Question
Ken:

I will describe the situation, and follow at the end with a question.  I have several thatch-related questions, but will send each one separately so they will index better in "experts."

I have thatch in two parts of my lawn.  The thatch is 3/4 inch thick or more.

First, a word about my lawn practices.  I fertilize Labor day, around Halloweeen, and sometimes on Memorial day.  I use Turfbuilder and the standard (not heavy) rate.  I water deep at least once a week* if needed.  I mow with mower set at 3 inches.  I usually use mulching attachment (but have bagged this season because I have annual bluegrass).  

I aerated 3 of the last five falls, but couldn't schedule around rainy weekends last fall.  The areas with the thatch are areas that have a lot of slope, face south and west, and have sand backfill under the topsoil.  I have not topdressed after the aerating.

The north side of my house also was backfilled with sand.  There is no thatch problem there.  So it appears to be a combination of sand AND sloping toward west/south sun that causes the problem.

*I believe that when I water these areas, between the slope and the thatch, the water just runs off, so I have switched to water a little at a time throughout the day.

This year the thatched areas (and the rest of my lawn) look great.  I suspect we've had so much rain that the thatch has stayed moist.  I dug down in the thatchy areas two days ago and they are still thick with thatch--in fact, I could barely get a sharp hand trowel through the grass/thatch/roots to take out a core sample.

I will follow this description with a series of questions.  But my first question is:  do you think my assessment of hot/dry/sandy/sloped areas--maybe with some earlier years of watering poorly--is the cause of the thatch?  

Answer
Thatch typically does not develop on most cool season grass but can on kentucky bluegrass especially if fertilized with high levels of nitrogen in spring. Your mowing, fertilizing and water practices sounds right and should not be changed. You can look for a slow release fertilizer product for spring applications. Some grass cultivars are more thatch prone than others.

I suggest core aerating if thatch is only 3/4" thick. Up to 1/2" it is typically fine, but more than that you should try to improve. If you rent a core aerator then run over the lawn area in question in various directions several times. If done in early september you can not really make too many holes.