Grass clippings do not cause thatch. Thatch is a tightly intermingled layer of living and dead stems, leaves and roots of grass that develops between the green grass and the soil surface and is caused by short, frequent watering. Use an empty tuna can to measure the amount of water provided. Water during the cooler parts of the day, like in the early morning or late afternoon, to reduce water loss due to evaporation. Avoid watering too late in the day, when the grass is unable to dry off.
Damp grass invites diseases.
Stay off the grass while the lawn is wet. Wet soil compacts more easily, and grass does not grow well in compacted soil. Mow only when the grass is dry. Wet grass cuts poorly, clumps and spreads diseases more easily. If the lawn is too long or wet, use clippings in your compost pile or as mulch.