What percent weeds does a lawn have to be to kill with Roundup and start over. vrs. a 3-way then overseed. I would go with 50 % or worse, just the totally lost causes. Also, Roundup once then seed, or twice to make sure u get it all. Ok just one more, the lawns that get as bad as 50 % weeds are usually growing in crap, try very hard to sell topdressing? Is complete kill with Roundup, then topdress with compost, then seed, the best lawn reno possible?