Clay soil is composed of small particles of soil that fit so closely together that there is little room for air. As a result, clay soil is poorly drained soil. Clay soil is wet and slippery in spring and rock hard in summer. Plants that are grown in heavy clay soil often have stunted roots and might develop root rot from standing water. If you wish to plant grass in heavy clay soil, you must first amend the soil with organic amendments to make it loamier and improve the drainage.
Touch clay soil to make sure that it is barely moist before breaking it up with a rototiller. Wet clay soil is sticky and will adhere to the tines of your rototiller. It also clumps up to form rock-hard clods. Dry soil is so tough that it will not break up easily. But barely moist clay will break up quite well. Loosen the soil to a depth of 6 to 8 inches with a rototiller.
Spread 4 inches of peat moss and compost over the soil. Mix these amendments into the soil with the rototiller.
Smooth and re-grade your soil with a landscaping rake. Your soil should slope gradually away from your home to carry water away from the foundation. Smooth out any large divots to avoid creating areas where water can collect.
Select a grass sod that grows well in your climate and in clay soil. In the south, warm-season grasses such as Zoysia or Bermudagrass grow well in clay soil and provide green color from spring through fall. In cooler climates, select a wheatgrass or ryegrass for heavy clay soils.
Place sod in strips over the soil so that each strip touches its neighbor and there is no exposed soil. Lay the sod in staggered rows so that there is no uniform seam over your lawn.
Roll over the sod with a sod roller to force the grass roots into contact with your soil. Water your sod several times daily with 1/4 inch of water per square inch of soil during each session to keep the sod moist and help establish roots. Use a rain gauge to measure how much water you have used.
Decrease the amount of water you use over your lawn until you water with only 1 inch of water every 10 days.