Drywall Cracking Under Scissors Trusses
Question:
We keep having problems with dry wall joints raising on the vaulted bottom chords in the track homes that we are building. The joints run from front to back of the house and the trusses bear on the side walls. The trusses are mostly 2-point bearing (a few in the front and back of the building are tri-bearing). The trusses span 40 ft. and have a 4/12 pitch with a tile roof. Is it common to have dry wall problems as the trusses deflect? How long should the roof be loaded before hanging sheetrock? Should the heel of the truss be allowed to slide out on the top plate?
Answer:
There are a couple things that could be going on in this situation. The first is partition separation, which describes the gap between partition walls and truss bottom chords.
The second thing to consider is horizontal deflection in scissors trusses. This is an outward thrust that occurs in all vaulted framing. This seems less likely as it is not clear how this would manifest itself as raised drywall joints.
Make sure that the truss manufacturer knows that there is a tile roof on these houses, as the loads for them are quite a bit higher than for asphalt shingles. Make sure that all necessary permanent bracing is installed. Keep in mind that you can always request a higher deflection criterion on the truss. This means that you will get a stronger truss that will deflect less under load, but it may increase production costs.