What’s the purpose of the rocks sometimes seen on flat roofs?

Dear Cecil: I am 10 years old and I ask my mom a lot of questions. She said to ask you because she asked you questions before and you know everything. My question is, Why do gas stations sometimes have rocks on the roof? Owen K., Seattle

Cecil replies:

Dear Owen:

I assume you’re not talking about rocks meaning boulders, but rather the layer of small stones you see not just on gas stations but on many flat roofs. Ideally the stones are river gravel, which has smooth edges and won’t cut into the roof when somebody walks around on it.

Talk to ten roofing contractors and you’ll get ten answers on what the gravel is for, but this seems to be the consensus:

(1) Stabilization. In a typical tar-and-gravel roof (which, by the way, is gradually being supplanted by gravelless roof systems), you put down a layer of tar paper, then tar, and finally gravel. Tar has a low melting point. In the summer it gets pretty soft and if left to its own devices would flow to the lowest point on the roof. The stones force it to stay put.

(2) Protection. The gravel prevents sun, hail, errant baseballs, etc., from damaging the “membrane,” the all-important moisture seal created by the tar paper and tar.

(3) Ballast. The stones keep the tar paper in place and prevent it from buckling or blowing away in a high wind.

That’s all there is to it. My regards to mom.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.