1% to 17% in
eleven years.

Topsoil does, however, generally make it easier for the average
homeowner to grow grass because it holds moisture longer than sand, but
topsoil is not a requirement.

Bob Smith wrote:
It's the first time I have looked on this group, so forgive me if I ask
something straight out of the FAQs, but here goes:

Can anyone tell me how deep topsoil needs to be to grow a lawn?

TIA

Bob