Roof purlins

When you find roof purlins not the size of the rafters they support, also having narrow edge laid on side instead of standing up - do you write up as comment or repair?

Does your comments/repair also depend on the age of the home?

Hi Linda,
that is a knee wall not a purlin.
you should recommend that there is a “stud” at each rafter.
the plate that is on the flat is normal for a knee wall construction.

It’s true, that is not a purlin assembly. A purlin assembly would have a horizontal member with it’s edge against the bottom of the rafters and be supported by braces (preferably) every other rafter. Each brace must bear on top of a wall, not on joists.
This is a “knee wall” which is basically any short wall and has different names in different parts of the country. Because the studs are so far apart and because the top plate has the width of the board against the rafters, it’s pretty ineffective and may cause the ceiling to sag in areas since the load is borne by ceiling joists instead of by the proper load path which would be walls/ floor assembly/ foundation.
Bad Carpenter!
Modern codes call for the strongback member of purlin assemblies to be the same dimension as the rafters. This was not required for many years and many, many roofs have been framed and have performed successfully over the long term with purlin strongbacks smaller than the rafter size.

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:]( 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:]( 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.

Glossary of terms

Definition of words generally found within this website. Scroll down for the full list
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).

Good information. I learn something every day.

My understanding of knee wall - there would be studs framed as the joists above / below, every 16 to 24 inches, and placed above a load bearing wall to transfer the weight to the foundation. This wasn’t made that way.

House built in 2005 on slab foundation, 2,716 sq/ft with hip roof, pretty long span from eave to ridge. Below are a couple of pics in the attic. The ‘knee walls’ were in the lower third of the span. 2x8 ridge, 2x6 rafters on 24 inch centers, 7/16 osb decking.

The roof from outside looked pretty good (speaking of deflections) except over the double overhead garage door, on end of house. Had noticeable sag in center of header and the fascia at that point.

I really thought it could have used some more bracing. Of course, if I build anything it’s usually built pretty ‘butch’ as one of our customers has been known to say (as in lots of bracing).

That’s what is so great about the message board. Sharing of info !! :mrgreen:

101_2707 (Small)a.jpg

101_2722 (Small).jpg