roundup is glyphosate it only works on leaves, its inert where it soaks into the soil as roots cant absorb it that way
I have a really dodgy drive that has patched of soil in it now -I use gallop a commerical glyphosate, I wiat till there are sufficient leaves on the weeds then spray, 2 or 3 times a season is sufficient.
there are 2 problems: perennials like dandilions that can grow back if not killed down to the roots with glyphosate. A gas burner wont kill the roots of perennials , they will grow back
and seeds newly germinated -which glyphosate wont kill, although it does kill tiny seedlings, so it will be while before it grows back