Truss bow, ever seen it?

Working in CA and CO I never have. Just wondering in what parts of N. America it happens.

Better yet, does anyone have a photo?

Wanna explain what truss bow looks like. I’ve set trusses in Florida. If they aren’t protected and lay out for too long they certainly will bow a bit more in a longitudinal manner and not necessarily deflecting sending things like roofs or ceilings out of plane.

With almost all truss roofs, trusses are designed to bear on the exterior walls only. This includes trusses designed for vaulted ceilings. Truss clips allow interior partition walls to be secured to truss bottom chords while still allowing for the possibility of vertical bottom chord movement from changes in moisture content or to a lesser extent, temperature changes. It’s called “truss bow”. **The most common sign of trouble related to truss bow is cracks and gaps appearing where ceiling drywall meets drywall covering the walls. You may also see gaps appearing where upper cabinets meet- or terminate close to- the ceiling. **
This condition is due to truss uplift; more common in some parts of the country than in others. Moist air from the home leaking into the attic will condense on- and be absorbed by- wood truss members, causing them to expand and elongate. The bottom chords, buried in insulation, will remain dry and stable. This differential moisture content in different members of the same truss creates forces that cause the bottom chord to bow upward in the center. This condition is likely to be worse in the winter when the attic is colder.
In many parts of North America truss bow is uncommon and you would not recommend correction.

Interesting there Kenton. Florida has a high humidity and attic temperatures can be extreme.
I’m not so familiar with cold environments where there would be higher differentials in moisture within the insulated space where the chords are buried beneath insulation, as you say. I don’t recall having seen any deflection in Florida where tray ceilings are popular as well. California is s moderate climate here in the San Francisco Bay Area.

would a graphic work or do you need an actual photo?

I need a photo for a course, Les.

I just had that on tonight’s inspection except that it was on one of the middle cords and it looked like it was built that way.

None of the other trusses were damaged and the one that was wasn’t separating where it met the top and bottom chord but was out of line by about an inch.

I have installed and inspected thousands of trusses and never seen one like this.

There are several in this thread.

Just asking where you got that term. There is plenty of information for “Truss Uplift”, but I have never heard of “Truss Bow”.


He is one photo, I have more just have to find them

It’s just one of those terms that’s been floating around the industry for a long time, Brad. I think I heard it for a couple of years before I heard it called “uplift” so that’s sort of what stuck. Bottom chords bow upward so it’s not a much of a stretch, in fact I think it’s actually a better descriptive term since the process has nothing to do with uplift of a truss as is commonly used when talking about the effect of wind on a roof. But if you use Google to look, “truss bow” has almost no results, so for purposes of general communication it’s always better to stick with the most widely understood term.

Thanks Randy! And Les!

Truss Bow, this is from and old InterNACHI tread



Here is the best photo I have showing truss uplift. I don’t know what your working on but I should mention this same circumstance can be observed in a house with engineered trusses where the center support wall settles. Could be crawlspace piers sinking in the mud or basement slab settlement where the center support wall rests on top if the basement floor, i.e. no posts with independent footings.

OK Randy we are waiting for one of your cool illustrations now.