I will be rehabbing a house this summer on a very limited budget and will need to dispose of a few rooms of 40-year-old awful, very thin cheap carpet. (It is far too nasty to be recycled in any way.) The county charges a fee for what doesn't fit in the trash can. It has occurred to me that several layers of carpet might be useful for filling in a long low area of lawn that will probably never be planted as a flower bed that is alongside the street. If this is a feasible idea, how thick should the layer of topsoil be over it for grass to grow successfully?