Killing your old grass with Roundup can be an effective way to prepare your planting area for a new lawn. After using Roundup, you can skip tilling your lawn and seed directly into the old grass. Your new seed can use the old grass as mulch to prevent the seeds from washing away in rain. Also, by not tilling your planting area, you won't be bringing any weed seeds to the surface. Your end result will be a lush, new lawn without weeds.
Mow the dead grass and weeds down to a height of 1 inch and rake up all the clippings.
Rent a slit seeder from your local rental store. Read the instruction manual for the slit seeder to learn how to properly operate it.
Fill the seed hopper on the slit seeder with your intended grass seed. Consult the seed bag to see how many pounds of grass seed should be applied per 1,000 square feet.
Set the slit seeder to apply half of the indicated rate. Drive the slit seeder over your yard twice--the second pass should be perpendicular to your first pass. This will prevent your grass from looking like it was planted in rows.
Set up your sprinklers and irrigate the grass seeds two to three times per day to keep the soil moist. Do this for 10 to 14 days or until germination is complete. Then slowly back down watering over a few weeks until you are following your normal irrigation schedule.
Mow your grass when it reaches 3 inches in height. Once mowed, continue to mow as needed to maintain it between 2 and 3 inches high.