Have you ever seen a yard with random dead spot all over it? It could be an insect, fungus or disease...but often times it's self inflicted! Don't spray round up on your turf!
There are other options out there that WON'T kill your turf...just a little research to identify what weeds you have and what will kill them will go a long way!
Digging up problematic weeds and patching in sod is a much better alternative to round up.
If round up is a must, it is better to wait until fall. That way the the grass will be dormant soon after the weeds die, and you won't have those dead spots all over your lawn all year!