it sounds like they are decorative styles over plywood or other
construction instead of a plain 'Five V' roof on perlings and rafters.

The Collaborative International Dictionary of English v.0.48 (gcide)
Purlin Pur"lin, Purline Pur"line, n. [Etymol. uncertain.]
(Arch.)
In root construction, a horizontal member supported on the
principals and supporting the common rafters.
[1913 Webster]