A little thatch is beneficial. It can help regulate the soil temperature and moisture levels that nourish the root system of your lawn. However, once it accumulates beyond half an inch thick, it can be detrimental. When this happens, the roots of your grass start to curl up, trying to reach areas that have greater nutrients, water, and oxygen. To check how thick the thatch is, use a trowel to dig up a small section of your lawn. Look at the cross section of your grass. If there is more than a half inch of accumulated thatch, it is time to dethatch.