Truss bow is caused by expansion and contraction of the truss web members due to fluctionations in moisture content in the attic. This causes the bottom chord to bow and recover slightly.
I’ve never seen it but I’ve always lived in dry climates (CA and CO).
Do any of you ever see it and if so, what type of climate zone have you seen it in?
Maybe it’s an urban myth like that one about the woman who… well… never mind.
Is the truss bow you are talking about the same thing as truss uplift? I have seen uplift here in Washington. It is also a function of drying out of trusses.
Hi David, yep, same thing. I was in a truss manufactuing plant on Wednesday and asked them if they checked moisture content of the wood used in assembling trusses and they said no.
It’s dry enough here in Colorado so that material dries quite a bit by the time it leaves the yard and comes in through the door, but in your neck of the woods, it could be very different.
Does it just show up as sections of raised ceiling?
Kenton,
It’ll look like this…
Here in MI, "truss lift" is common on new houses due to snow load. (Last winter, we had 2.5 feet standing, and buildings were being crushed) Back in the construction days, we would combat this by placing a nailer for the drywall on the wall, and intentionally not screwing or gluing the drywall to the truss within 2 feet of a wall. I found that “truss lift” is more common when you use “Single W” trusses rather than the more expensive “Double W”.
More often than not its a combination of camber on the bottom chord as well as the trusses being misaligned and then cut at the eaves to make it appear that the truss was not out of align.
The rare times I use trusses, I have had a couple companies put way to much camber on the truss to the extent that the bottom chord was off the interior wall top plates by as much as 1 inch. What framers also mistakenly do is to nail the bottom chords to interior walls which are a no-no. There are special clips for this however the truss companies never tell you about his nor do they send them out. The also don address the fact that when you use such clips you drywall will look like the picture that David posted (not to say that was the reason behind those cracks).
Speaking from experience, engineering and application do not always go hand in hand in the field.
In the case where my trusses were over 1 inch above the plates, I simply had my framer cut the chords, properly sister it, had the truss engineer stamp it and then back charged the truss company over $1000.00 for wasting my time.
Most of the time if I look long enough I can find problems with truss installations.
I am not a big truss fan.
Jeff
That sounds like “compression”, Mike. 2.5 feet is a lot of snow, but not uncommon in some part of the country, like ski towns, where they often use a lot of trusses. It may be that engineers are used to designing for certain expected maximum loads and that the 2.5 feet exceeded that expected maximum.
Are we talking about two different causes with similar results? FYI Around here roof snow load must be 200 pounds per square foot.
I think so…
Cause 1: trusses built using wood with excessive moisture levels or which have absorbed largte amounts of moisture.
Cause 2: excessive roof loads.
Colorado gets humongous roof loads occasionally… way more than 3 feet. in '94 we had a storm in Nederland that left about 5 feet sitting on everything. Didn’t hear of anything but real old roofs caving in.
So Michael R,
A 10’ X 10’ roof would need to support 20,000 lbs. ?
That seems to be excessive considering that I live further North and much less is required with regard to snow load.
I just looked up the code, and for MI, required is 25 pounds per square foot snow load, That would put a 10 x 10 roof at 2,500 pounds. Not sure why that 200 stuck in my head.
