Do you need a rooster to have a hen lay brown eggs?

Hmmm, I don't know. Do men mate with women to produce children?

A somewhat less sarcastic answer

Yes, roosters are the only way a Hen can produce a fertile egg. The eggs you buy in stores are not fertilized. A flock of chickens with no rooster will still lay eggs and some of the hens may sit on the eggs but nothing will hatch.