Thatch is the naturally occurring layer of decomposing organic material that accumulates between turf and soil. A healthy amount of thatch is good for your lawn as it enriches the soil with nutrients. But too much water and fertilizer can cause excessive thatch buildup, which is considered to be anything more than 3/4 inch thick. (See References 1) It's very common for lawns to get too much water in the rainy Pacific Northwest, but fortunately, dethatching is fairly simple as long as you're prepared for some hard work.
Cut out a section of turf about 6 inches square from any problem area of your lawn using the spade. It should be deep enough to provide a good vertical cross-section of the turf and soil. The light brown layer of organic material between the turf and soil is thatch. Measure this layer to find out if you have a thatch problem. If it is more than 1/2 inch thick, you should give the area a light dethatching. Remove more thatch as the thickness of the layer increases. You should dethatch in the spring when turf grass is in peak growing season.
Mow the area to be dethatched as short as possible and remove all of the grass clippings. A double-sided dethatching rake has V-shaped tines with sharp edges designed to cut through the thatch. Press the tines into the ground to get just below the thatch layer and maintain pressure while pulling the rake toward you. You should see a lot of light brown material coming out as you rake.
Rake each area several times or more depending on the thickness of the thatch layer. If you are pulling grass up with the thatch, it means the tines are too deep and are cutting through the roots. Raise the rake slightly and continue. Dispose of the removed material in leaf bags or compost it if you have a pile available.