What is the meaning of "tops" here?

Question

we would see the Sun on his journey, crossing between the building tops from our side over to the RPO Building side.

Does it simply mean "roofs"? So why "tops" and not "top"? Or why building and not buildings?

Could you explain it to me?

The fuller text:

when we were new, Rosa and I were mid-store, on the magazines table side, and could see through more than half of the window. So we were able to watch the outside – the office workers hurrying by, the taxis, the runners, the tourists, Beggar Man and his dog, the lower part of the RPO Building. Once we were more settled, Manager allowed us to walk up to the front until we were right behind the window display, and then we could see how tall the RPO Building was. And if we were there at just the right time, we would see the Sun on his journey, crossing between the building tops from our side over to the RPO Building side.

Answer 1

Tops means roofs, or the upper edge of the buildings that the narrator can see.

It is tops because there is more than one building that the author can see, and the sun is moving from one to another.

It is building and not buildings because this is a construction called a noun adjunct, using one noun (building) to modify another (tops). When we use a noun adjunct, we don't pluralize the noun adjunct: for example, a horse race, not a horses race, even though there is more than one horse.