In some areas of the country, thatching is an annual lawn maintenance chore, but in many areas, it is resorted to only when a lawn has been neglected, over watered or over fertilized. Heavy clay soils can also cause thatch, that dense layer of living and dead grass that keeps sun, air and water from getting to the roots of lawn grass. Since thatching can be a destructive process for plants and roots alike, be sure that it's thatching, not some less invasive treatment, you need and proceed carefully.
Thatch after a light rain (or after watering) but wait until the lawn dries out a bit after a heavy rain. The soil should be moist but not so wet that thatching pulls the roots out of the soil along with the thatch.
Mow the lawn to the lowest recommended height for that grass. Kentucky bluegrass should be mowed only to about 2.5 inches but some grasses, like Zoysia grass, can be mowed as low as 1.5 inches.
Use a hand rake for spotty or light layers of thatch. Use a rigid-tined lawn rake to start at lawn edges or thatch around trees or other shallow-rooted plants. Use a thatching or cavex rake with half-moon-shaped tines on light layers of thatch. Pull the rake across the lawn just enough to loosen the layer.
Rent a power thatcher and have the rental agent give you complete instructions on how to set the blades or tines before operating it. Set the blades just above the surface and push or pull the machine across the yard one way, then again, at right angles. Reset the blade if necessary for the second pass but not so low that it slices into the ground.
Rake up all of the material pulled out by the thatcher with a garden rake. Add it to your compost heap or mulch pile. Fertilize using a fertilizer without herbicides and water the lawn immediately.