How do you call one part of a pitched roof

The pitched roof consists of two sloping sides that have some angle between them. So how would one call such a side?

I would like to replace side of the roof in the following sentence with one word: "There is a chimney on one side of the roof and a dormer on another"

Solution 1:

Side or Slope

A pitched roof has names for many parts, and names for each layer of the roof. The big wide part that would be called a side does not have any more specific name. It is called a side or a slope.

In your sentence:

The roof has a chimney on one side and a dormer on another. The roof has a chimney on one slope and a dormer on another.

See here for an example: https://www.caaquebec.com/en/at-home/guides/reshingling-your-roof/identifying-the-parts-of-the-roof-and-understanding-their-functions/