Use of a slit-seeder, which is a specialized machine that will cut slits into existing grass and then drops seed into those slits, is an ideal way to overseed lawns. Seed may also be broadcast over thin lawn areas, but there needs to be good soil to seed contact. Dethatchers or vertical mowers can also be used to tear out excess debris prior to overseeding. All of these types of overseeding procedures do not require additional soil modification. When soil problems exist under a lawn, there are ways to address them without tearing up the lawn.
Core aerifying is suggested for problems such as thatch and soil compaction.
Core aerifying machines will pull up numerous plugs of soil about the diameter of a pencil, making holes into the lawn. Allow the plugs to remain on the soil surface. Unfortunately, some lawn problems, such as soil problems of severe compaction, high clay levels, or poor drainage, require starting over. Remove existing grass or rototil it. High populations of perennial weed species may require use of a nonselective herbicide, such as glyphosate.