Roundup is a non-selective herbicide. Because Roundup is non-selective, it will kill any grasses or weeds it comes into contact with, although it must land on a plant's foliage to be effective. It is not effective when applied to the soil. For that reason, you can plant grass seed fairly quickly after using Roundup, although it is best to wait until the dying weeds decompose, as they enrich the soil.
Things You'll Need
Hand-held or rolling sower
Apply the Roundup, then wait until the weeds are dead (completely brown). This could take two or three weeks, possibly even longer.
Remove large rocks or anything else that will interfere with a lawn mower from the sowing area.
Mow the dying weeds and grass as close to the ground as possible. Then, till the top 2 or 3 inches of soil, working the decaying weeds into the dirt.
Use a metal rake to level out any areas of the lawn that might collect standing water, as standing water can cause grass seeds to rot.
Spread a layer of grass fertilizer onto the soil using a rolling or hand sower. Work it into the soil, again using a tiller.
Plant the grass seed. Using a hand sower will let you get into the corners of oddly-shaped lawns. Work in a pattern, just as you would mowing the lawn.
Flip a plastic rake over and use the wide back of it to gently work the seeds into the soil. You do not have to press them very deeply into the soil.
Water gently. Too much water may cause run-off, which can wash away the seeds. Keep the soil moist and cool for optimum germination.
The best time to plant grass seed is in early fall, when the temperatures are cooling off but there is still no danger of frost.