Before roof trusses, builders framed the roof structure using dimensional lumber as rafters. Known as stick framing, this type of roof construction still is in use, but today contractors also may use trusses to construct the roof. Truss manufacturing companies rely upon blueprints to engineer roof truss systems. In some cases, the trusses might attach to an interior wall, while in other cases they might not. The reason most homeowners want to tie the trusses to the walls is to stop ceiling cracks caused by truss uplift. It’s not the best solution, however.
Roof trusses feature a framework of smaller dimensional boards, designed to support roof load like larger dimensional rafters. Trusses offer a number of benefits, including the ability to span greater distances without requiring support from interior walls. Trusses are lighter in weight than rafters, which reduce the load on the perimeter walls. Trusses are more expensive, material-wise, than the dimensional lumber used in rafters, but the cost of labor to install trusses is less because there are no angles to figure and cut.
When the truss manufacturer delivers the preassembled trusses, they will come complete with a diagram that tells exactly how you must install them. Depending upon the span and the design of the individual truss members, the instructions may tell you to attach the bottom truss chord to the top of a load-bearing wall. Do not attach a truss to an interior wall, however, if the diagrams do not require you to do so. Trusses are engineered to move as a unit, if you connect the bottom chord any place other than designated, when the truss shifts, it can create excessive pressure on the bottom chord or on the other truss members. In addition, attaching the chord anywhere other than directed can void the truss warranty.
Because trusses do not attach to non-load bearing interior walls and some load bearing walls, they may contract and bow upward in the center when subjected to temperature and humidity fluctuations in the attic. Known as “truss uplift,” this is quite common as the new or green wood used to manufacture the trusses dries gradually. The roof trusses are designed to move without compromising the integrity of the roof structure, but homeowners are dismayed to find cracked ceilings if a truss pulls upward.
The drywall installation technique used to counter cracks caused by truss uplift is called floating drywall. The drywall doesn’t actually float, but the contractor installs blocks on the top sides of an interior wall, level with the bottom truss chords. The drywall attaches to the blocks and not to the trusses at the edges of the walls. When truss uplift occurs, the truss bows upwards, but the ceiling drywall remains firmly in place because it attaches to the blocks and not to the trusses.
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