Technical Booklet D Structure·Page 36·4B.13
Purlin installation and thrust restraint
Tables apply to purlins installed perpendicular to the roof slope, with horizontal thrust from rafters restrained by ceiling joists or other means.
the tables are applicable to purlins installed perpendicular to the roof slope, whereby any horizontal thrust sustained by the rafters is restrained by the ceiling joists, or by other means
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet D - Structure.pdf