Is snow load a possible factor in truss uplift?
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I'm not sure what you mean.
Truss uplift means that the trusses go UP. Snow load would tend to mean they go DOWN.
Have you seen my post on truss uplift? It tells a lot about what I think about the problem:
http://forums.taunton.com/n/mb/message.asp?webtag=tp-breaktime&msg=24607.1
In that post I mention that I think that poor ventilation contributes to truss uplift. I suppose if a roof was covered with snow and the vents were blocked for a long time it could contribute to the problem. But I've never given it much thought, as snow doesn't stick around very long in this part of the country.
Is there some particular reason that you're asking about this?
Truss uplift occurs during the winter... right?
Yup.
If at first you don't succeed, it could that you're just not be very good at it.
I thought I had read somewhere that the added weight of snow on a truss roof can cause additional tension on the bottom chord, causing uplift. Maybe, maybe not, I dunno. It was being discussed elsewhere, which is why I ask.
I believe you are right. Some part of the bottom chord are in compression and some parts are in tension so with a heavy snow load possible movement would be amplified. When I was a draughtsman I used to know all that #### but forgotten a long time ago. A very long time .............so I might be wrong.roger
A draughtsman.... a guy sitting at the pub swilling large mugs of frothy brew?
Nah! Sometimes I forget myself and spell tyres and draughtsman the old way. I get a kick out of all the red underlining in spell check. At least phonetically they sound the same.roger
Really - I'm there.... I'm on this imperial stout kick now... it packs a punch... :-)
Seriously though, I thought that truss uplift was largely caused by the webs shrinking - ie drying out of the lumber. Here for example we have about 90% RH during the summer and 20% RH during the winter...
Edited 5/3/2008 12:22 pm ET by Matt
I lived in Ann Arbor for 25 years. Get out of the shower in the summer, towel off, and the exertion causes you to sweat like a pig all over again. Oh yeah, love it.
As far as uplift goes, I plead total ignorance. I know enough to order and use the clips. I ask not why. In another thread there was talk about why uplift occurs. I mentioned having read about roof load as a factor. A couple of guys sniffed at me. I thought we'd see what Ron says.
I'm no expert by any means, but the clips specifically recommended do NOTHING to help the problem - as far as I'm concerned. What they do is allow the truss bottom chord to ride up and down - unencumbered... Ahhhh.... that IS what the problem is - the bottom chord going up and down. I just make sure my sheetrock guys don't nail too close to the partition walls and hope for the best. I think a lot of air leakage in the top ceiling and poor attic ventilation contribute too. Possibly moisture pointloads in the interior such as poorly vented or vented to the attic bathrooms or kitchens too. I've only seen it pretty bad on one house I built. I think part of that was though that the floor trusses below had a clearspan of something like 24', so I couldn't really say if the 'lid' was raising, or the floor was sagging. As it turned out I didn't have to pursue it too much. Hate to admit it but I caulked the ceiling/wall joints with paintable silicone which was the most flexible caulk I could come up with threw some paint on it and never heard back on it. It did teach me to always check floor loading, etc on truss designs though...
"the clips specifically recommended do NOTHING to help the problem"
They don't cure the problem - Just hide the symptoms. They're the best option available, as far as I'm concerned.
"I think a lot of air leakage in the top ceiling and poor attic ventilation contribute too."
I agree, and I mentioned poor ventilation in the thread I referenced earlier.
"I think part of that was though that the floor trusses below had a clearspan of something like 24', so I couldn't really say if the 'lid' was raising, or the floor was sagging. "
Good point. I think the truss industry has only done enough education on truss uplift to convince everyone that ANY drywall crack might be truss uplift. There certainly can be other causes.
That which does not kill me only postpones the inevitable.
Ron - I'm hurt - you don't want to argue with me???
OK - so I had a few.... :-0
I was up most of the night for prom and post prom. (Pics are in the tavern)Don't have the energy to argue with anyone. (-:
Married men should forget their mistakes - there's no use in two people remembering the same thing.
Wow, I didn't realize you were a high school senior. Did you have a cute date for prom? Dad lend you his convertible? You definitely post like a guy with a lot of actual experience. You just never know....
"I thought that truss uplift was largely caused by the webs shrinking - ie drying out of the lumber."
The webs may have a little to do with it. But I don't think anyone attributes very much to them. It's assumed to be mostly the top and bottom chords.
Children and the simple-hearted are nearer to God than most of us.
" Some part of the bottom chord are in compression and some parts are in tension..."
The entire bottom chord is in tension in most cases. The only exception would be if one end of the truss was cantilevered.
Colvard's Logical Premises: All probabilities are 50%. Either a thing will happen or it won't.
You're right and that's what I remember BUT according to "Framing Roofs" a Fine Homebuilding book they show a picture of a truss with arrows pointing the direction of the loads and they show that the bottom cord is in fact in tension but in different directions. The tension is pulling towards each gusset plate. If you think of a gusset plate with arrows on both sides pointing towards it, the point where the gusset plate must therefore be in compression.
To be honest I always thought that the tension started in the middle and went out equally to each side.
The web braces are also either in compression or tension so with all this pushing and pulling at different locations I don't see why in some cases the truss doesn't lift with load especially if the the compression webb compresses more that anticipated, thereby lifting the bottom chord.
Either way, I liked it better when I hadn't read all this extra stuff:)roger
"...they show a picture of a truss with arrows pointing the direction of the loads and they show that the bottom cord is in fact in tension but in different directions. The tension is pulling towards each gusset plate. If you think of a gusset plate with arrows on both sides pointing towards it, the point where the gusset plate must therefore be in compression."
You misunderstood the drawing. The arrows only show that the indicated chord segment is in tension. There is no tension in a typical bottom chord. (in most situations)
The amount of tension in the bottom chord of a truss does not change until it hits a joint.
"I always thought that the tension started in the middle and went out equally to each side."
Tension is highest in the end panels, and less in the middle.
"I don't see why in some cases the truss doesn't lift with load especially if the the compression webb compresses more that anticipated, thereby lifting the bottom chord."
I'm not sure why you would think that. When the top chord is in compression, it shrinks a little in length. The bottom chord is in tension, so it stretches a little. Both of those things would allow the truss to sag a little - Not make it move upwards.
Born free... taxed to death.
I'm talking about the 4 web braces. Two are in tension and two are in compression(according to the drawing). I'm surmising that if the compression one compresses more than anticipated it could lift up the bottom chord. Stranger things have happened.roger
"I thought I had read somewhere that the added weight of snow on a truss roof can cause additional tension on the bottom chord, causing uplift."
Nope.
Taken from the thread I linked to:
"The most widely accepted theory is that it's caused by a moisture difference in the top chord and bottom chord during cold weather. The bottom chord is covered in insulation, and is warm and dry. It dries out a great deal and shrinks some.
The top chord is cold so moisture condenses on it. Since its damp, it expands. Since the top chord expands and the bottom chord shrinks, the truss arches upwards."
Children seldom misquote you. In fact, they usually repeat word for word what you shouldn't have said.