DON'T WASTE YOUR MONEY ON DETHATCHING.
9 times out of 10 it is not necessary.

Thatch is the brown mat or layer that is located directly at the bottom of the grass blades and on top of the soil surface. Thatch is part of the root of grass that grows above the soil, not the part that grows down in the soil. The top layer is where the green portion of the grass grows. If the thatch layer is too thick, (spongy) it will reduce the ability of water, air and nutrients to get into the soil and conversely the water moisture to be released from the soil line into the air. It also stifles good grass grows. While thatch is not made of mowed grass clippings, excessive "hayfield mowings" slow down the natural decomposition of thatch.

Thatch is not usually a problem if the yard is mowed, fertilized, watered, and core aerated correctly. If, one or more of these items is out of balance then thatch can get heavy and negatively affect the lawn. If the thatch is too thick, it not only needs to be reduced, but a determination as to why it is a problem must be resolved.

To measure thatch we suggest taking a shovel and digging down into the soil. Wedge the dig back and forth. Look at the brown area after it has decompressed a bit. Take an average of six to twelve samples in the yard. If the yard is a well-maintained yard and the average thatch is 1/2" to 3/4" then we think you are okay. If the yard is thin, has lots of weeds, or disease, or in decline, then the average should be more to the 1/2" mark and other solutions to the weak yard should be reviewed.