purlin
A longitudinal structural member bridging two or more rafters of a roof.
The loft was broad and high, with splendid rough-cut oak purlins and three large Veluxes.
Where it comes from
Uncertain. Information on the etymology is scant at best. According to Websters it comes from 15th-century English. According to the Oxford Concise Dictionary, it is "Middle English, perhaps of French origin". Other sources reference Middle English or 15th-century English.
Adapted from the Wiktionary entry purlin (CC BY-SA 4.0).