Thatch is that layer of dead and dying material just below the surface of the green sward of grass. Laying on the soil. All established lawns have it, and it is necessary to a certain extent is giving your lawn a bit of bounce.
Thatch in lawns is generally made up of horizontal stems of grass, old lawn mowings, decaying leaves and anything organic that has found its way under the
green sward.
A layer of thatch with a thickness of up to 1cm (half inch) is desirable and will cause no problems. This thickness of thatch is almost necessary for your lawn to have a bit of 'body'. At this thickness, water can still penetrate down onto the soil - as can nutrients. This thickness of thatch on your lawn will also act as a mulch in drought conditions.
However, once the thatch builds up to a thicker layer - say 1 inch (2.5cms), the problems start. Rain water is impeded from percolating downwards as the thatch forms a waterproof layer. Rainwater runs off the lawn instead of finding its way down into the root zone.
The image shows a typical layer of thatch in the earth core that has been removed.
Be aware that there will be a natural - desirable - layer of decaying organic matter below the surface of the turf. This should not be confused with 'thatch' which is a denser layer of dead grass nearer to the lawn surface.
Any thickness of thatch in the lawn over the 1 cm, will also cause problems with the physical growth of the grass. The roots of the grass are obviously growing down below the thatch - into the soil, so the grass plant then has to grow up through the thatch to reach the light, and subsequently turn green. The stem of the grass which is actually growing through the thatch becomes blanched, and this soon shows through in the lawn. Thick layers of thatch effectively 'strangle' the grasses, weakening them and leaving them open to fungal attack.