I think that “kneewall” or “intermediate support assembly” would be the more correct terms here. The use of purlin as a support beam at this location goes back to the 15th century and is used in timber frame construction and British definitions. In North America, purlin (other than in timber frame structures) is used to describe the horizontal members installed on top of trusses (and some times widely spaced rafters) to support roof sheathing or roof materials such as metal panel roofing. In stick framing, the term “purlin” would never be used for this support structure (unless we continue to bastardize building terms like we have with the terms “lintel” and “header”).
Archaeology Dictionary:](http://www.answers.com/library/Archaeology%20Dictionary-cid-2235220) purlin
A horizontal beam running along the length of a roof to support the rafters or roof-boards. The purlin is supported at the ends by gables and at various points along its length by crucks or frames.
Wikipedia:](http://www.answers.com/library/Wikipedia-cid-2235220) purlin
In architecture or structural engineering, a purlin (or purline) is a horizontal structural member in a roof. Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters and/or the building walls. The use of purlins, as opposed to closely spaced rafters, is common in pre-engineered metal building systems and some timber frame construction.
In timber roof construction prior to the introduction of trusses, under purlins were used to support rafters over longer spans than the rafters alone could span. Under purlins were typically propped off internal walls. For example, an 8"x 4" under purlin would support the center of a row of 6"x 2" rafters that in turn would support 3"x 2" roof purlins to which the roof cladding was fixed.
In metal building roof systems, purlin members are frequently constructed from cold formed steel ‘Z’ sections. These sections can be lapped and nested at the supports which creates a continuous beam configuration between the bays.
References
- This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.
Glossary of terms
Definition of words generally found within this website. Scroll down for the full list
http://www.google.ca/search?hl=en&rlz=1T4GGIC_enCA284CA285&defl=en&q=define:Purlin&sa=X&oi=glossary_definition&ct=title
Word: Purlin
Definition: Timber used to support roofing sheets. Usually fixed on top of rafters.
A horizontal structural member spanning between beams or trusses to support a roof deck
A horizontal member attached perpendicular to the truss top chord for support of the roofing (i .e., corrugated roofing or plywood and shingles).