I rarely do any framing, and none of the little framing I do involves trusses. But I do install crown molding and the ilk, and have learned that I need to pay attention to uplift in these cases.
So I’m wondering why uplift is a problem with trusses but not with stick built rafters?
Anyone care to shed some light on this for me?
Justin Fink – FHB Editorial
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Replies
I did a thread on Truss uplift quite a wile back. Some of the links are outdated, but the info is still sound.
I really need to update that thread...
A. Call and tell her.
Thanks for the help Boss, I actually toyed with the idea of addressing the post directly to you - but I guess you found it first anyhow.
by the way, you're gonna have to lose that signature tagline.Justin Fink - FHB Editorial
This would make a good subject for an article, wouldn't it? It's been discussed several times. BTW - No one from Taunton has ever complained about any of my tag lines before. ..
A man who finds it painful to smile should not open a shop. [Confucius]
Boss,
You and I should talk about the possibilities of a truss article - I'm not sure what would be the best focus, perhaps you have some ideas in mind? Justin Fink - FHB Editorial
Why don't ya email me, and I'll tell ya everything I know in 30 seconds or less.(-:
If you can't say something funny about someone, don't say anything at all.
Here is another thought that occurred to me when I posted this link above: Do timber framed roof systems ever uplift? Admittedly I don't know jack about timber framing but that howe truss sure does look like some timber framing - doesn't it?
Boss,
Maybe you can answer something for me along these lines.
When I framed in Florida it was strictly forbidden for a truss bottom cord to be nailed fast ( Toe nailed, with no chance of moving) and to bear on any wall unless the truss had a tag identifing a bearing spot on it.
The reasoning given to us was that because of both uplift and how a truss distributes weight, that doing so could cause it to break.
All of our interior non-bearing walls were framed single top plate and about 3/8 shy of the bottom cord. Then they were nailed in a manner that the bottom cord could move up and down.
My first winter in the north I watched some guys let bottom cords bear on, and get toe-nailed to each and every interior wall they crossed. I did get to see one truss break and the company refused to cover it as they said it was an improper installation.
What is your take that?
What about "Float-r" brackets on interior partition walls? That is what we use, and we are careful about how we nail drywall to the trusses(no screws within 8 inches of "Float-r" bracket. Rarely do we have any cracks or problems with uplift.
Thats what I know in less than 30 seconds.
An inch too short. That's the story of my life !
bstcrpntr --- I hope to grow into this name.
The "Float-r" brackets are fine. But so is wood blocking. As long as the truss bottom chord can float, either one is fine.
Age is not a particularly interesting subject. Anyone can get old. All you have to do is live long enough.
Boss,
What do you mean by "wood blocking"?Justin Fink - FHB Editorial
"What do you mean by "wood blocking"?"
From the WTCA truss uplift documentation:
View Image
The difference between theory and practice in practice is greater than the difference between theory and practice in theory.
If the truss wants to go up, it's gonna go up. Something has to give.I've seen trusses pick up interior walls and make some pretty impressive cracks in the drywall. Homeowners don't tend to have much of a sense of humor about that. So I'd say the guys nailing trusses down probably just haven't been burned yet.
One mosquito to another: "Sure, I believe in reincarnation -- in my previous life I was an IRS agent!" [Bob Thaves] (Frank & Ernest)
SO if your company field service guys was called to a house with a broken truss where all of the bottom cords were nailed tight to the walls, Installation error, no warantee?
"SO if your company field service guys was called to a house with a broken truss where all of the bottom cords were nailed tight to the walls, Installation error, no warantee?"
Actually, I've never seen a truss that was broken from being nailed down. I've seen more than one where the truss picked the wall up.
The company policy is this: We send out info about truss uplift with EVERY order, and have done so for many years. (Of course, the first thing framers generally do is throw this stuff away)
We've done presentations about it at local home builders forums. We've passed out literature about it at trade shows. So we've gone out of our way to let our customers and the public know about it and how to prevent/deal with it.
So basically, the answer is yes - Truss uplift is an installation problem.
I went to a ball game and visited the concession stand. I got three concessions and a compromise.
I think it's the truss webs that make the difference. These members transfer any movement in the top cord to the bottom cord (ceiling) and visa versa. Also any length wise shrinkage in the webs will very possibly show up directly as "truss uplift". Basically the truss shrinks and expands as unit - be it 38' long or whatever. Generally the individual members a stick framed roof can shrink and expand independently.
Granted, some stick framed roofs include different types of angle bracing, but it would be far the exception to see a stick frame constructed like either of the pics shown on this web page.
Yeah, but eliminating 26 and 47 on the Howe decreases the "sagging" stiffness of the lower chord, and relates to the ceiling weight. In the example given, that leaves that member like 20 feet long. So, upsize to compensate, and no material savings . . .
I don't think that's "Howe" I would do it.
Forrest
Edited 3/28/2006 7:10 am by McDesign
>>
Yeah, but eliminating 26 and 47 on the Howe decreases the "sagging" stiffness of the lower chord, and relates to the ceiling weight. In the example given, that leaves that member like 20 feet long. So, upsize to compensate, and no material savings . . .<<
OK - I agree with that.
Not sure what your point is though. I was not suggesting prescriptions to modify truss design to reduce/eliminate truss uplift, I was just simply stating that the very essence of the way webbed roof trusses are built is what causes them to uplift the bottom chord. I think you are reading something into my post.
The purpose of the link I posted was just to get the reader to think of a stick framed roof that would be built like one of the more common truss designs.
BTW - I bet Boss could tell us if some roof truss designs are more likely to uplift than others, and further I'll guess that there has been some modification of truss design over the years to reduce this "black eye" that trusses have.
Another BTW - do floor trusses ever uplift? I've never heard of it...
Edited 3/28/2006 8:00 am ET by Matt
"I was not suggesting prescriptions to modify truss design to reduce/eliminate truss uplift, I was just simply stating that the very essence of the way webbed roof trusses are built is what causes them to uplift the bottom chord."
I understood what you meant, and I'd say you're right. Maybe it could be further explained by pointing out that in stick framing, the ceiling joists actually bear on interior partitions. (Unless it's a really small roof) So there's no webbing to pick them up off the interior walls.
"I bet Boss could tell us if some roof truss designs are more likely to uplift than others..."
Not that I know of.
"I'll guess that there has been some modification of truss design over the years to reduce this..."
There's nothing in the truss designs that you can modify to make any difference. Truss uplift is caused by wood expanding and contracting due to changes in moisture content. Nothing that you do to a truss design will change that.
"do floor trusses ever uplift?"
Did you read the thread on truss uplift that I linked to above? It explains the theory on how truss uplift works. Since floor trusses aren't in an attic, they don't have this problem.
The bigger the bankroll, the tighter the rubber band.
Truss uplift happens because the exposed truss lumber in an attic space absorbs moisture. This acceptance of moisture causes the lumber to swell. The swelling lumber lifts the horizontal bottom chord of the truss causing it to bow upwards in the middle of the truss. Without the proper allowances made for this "swelling" drywall corners, crown moulding and the like will seperate at the ceiling line.