I’m looking for insights from others (and/or references) on the subject of truss rise. If it’s been discussed at length on an earlier post, please let me know.
I’m interested in a few things:
First, what it is I’m referring to is the tendency for roof trusses to rise in the winter and lift off of any interior partition walls, sometimes causing ceiling drywall cracking. I know all about how you are supposed to isolate the ceiling drywall attachment away from any walls but I’m interested in the causes and frequency of occurrence.
Is it somewhat random?
Is there a set of conditions that will always result in the truss rise behavior (excluding consideration for whether or not the drywall is fastened suitably)?
In two identical houses, could one have it and the other not? -or- Does it always occur and we just don’t see the problems unless the ceiling happens to be attached unfavorably?
I have trouble understanding the explanation that the bottom chord is covered in insulation and the top chord experiences the changing environment. My trouble with that explanation is this: If the bottom chord is warm and the top chord is cold, wouldn’t the top shorten and the bottom stay long and wouldn’t that tent to flex downward? Similarly, if the top chord gets dry due to the relative humidity drop in the winter, wouldn’t that top chord shorten, again leading to a drop of the truss?
If anyone has experience observing the various conditions that cause this behavior, I am interested in hearing about it.
Thanks
I have another question about this problem. (BTW I’ve tried to search out other posts on this subject. Using the term truss lift was helpful)
My additional question is this:
I have a question about the use of slotted angle brackets on top of the interior walls. Except to help stabilize a long straight wall, are these really beneficial? with many conventionally sized rooms I would expect that perpendicular walls will be adequate to stabilize each other.
Also, how common is it for the truss manufacturer to specify these? (I’ve never seen the truss maker spec them and would expect it would be on the building designer to know whether or not they are needed.
Any experience with these components?
Also, any feedback about the specific conditions in my original post are appreciated.
Edited 12/30/2005 5:36 pm ET by houseboy
Replies
You need to move this post.
It belongs in the Mysteries of the Universe section.
Joe H
I have always assumed that it was caused by the wood in the truss expanding and contracting as they absorb and give up moisture. Expansion and contraction due to temperature changes would exacerbate this effect in that when the air is wet it also tends to be hot and when dry, cold.
So truss members are expanding and swelling in warm-wet weather and shrinking and contracting in cold-dry weather. The effect being more clearly apparent on longer trusses.
I don't know and have based this on an imperfect understanding of the effects of these basic physical dynamics on trusses. Perhaps Boss Hog, the resident truss expert, will weigh in on the subject with more learned insights.
The effect being more clearly apparent on longer trusses.
Not necessarily true 4Lorn1
I've lived in several houses with trusses, all in our cold MI climate. We have about a 30' span now and I've never noticed any truss uplift effects. I have both flat ceilings and cathedral ceilings here. I don't think this house had any special techiques to deal with truss uplift used, but they may have floated the drywall in on the ceilings. We are showing nail pops in a few spots.
The worst uplift effects I 've seen was in a house that had only 26' trusses. The effects were so bad that it would be impossible to paint the walls and ceiling in winter, if you were trying to cut in two different colors at the ceiling/wall junction becuase the drywall tape was rounded almost looking like chamfer. In that same house down stairs (it was a quad level), the lower trusses showed no signs of uplift. They were only 24' span however.
I also lived in a small house on a lake. That had about 25' trusses and zero attic venting. No signs of moisture problems or truss uplift problems.
On that same lake, my neighbor had a smaller cottage with about 24' trusses. the carpenters had notched a lookout block into the first truss. The truss snapped at that notched area.
It's a mystery.
blue
Re: "I also lived in a small house on a lake. That had about 25' trusses and zero attic venting. No signs of moisture problems or truss uplift problems."Several thoughts:A lake can sometimes have a stabilizing influence on moisture and temperatures. As sweating breathing humans we feel the wind and the moist wind off a lake can make it feel colder but trusses only 'feel' the temperature and moisture.Also you say: "zero attic venting" which might tend to stabilize the temperature and moisture in the attic. Especially considering your northern climate. Unlike a sealed attic in much of Florida where temperatures can soar in a sealed attic higher than the experts seem to think. The sun down here is much higher in the sky and is going through considerably less tempering atmosphere.As for size I suppose the design of the truss itself might have a greater bearing on its sensitivity to temperature and moisture changes. But given two of the same design I would still think the larger would have more movement and uplift.
i'm not a truss guy so I wonjh't contribute much to your knowledge base.
But since Boss is on the road, i'll add a bump here, and mention that this is also known as truss-lift. you might try the advanced search button with that phrase in different configurations.
I tend to agree with your reasoning except that a truss is not made of a top chord and a bottom chord acting independently of one another. Instead, a truss is an engineered unit containg otjher members also, and anything that effects any of them, effects all of them as a singular unit.
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I think they have it wrong. I think the major component that shrinks in the winter is the struts. Think about it, it the top chord shrinks the ends would have the tension and since they have the load and can't rise, the bottom chord should press down.
But I think the struts are shrinking causing the bottom chord to rise.
do a search for "truss uplift" or "truss up-lift".
As mentioned above moisture is a factor as it contributes to the expansion/contraction of the wood truss components. I think limiting/ceiling penetrations in the "lid" helps a lot by reducing/eliminating the migration of household generated moisture that travels on the warm air that ends up in the attic via the "stack effect".