Why Do They Put Rocks on Roofs?
Climate had a bearing on construction. Wind blowing across a roof will cause uplift and, in severe cases, blow the roof totally off. Roofs are typically mechanically fastened (screws) or affixed with some type of glue.
However, some roofing systems use rock ballast to keep them in place. As ballast, the gravel will help anchor the material underneath and lower the risk of it being blown off in high winds.
UV protection is provided. Some roofs require UV protection, much as the shingles on your house have small small rocks in them to protect the asphalt in the shingle from UV degradation. UV protection is required for almost everything made of asphalt, such as a hot tar roof.
Why Are There Rocks on Japanese Roofs
Gravel and pea shingle have two key functions on a flat roof: they act as a ‘loading coat’ and give UV protection. The gravel, acting as ballast, will help anchor the material beneath and reduce the chance of it being blown away in heavy winds.
Second, climate influenced building & construction. In Kyoto during the late Heian and Muromachi periods, roofs were wrapped in thin wooden shingles, thus owners would place stones on top to keep the shingles from blowing away in the wind. The size and complexity of the building denoted the social rank of the minka owner.