Because the bottom of the roof is easy to accumulate dust, it is not easy to clean, and dust often falls down. So use a flat flat roof to support it under the roof. As a result, the structure of smallpox was produced.

Smallpox is also called "bearing dust", and this name illustrates its function.

The skeleton pattern of smallpox is not only square or rectangle. There are also hexagonal and octagonal grids.

In ancient times, directions were divided into east, south, west, north, heaven and earth. Besides its function, the flat ceiling in the sky is usually decorated with various beautiful patterns, so it is called smallpox.

This is a common explanation in the field of decoration.

Just checked in Baidu Q&A and found that it was mostly plagiarism: the decoration on the board was like smallpox, so it was called smallpox. Personally, I think this statement is inappropriate. This smallpox should have nothing to do with smallpox virus.

References:

Ancient decoration