how soon after using roundup can you re-sod
Roundup is not persistent in the soil. You can lay your sod one week after spraying the weeds. The herbicide will get into the growth of any plant it touches, not anything the chemical does not touch. I don't know of a pre-m herbicide that controls germination of any weeds likely to grow this time of year. Pre-m only prevents weed seeds from germinating.