Gardening

For a 10,000 sq ft yard, adding 12 inches of topsoil would require over 1,000 yards. How deep the roots go also depends on how compacted your soil is. If your soil is compacted (which most are if it sees much traffic), grass roots just aren't going to go that deep because water isn't going to penetrate that deeply.

Not all lawns should be mowed 3-4 inches. The longer your grass is, the more water it requires. Some grasses have significant horizontal growth, so letting it grow longer to provide shade that is already there is counterproductive. So it's best to figure out what kind of grass you have and mow accordingly.