What causes roof truss to uplift?
The best available evidence indicates that roof truss uplift is caused by lengthening of the truss chord as its moisture content increases in the winter. Cold winter air, whether outside or inside an attic, can hold very little moisture, so it is almost always close to saturation; that is, it has a high relative humidity. This causes the moisture content of the truss top cord, which is surrounded by that air, to increase. This in turn makes it try to get longer. The bottom cord, which these days is completely covered by insulation, is in a much warmer and dryer environment and therefore tends to stay the same length. In order for the top cord to get longer without breaking the connections to the web members the truss as a whole must arch upward.
Why doesn't roof truss uplift happen with every truss?
The shrinking or swelling of normal wood along the grain, due to changes in moisture content, is very small so that even when the above phenomenon occurs it is not noticeable. However some abnormal types of wood shrink and swell much more along the grain. These include “compression wood”, “juvenile wood”, and wood with high slope of grain. If the lumber used in the top chords of a truss is any of these abnormal types, the truss will tend to arch to a greater extent.
Can truss uplift be prevented?
There appears to be no practical way to prevent truss uplift. At current and likely future energy prices it would not make economic sense to reduce the insulation in order to expose the bottom chord. Nor would it be reasonable to expect the workmen in a truss plant to be able to cull out the abnormal lumber – even the experts have trouble telling the difference. Attempting to tie the trusses down so they can’t arch cannot be recommended. The forces involved are very high and the trusses might lift the entire partition so that the problem appears at the bottom rather than the top. They might even pull the top plate off the partition. Nor does the type of truss make much difference. Howe and Fink trusses tend to arch about the same amount. However, the Fink truss has a more flexible bottom chord which may tend to bend and thus lift the ceiling less.