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MAIN REPORT
Description
The subject of our investigation was a one-story frame dwelling. The building was reported to be approximately 5 years old. The basic construction type consisted of a wood frame superstructure supported by a concrete slab-on-grade floor system within masonry stem walls.
Observations
The site investigation was conducted on Wednesday, April 3, 2002. A follow-up inspection was conducted on April 10, 2002.
We observed slight to moderate damage and movement of the interior finish surfaces, including:
- There was a horizontal crack in the drywall of the tall wall at the cathedral ceiling in the family room. The crack ranged from a hairline to 3/4" in width and was located at the level of the wall top plate.
- There were gaps and distortion of the drywall tape between the flat ceiling surface and the top of the wall. The gaps were a continuation of the crack described in item 1 above.
- There were hairline to 1/16" wide gaps and minor distress along the interface between the cathedral ceiling surface and the top of the sloped interior wall.
- There were hairline to 1/32" wide cracks and minor distress in the gypsum wall board on both sides of the door to the rear middle bedroom. There were cracks and distress in the ceiling-to-wall tape joint along the wall at the closet. There was a gap of up to 3/4" between the wall base plate and the concrete floor slab.
- Several of the interior door units are out of square and have been cut down to permit them to close.
- The roof framing system is typical prefabricated 2x4 wood trusses. The transition area between the cathedral and flat roof areas is made by attaching wall board directly to a truss rather than extending the wall framing to the cathedral ceiling line.
We performed a follow-up inspection on April 10, 2002 to take floor level measurements with a laser leveling device. The level measurements are noted on the attached sketch.
Level measurement of the floor surface shows that the difference in the floor elevation is approximately 13/16" between the fireplace hearth in the family room and middle of the rear center bedroom. This is a distance of about 22 feet.
Level measurements of the ceiling surface that the difference in elevation is approximately 1-1/16" between a point just inside the hallway from the great room to just in front of the attic access stairs. This is a distance of about 7 feet.
Discussion and Conclusion
The distress and movement we observed is the result of normal seasonal movement of the roof truss system; a condition which is commonly known as truss lift. Truss lift is primarily due to changes in moisture content of the wood truss members, although temperature may also play a role. Basically, the roof truss system “flexes” with changes in temperature and humidity. This is a normal response by the roof truss system and does not represent any significant structural problem.
Trusses are constructed of 2x4s which are arranged in triangles and fastened together with metal connector plates. Roof trusses are a very important and common construction component and are also a very efficient use of building materials.
Structurally, trusses are very rigid and each truss behaves as an individual rigid component. The top of the truss (the roof rafter) supports the roof plywood. The bottom of the truss (the ceiling joist) supports the ceiling drywall, attic flooring and, important to this discussion, the insulation. The insulation keeps the temperature of the bottom truss member between 60 and 90 degrees or so all year around and isolates the wood from seasonal humidity cycles. Consequently, the bottom chord gradually looses moisture and shrinks to an equilibrium point. The internal (web) members and top members are exposed to these humidity changes and temperature extremes from around zero to over 140 degrees. From this it is apparent that the top chord and internal web members will change length and width seasonally while the bottom chord member stays relatively constant. Since all the members are rigidly fastened with the metal plates, even small changes in the member lengths will result in minor warping or distortion of the entire truss.
Under normal conditions, this does not result in any significant structural problem, but it does cause the types of cracking and distress we observed at your home.
For your convenience in gaining a better understanding of this issue, we have attached several brief articles on this subject.
Our level measurements show that the level of the floor slab is within excepted normal tolerances. The ceiling surface exhibits a much greater degree of movement. While there may also have been some settlement of the floor slab, the amount of settlement is minor and would not suggest a significant problem.
Much of the cosmetic damages could have been avoided had some simple alternate details been used during construction. With the home now finished, the practical solutions are a little more limited. Below we have outlined some steps which may be taken to minimize the cosmetic effect of this problem.
- Ideally, the ceiling gypsum boards should not have been fastened to the bottom of the trusses within 16 inches or so of the walls. This provides enough distance from the wall that the gypsum boards can flex with the truss movement and no gap or tearing will result. This may be corrected now, but the ceiling will have to be refinished.
- The interior wall was pulled up from the floor by the roof trusses. The trusses should not have been nailed down to the top of the wall. Wood trusses are not intended to be rigidly fastened to interior non-load-bearing walls for just this reason. The top of the wall should be secured with metal connection clips which permit the truss to flex up and down freely. We have included a page from a Simpson catalog illustrating this device. The trusses can be loosed from the interior walls and these clips retroactively installed. In addition to installing the metal clips, the wall will need to be resecured to the floor slab.
- It is difficult to make an adjustment at the cathedral wall detail at this time. A better construction detail would have been to frame the wall studs fully to the sloped ceiling rather than transitioning to the truss at the upper half of the wall. It is possible that the individual truss at the wall can be altered to eliminate most of the movement, but this will require engineering design which is beyond the scope of services you have authorized at this time.
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