couple of questions from a newbie

I’m not sure what the proper term for the horizontal members in the pic are but i’m pretty sure they should not be terminating under a rafter correct?

Also, is it normal (and ok) to use two seperate rafter supporting methods for the front and back? in one pic the supports are pretty much equally spaced … in the other they are originating from too points.

Help please!

The first pic is FUBAR. If it looks wrong it probably is.

The second pic, I will need to pass on to my more smarter colleges.

I think it all looks F’ed up, but like I said… There are way smarter folks to answer your question…

I believe the member you are referring to is a UNDER purlin. Do a search and see a few recent threads on that. The fan bracing in the second pic is probably done to hit the top of a post or a corner in a wall below.
They could have done better, but that first pic is definitely a structural defect. Easily fixed however, IMO. :stuck_out_tongue:

John Kogel


From previous thread:

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/rafters (and some times between widely spaced rafters/beams) 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”).…ition&ct=title

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.

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…ition&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).…ldings_101.asp

Mr. MacNeish, I tried making the same point about the use of the word “purlin” a few months ago, and got my head bitten off. I hope you have a better fate. =)

My impression of the pictured roof framing is that no one would frame a roof that way out of incompetence…someone must have had a reason for designing it as we see, and I believe Mr. Kogel has accurately described the probable reason. To understand the roof framing, you need to look at what’s below, and trace the load path from the roof through the horizontal rafter brace (how did I do there, Mr. MacNeish?), through the supporting members of the brace, and through whatever exists on the floor below, and on and on until you come to the earth.

The condition shown in picture #1 may not be a problem at all, but we can’t see enough to know. The end of the right-hand horizontal member is supported by a post, so the question is whether the left-hand horizontal member is supported by a post directly under whatever load it may be carrying, and we can’t see whether it is or not. It may just be a meaningless cantilever, and could be cut off back at its support without harm to the structure. On the other hand, if it’s carrying two or three rafters since its last support, it would need to have a good splice applied to connect it to the right-hand member. Best to kick this issue to a qualified professional to evaluate and possibly repair.

Hi. John and hope you are doing well.

I looked at your pics and brings back old memories of the inadequate builders in my younger years. ha. ha.
My old Man would have freaked out.

The first picture I can not help you much because I don’t know who is supporting who, too small a vision.

Second picture, ah, the fan support. Well obviously the fan is supported at the bottom to something solid that the builder at the time new and figured would fan out to pick up the load.

Well he was smart enough to know that his fan members were in compression also, so he added the 2x4 on to the fan braces to provide the deflection reaction to the compression on the members.
Well now we have all this compression due to the undersized rafters for the span that I see distributed onto this fan support.
All the compression from the deflection weight of the dead load are on the shear stress of the nails they used to nail those fan braces and the stiffner braces.

Problem is, as you apply a downward pressure on this fan brace and its horizontal support at the underside of the rafter, where is it going to go? In the least resistant direction. And that is towards you looking at the picture.
I do not know how it was attached to the rafters, but it is only worth as much as the fastners used at that point.

And I am still curious as to what the hell that big mound of insulation or what it is in the middle of the attic. :mrgreen:

Hope this helps or confuse you more. ha. ha.

Write hard and speak softly.

Marcel :):smiley:

[quote=“Brian_A.MacNeish, post:4, topic:33813”]


From previous thread:

I think that “kneewall” or “intermediate support assembly” would be the more correct terms here.


Mr. MacNiesh, please call Allan Carson and tell him he is mistaken in his use of this most ancient of words. :stuck_out_tongue: I shall henceforth dub said member a 2X6. :stuck_out_tongue:

John Kogel](“”)

Under Purlin:):smiley:

I believe the term “strong back” and “under purlin” describe the same thing.

Around this area the term strongback is used to describe addittional vertical members added to a whaler stystem on concrete forms higher than 8’.

It is also a term used in floor truss systems where strongbacks are known to be lateral braces that help reduce the dynamic response of a floor framing system by increasing stability. They are not considered structural elements, altough they can help distribute the load on a floor truss to adjacent trusses, helping the floor system act as a unit.


A strongback, to me, means a beam installed above and in contact with ceiling joists, usually suspended from rafters or ridge, to stiffen ceiling joists which otherwise would be insufficient for their span. “Under-purlin” seems to be a good term for the pictured member, as it simply adapts what some believe is the true meaning of “purlin” to the pictured situation.

Question: Wouldn’t it be cheaper and easier to just make the rafters the size they should be for the span?

Another question: There are obviously posts or something similar under one side of the roof, where the under-purlin =) supports fan down to a point, but on the other side, is there something under the ceiling joists which carries the load from the vertical under-purlin supports? Or did they just forget the load path to the earth?

Here are a few pictures of strongbacks;

Now Purlins;

Hope this helps

Marcel :):smiley:

This illlustrates the strongback I had in mind. Apparently the word is used for similar applications in many different fields.

You are correct in that Richard, it all explains the same function though, just differently applied.

Marcel :):smiley: