Grass seed needs contact with soil to germinate. According to Texas A&M Cooperative Extension, spread at least 4 inches of topsoil in areas you plan to seed (6 to 8 inches is ideal). If there are low-lying spots or dips, use the soil to level the area. Once the seeds are sown, rake them until they are covered with 1/4 to 1/8 inch of topsoil.
Use topsoil on top of an existing lawn to plant new grass seed. Spread fertilizer first, followed by 2 inches of soil. Rake and level the new topsoil, then sprinkle grass seed on top of it.
Weeds can invade a newly planted lawn. Use quality topsoil that is made from heated compost. The heating process kills most weed seeds.