I think the activity near the fence has increased because of the person feeding them rats. It's caused them to be more agitated knowing theres food inside, where previously they were more docile unless they saw people nearby. But I agree that they should be thinning the herd.
In the past the walkers have shown they aren't very adept at climbing, plus there weren't any humans on the roof to go after, so the group wouldn't think they would be up there. I assume the walkers on the roof were from the crashed helicopter, and the helicopter weakened the roof when it crashed. Some of the debris and possible fire could have damaged the roof to allow rain water to penetrate and rot it.
Wait until this week. The preview on Talking Dead showed Daryl and others in a car coming upon a herd of walkers coming their way that Greg Nicatero said had over 7000 walkers.