Most guinea pigs can't make it over the grid and usually have no desire to, as guinea pigs aren't climbing animals. However I think a few people have had a pig escape from the cage, and the only answer could be that they took a flying leap over the grid! I don't think there's any need to make a roof, since hardly any pigs try and get out.
If you have a cat or dog you'll probably want to put a top on, as they are predators by nature and it's only their instinct to chase a small, furry scurrying animal.
Hope this helps
