Instant gratification -- that what landscapers love about sod. Sod installation is easy, but like a lot of landscape work, most of your time will be spent on preparation that nobody sees when it's finished. What is visible is the result, a healthy attractive lawn for years to come.
Most plant material does best in a topsoil base, and the same is true for sod. It may not be necessary to add topsoil, if there is existing material on the site that can be amended suitably. But the better the base, the better the results.
Time spent on preparation for sod means lower maintenance and lower water requirements in the long run. Preparation includes attention to what goes under the sod.
Domestic grass seed needs an optimum 6 inches of topsoil. Topsoil settles over time so 6 inches will generally compact to 4 inches
Sod is grass that's already established. The supplier cuts it into manageable units; the sod arrives with some topsoil attached, usually about 2". Check the depth before you calculate the amount of topsoil you'll need for a base. In this example, 4 inches topsoil is added to reach 6 inches total.
A Handful of Information About Topsoil
It's important that the base for sod be in a condition that allows the roots to penetrate, and for proper drainage. A simple test for topsoil texture is to squeeze a handful. It's a good sign if some falls apart and some consolidates like a half-formed snowball. That means it can retain some moisture and nutrients, and also permits drainage.
If there is enough topsoil on site to provide the depth needed, till it and remove all stones, debris and clumps of weeds.
One or Two More Things
The topsoil needs to be rolled so that it settles evenly. Professional landscape specifications describe the desired condition as "firm against footprints". Once the sod is placed it needs to be rolled again lightly so that the roots make contact with the soil. Sprinkling topsoil or compost between the joints will keep the edges of the sod from drying out as it gets established.
Stand back and enjoy your instant lawn -- for an instant. Then you need to go get the watering hose.