Thread: purlin-brace system

The 2x4 purlin was lying flat, rather then on edge.

I'd think this would be a defect , not carrying the load properly, but I didn't any roof sag, etc... 1981 home.

What do you all think?
Clay

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Clay

Purlins to my knowledge are always on their side - 3.5" side in your case with a 2x4.

Clay,

To clarify, your title is "purlin-brace system" but your post was about the "purlin"?

Or was your post about the "purlin brace"?

Purlins are required to be the same size as the required size of the rafters, which would mean that the pulins were installed vertical (1-1/2" wide by the rafter height for the height) and not flat-wise (1-1/2" high by the rafter height for the width).

What was the size & span of the rafters?
Some times you will find that the size & span installed are well within the limits of requiring mid-span support.

As a Structural Engineer, that sounds really strange to me - not normal at all.

Is there any way to post a picture or sketch of the condition?

And, in reference to the "purlins", are you speaking of ceiling joists in the attic, or rafters at the roof as being 2X4 flat? Or something else?

No one has discussed the main point. The roof is performing. If we did all our inspections on a perscerptive basis ( as related to attic framing) Then every home with 2x4 rafters would need repair.

Originally Posted by Stacey Van Houtan

No one has discussed the main point. The roof is performing.

That is anything but the main point.

If it is not constructed properly, then it is likely "under-designed" and "under-built" and "the test of time" to which you are referring to without naming is not a "test of time for the better" but a "test of time for the worse" to see if it has failed ... YET.

If we did all our inspections on a perscerptive basis ( as related to attic framing) Then every home with 2x4 rafters would need repair.

They very well might.

Likewise, and to the opposite, any home built with 2x4 rafters SHOULD be addressed.

The purpose of a home inspection is not to look at a tin shanty with no framing and say 'Yeppers, it sure looks like the tin is still standing.', but to look at that tin shanty with no framing and explain to your client that 'I really have no idea why it is still standing, there is no structural aspect or components to hold it up. I doubt you will be as lucky as the previous owners, resulting in you having to pick up the pieces as it falls down and rebuilt it properly - which could become quite expensive.'

It seems that most of the time the purlins are wrong & when they do get the purlin & support spacing correct the support is placed under the flat 2x4 nailed to the bottom side of the rafters instead of supporting the "on edge" part. This puts the stresses on the fasteners instead of the member.
I did see a roof structure yesterday that was textbook, & I mean in a good way. I was impressed & asked who had done the framing & they said he had gotten too expensive & were no longer using him.

Originally Posted by John Goad

It seems that most of the time the purlins are wrong & when they do get the purlin & support spacing correct the support is placed under the flat 2x4 nailed to the bottom side of the rafters instead of supporting the "on edge" part. This puts the stresses on the fasteners instead of the member.
I did see a roof structure yesterday that was textbook, & I mean in a good way. I was impressed & asked who had done the framing & they said he had gotten too expensive & were no longer using him.

And we wonder where all the good carpenters have gone!

I did not mean that the test of time is always the correct method. BUT If you think that all homes with 2x4 rafters need repair i disagree. Do you know why for the same spans the code chaged from 2x4s to 2x6s for rafters and for that matter for the same span 2x8s to 2x10s for floor joist?

Originally Posted by Stacey Van Houtan

I did not mean that the test of time is always the correct method. BUT If you think that all homes with 2x4 rafters need repair i disagree. Do you know why for the same spans the code chaged from 2x4s to 2x6s for rafters and for that matter for the same span 2x8s to 2x10s for floor joist?

I am just getting into this, but I'll take a stab at this one. Was it because of increased insulation?

Some framing methods and size of materials have changed over the years. NOT becuase of structural failure but becuase the fiber creep of the wood caused bowing and saging or excessive bounce in floors. A change in how we define servicabiltiy. This is cosmetic. As I tell my clients you can't expect a old house to be a new house.

Originally Posted by Stacey Van Houtan

Some framing methods and size of materials have changed over the years. NOT becuase of structural failure but becuase the fiber creep of the wood caused bowing and saging or excessive bounce in floors. A change in how we define servicabiltiy. This is cosmetic. As I tell my clients you can't expect a old house to be a new house.

"but becuase the fiber creep of the wood caused bowing and saging or excessive bounce in floors. A change in how we define servicabiltiy."

More accurately, how one defines lack of serviceability.

"This is cosmetic."

No, that is a defect. You said so yourself: "becuase the fiber creep of the wood caused bowing and saging or excessive bounce in floors"

That is not "cosmetic", that is "under-designed" and "under-built".

Does that mean it was "under-designed" and "under-built" intentionally?

Nope. Just means that "it was", intentionally or not.

Older homes show the learning curve of acceptable building practices, and of UNacceptable building practices ... "fiber creep of the wood caused bowing and saging or excessive bounce in floors" is NOT the result of an acceptable building practice, it is the result of an UNacceptable building practice.

Where I live, we went to 2X6 because you can't buy a decent 2X4. The good Douglas fir gets shipped out, so we build with spruce and pine. Check out this 1950's attic. I call this a knee wall, and the flat top plate is correct here. I never use the word 'purlin'. Nobody knows what that is.

Originally Posted by John Kogel

I call this a knee wall, and the flat top plate is correct here.

'Ceptin' that the top plate should have been doubled when laid flat.

Originally Posted by Jerry Peck

'Ceptin' that the top plate should have been doubled when laid flat.

You are more correct than me. There could be a slight sag in that 2X4 between the braces. I don't ever see a double plate but it would no doubt be better. On the opposite side of this roof, there's a chimney and a valley so they just popped in one brace.

These three pic are from a 20 yrs younger home, lower pitch with plywood sheathing. This roof is like a trampoline to walk on.
I recommended an upgrade for this one.
Manufactured trusses put an end to the 2X4 rafter and that was a good thing.

Some framing standards are unclear, but the Perlin system is oneway only. Perlin is a 2x member that runs 90 degree to the rafter at 1/3 rafter span or less. A perlin brace is notched to fit the angle of the perlin and angled to a BEARING wall and nailed to the wall and ceiling joist. Collar ties are 2x members that rest on top of the perlin and are nailed to the rafters and perlin above perlin braces. The notching of the perlin braces to fit the perlin and the weigh load from the rafter is why it needs to be installed as a 2x4 not a 4x2. An excellent reference guide to building standards is the Architectural Graphic Standards

Brad Peterson
Tri-City Inspection Agency, LLC

Brad, I know exactly what you're saying and you must be from somewhere west of the Mississippi, because Ray Wand from out East does not agree with you.

100 yrs ago, a purlin was something else.

I have pics of west coast purlins and collar ties.

Originally Posted by Brad Peterson

Some framing standards are unclear, but the Perlin system is oneway only. Perlin is a 2x member that runs 90 degree to the rafter at 1/3 rafter span or less.

Brad,

Not quite right, and first, it is "purlin" instead of "Perlin", and second there are two totally different meanings and components of what a "purlin" is.

So everyone agress that the purlin should not be laying flat; should be installed on edge correct?

See photo

Rob

Originally Posted by RobertSmith

So everyone agress that the purlin should not be laying flat; should be installed on edge correct?

Not exactly.

Originally Posted by Jerry Peck

'Ceptin' that the top plate should have been doubled when laid flat.

If it is a concern, IMO an easy fix would be to add another purlin.

Quote:
Originally Posted by John Kogel
I call this a knee wall, and the flat top plate is correct here.



__________________

Originally Posted by Jerry Peck

'Ceptin' that the top plate should have been doubled when laid flat.

Ceptin if there is a stud under every rafter. Single top plate is allowed if every rafter (or joist) has a stud below. A case could be made that a single plate with a stud below is better than a rafter on a double plate between studs.

Maybe it's a Canadian thing but the first time I read a thread on purlins I thought you guys had gone bonkers. To me a purlin is on TOP of the rafters. Roof strapping are purlins where I'm from.

Originally Posted by Bert de Haan

To me a purlin is on TOP of the rafters. Roof strapping are purlins where I'm from.

Finally, someone who else who knows what a purlin is - Cool!

I've been telling these guys that for years, but all they know is the purlin which is underneath ... but there are two types.

The term "purlin" is used interchangably whether the member is above the rafters (for support of the roof covering) or below the rafters (for support of the roof frame). It all depends on what is being built and the framing method being employed. The addage that "poor people have poor ways" would seem to apply to some of the methods depicted in photos here. Some of these framers were poor in knowledge. Not that their methods didn't perform acceptably, but they weren't aware of how to do it most effectively. They needed a mentor, or an experienced inspector to show them a better way to ply their trade.

Art Dotson
Construction Code Services, LLC
permitexpeditors.com