Spliced roof rafters

Scarfed splice and recommended loading.

You’ll notice that the first diagram shows a structural support directly under the splice.

And if the splice is to be used in a ridge, a ridge itself is not a load-bearing member. It it’s a structural ridge, it’s a girder, and again would be more massive, such as multiple lvl’s.

By the way, I’m an architect, with well over 40 years experience in residential construction, plus wide experience in commercial construction, plus experience in civil, mechanical and structural engineering, land surveying, and real estate appraisal.

Im not a code inspector or an architect or engineer. But the IRC says
Purlin - R802.5.1 Purlins are permitted to be installed to reduce the span of rafters as shown in Fig. R802.5.1 (8) Purlins shall be sized no less than the required size of the rafters.

My comments - The purlin bracing is run to a load bearing wall for to provide the support. No wall no bracing and a beam has to be provided to pick up the purlin support bracing. RAfter transmit load to purlin then purlin support then beam if required the walls then foundation. The purlin does provide support for the rafter and thus reducing the rafters span. The rafter splice is not ideal. My first choice of further evaluation would be a structural engineer.

James, I don’t disagree with your comments but don’t we see purlin bracing terminating at, what I’ve always known as, a 'strongback"? A strongback would not neccesarily be on or above a load bearing wall but simply on the ceiling joists perhaps several feel away from the nearest wall.

Mcheal yes you do see that, but its actually wrong. You are not suppose to brace from the the strong back (If I understand you correcttly, also called rat run.). The load has to be transferred to grade one way or another. If they cant brace it to a load bearing wall the proper way is to build a beam. Strong backs (rat runs) are not acceptable. There good for straighting the ceiling joist though. And for rats.

Often, especially in older homes they used strongbacks which were insufficient and bowed the ceiling rafters. We were always taught to run purlin braces down to a wall wherever possible. The concensus seemed to be excessive brace angle (within reason) was better than bracing to a strongback.

I had to dig back into the old treasure chest for this one. Apprenticeship school '73, by cracky!

Thanks for the clarification guys! I’ll pay more attention now to where purlin braces end.

I pretty much agree with Kenton, probably because he was apprenticing about the same time I started building. Old school carpenters had to know a lot more than they do now. And yes, a purlin reduces rafter span and should be supported by load bearing wall.


I’m absolutely ‘right’ as far as framing these rafters go using the *‘prescriptive’ *requirements of the IRC building codes…

But you are also absolutely ‘right’ in that if this is an engineered design, then it can also be equally approved.

The question remains: Which one is it?

A butcher job or a code compliant engineered design?

I suspect the former…

But (as a Code Official) unless I had an architect or engineer’s structural calculations to verify that these rafters were framed via acceptable engineered methods…would have to reject it outright…

Unless specifically engineered this way and “clearly shown on the plans and drawings” the rafter should be continuous.

  • I would recommend further evaluation by a structural engineer.

Do not let anyone tell you anything different.

Kenton and all:

There are different definitions and uses for purlins:

In architecture](http://en.wikipedia.org/wiki/Architecture) or *structural engineering](http://en.wikipedia.org/wiki/Structural_engineering), a purlin (or purline) is a longitudinal structural member in a roof](http://en.wikipedia.org/wiki/Roof). Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters](http://en.wikipedia.org/wiki/Rafter) and/or the building *walls](http://en.wikipedia.org/wiki/Wall). 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.

The under purlins used in the picture are on their flat and will not support shot. The under purlin in your manual is on its edge and will give some support …but how much??? I see this latter strut supporting “under purlins” on their flat ocassionally but in this situation, each strut supports an individual rafter down through the purlin, then strut to the top of the interior load bearing wall. Not at all, like the picture. This technique was fairly common here in the 1920’s - late 40’s.

There’s some serious crap going on in the picture. Wouldn’t want it to be my house!!!


It strange, but the terms used by tradesmen can wind up with almost opposite meanings from the engineering terms describing the same component. In talking to an engineer about a complicated set of his trusses which we were trying to assemble, man we went round and round. I finally realized that what he called a girder, we called a hip (or something like that). It can be different in commercial and residential, in materials (“lintel” and “header”) and across the country.
When it comes right down to it, it makes no difference what you call it as long as you both understand that you’re talking about the same thing. Sometimes you have to back up and establish communication.

Ain’t that the truth and the whole truth.

What hurts the most is asking “why”, to what they are saying.

I have learned that Communication is #1 if we all try to resolve a situation, even if it means a step down on our comprehension.
When in Rome, you do as the Romans do, but at least you can ask why!
Then you can understand the meaning of their interpretation of the same outcome.

Marcel :slight_smile: :stuck_out_tongue:


deleted :mrgreen:

Thanks for all the input. This is why the message board is so great !!

I agree with you and homebild. Model codes like the IRC and similar standards have no provisions for rafter splices … and the IRC also has provisions that rafters must be connected to wall top plates and ridge/valley/hip rafters. So it would need an “engineered design” using IRC R301.1 … for good reason, as they are often done wrong.

Framing splices shown in current graphic standards and framing handbooks are typically for members that do not carry significant load (e.g. ridge rafter/board or top beam splice). Rafters can be spliced … BUT … the splices need to be carefully located and designed/detailed for the specific conditions. They are sometimes just field fabricated from whatever is on hand, using non-structural splice details/practices, and typically without enough strength. They may appear to work initially, but over time start showing signs of problems.

While it may appear to be a simple framing repair on the surface, I would probably go with the SE on this one and have them do a sketch/description of the repair for the record, as opposed to having a second framing contractor possibly wing it again.

JMO & 2-nickels … :wink:

P.S. In the engineering world a “purlin” is any horizontal roof framing member spanning between and perpendicular to roof rafters/bents/trusses.

It is typically used to describe horizontal members that directly support roof decking, and transfer those loads to roof rafters/bents/trusses. The term is also commonly used to describe horizontal members supporting sloped glazing.

While it could be used to describe a horizontal member below rafters (i.e. IRC Fig R802.5.1) … I typically wouldn’t use the term “purlin” or “under purlin” for that (e.g. I probably would just describe the members in the IRC Fig as rafter posts and braces).


Robert, you could not have said that any better.

You are absolutely right.

I have seen too many unqualified Contractors make that lethal attempt of repairs. I work for a Contractor, but have a lot of information and education under my belt.
Some undertakings by some Builders take on more than they can chew when it comes to right and acceptable in the Enginnering World.

There is never a bad time to make things right. If you know what I mean.