Longitudinal timber set in the plane of a roof slope and supporting common rafters. Purlins can be classified by their attachment to the principal rafter into five variants.
Subsidiary terms
A) Base cruck B) Spere truss C) Secondary truss D) Raised cruck E) Upper cruck
a) Wall plate b) Cusp c) Cusped windbrace d) Ridge piece (ridge purlin) e) Principal rafter f*) Plate g) Purlin h) Cranked tiebeam l) Ceiling beam
b*) Ridge purlin d*) Soffit (face) e) Upper side f*) Top (face) g) Lower side h) Inner side i) Outer side j) Upper face of truss