CAA: Christian Anime Alliance

I've noticed in lots of anime set at schools, like Azumanga Daioh or Kekkaishi, there's often scenes where the characters will eat on the roof. Is there any reason this happens in Japan? Is it the dense population?

Thy're usually on the roofs to be alone in those anime. From what I understand from my friend who's been over a few times, it's far more common in anime than in real life, where there's almost no access to the roof.

Yeah, I've noticed that too, Rock.
It's a perfect setting for the wind to blow through her hair and the sunlight to reflect off their polished brown book bags.

Incidentally, in anime, they don't present anyone as getting in trouble for being on the roof - I guess story writers don't plan on that.

You can climb onto our roof at school. You wouldn't be the smartest to do so, but you could. Our roof is dangerous. (Old school, stairs go almost all the way to the top, you just gotta jump).

You're right, it seems really really commonplace in anime, for whatever reason. (It gives us a cool perspective of Japan! )

Yeah, it's even common in older anime as well. Like Boys over Flowers. I always wondered that as well. Why it's such a common secret place in lots of "slice of life" anime especially.

Most Japanese schools do not have a cafeteria; students bring their own lunches and eat in the classroom (source). It would be pretty natural for students who want to be alone to seek out some other place to eat their lunch during that time.

I haven't been in very many Japanese school buildings and can't verify whether they typically have a roof access, but the church that I used to attend in Tokyo did indeed have a roof access that was just like what one typically sees in anime and manga, and I went up there on several occasions for exactly the same reason that those anime and manga characters do: to be alone. So it does not seem at all far-fetched to me that students in Japanese schools would do the same thing.

The rooftop scene adds to the character.
If you see a lot of anime involving this scene. They eat their lunch there, sleep there evading class and also smoke.
In Japan, it's prohibited to enter the rooftop. It adds to the anime character by showing a bit of authority (having access to the keys to the rooftop), shows that their rebellious (hiding from any teachers who might find them and going to the rooftop to sleep) like-wise for the smoking and delinquents that go to the roof top. The rooftop is also for the characters/group of characters that don't fit in, or don't necessarily like the presence of others. Then theirs also the possibility that directors are being lazy and don't want to add the "passers-by" characters, and create an isolated place where they can talk.

But to answer your question, this rarely happens in Japan. They do more of the joining tables and eating in the classroom.