by: Myron Ferguson
What is a Truss Rafter? Trusses are pre-engineered and manufactured assemblies that take the place of conventional rafter, ceiling joist – roof construction. Trusses are usually installed across the entire width of a home and transfer the load to the outside walls and through them to the foundation. Houses that are constructed using trusses seldom have internal load bearing walls. They are often a less expensive faster method than stick built roof framing. The trusses are a combination of wood members connected together with metal plates.
When a newer house shows signs of interior ceiling corner cracking at the top floor in the winter, this can normally be associated with truss uplift. Roof truss uplift occurs when the bottom chord of the truss is exposed to significantly different moisture and/or temperature conditions that the rest of the roof truss. The bottom chords of the truss are buried in heavy insulation, 12 inches or more thick. In the winter the warm temperature from the ceiling below and the thickness of the insulation keeps the bottom chord dry, causing them to shrink. The top chords (above the insulation) are absorbing moisture and being kept damp with the higher humidity in the attic. The dampness of the top chords of the trusses causes them to expand. This differential movement in the top and bottom chord of the trusses causes them to arch up in the center. Because the truss ends are secured to building exterior walls, (a location that resists outward thrust), as the truss bottom chord wants to expand along its length, the force pushes it upwards into the attic space. When the trusses arch up they pull away from the top plates at the ceiling-wall juncture of central interior wall partitions that run at right angles to the direction of the room trusses, (predominately at interior partitions in the center of a building). The ceiling/wall juncture cracks typically open in the winter and close in the summer.
As a result the problem is often an ongoing problem. The building contractor will be held responsible and in turn the drywall contractor will be called in to re-tape the corners and popped screws, only to have the problem reappear next season. If damage to the drywall occurs I am tempted to say that the drywall can't be just repaired and refastened. (See below for some possible preventative practices and ways to minimize the problem). Whatever the reason, the problem is real, but truss uplift is not a structural problem. This movement which can just cause a simple hairline crack or create large gaps and cracks along the corners is a cosmetic problem mostly in homes in cold climates. I think the best approach is to stop the truss uplift action from having any effect on the ceiling and wall drywall joints that exist throughout the home. Remember, truss uplift is not truss uplift if the owner can't see it. Let the trusses move. The truss moves, the drywall bends, no crack, happy customer.
Note: Most truss movement occurs over a period of time, the preventative techniques listed below will allow the drywall to flex near the corners preventing cracking. In other words the truss does not just pop up or slam.
When building a new house the framing contractor should secure the interior partitions to the trusses with truss clips. The clips attach to the tops of the interior wall partitions and are then fastened through the slots to the trusses. The fasteners are not driven tight so as to allow for movement of the truss. This keeps the partition wall stable and plumb and secures the rafters at the proper on center spacing. The drywall installer must then install the drywall properly to prevent cracking. A common method of attaching the drywall is to use floating interior angle method. This helps minimize the possibility of fasteners popping in areas adjacent to the wall and ceiling intersection and cracking due to the truss moving upwards. The drywall installers should not screw or nail the drywall to the trusses within 16 inches of an interior wall and within 8 inches of the ceilings on the interior walls. The ceiling drywall panel is fastened to blocks of wood nailed between the trusses to the top plates of the walls or to metal clips or continuous angles that are attached along the top of the interior walls.
Note: The key to eliminating truss uplift cracks and screw pops is to connect the ceiling and wall drywall while avoiding nailing or screwing the ceiling drywall panel to the bottom of the trusses at or near the intersection down. The movement is gradual so the drywall can just flex slowly as the rafter moves up but the drywall along the wall/ceiling intersection stays in place.
Figure 1- Ceiling and wall damage caused by roof truss uplift. Truss uplift occurs in homes when the bottom of the trusses separate from interior partition walls during winter months. This separation breaks the tapes joints between ceiling and wall drywall as shown in Figure 3. The wider the span of the truss, the more often the problem seems to present itself.
Figure 2- Roof truss in normal position.
Figure 3- Roof truss in "uplift" position. NOTE: The walls that are prone to the problem are the interior walls that run at right angles to th trusses, but walls that run parallel to trusses especially those close to trusses are also in danger of being effected by truss uplift.
1. The solution to an existing condition: The tops of the interior walls should be un-nailed from the trusses. Remove or cut free the nails from inside the attic. Then remove all the ceiling screws within 16 inches of the corners at the center partitions and all the wall screws within 8 inches of the ceiling along the wall. The holes in the drywall should then be patched with dry wall joint compound and the walls and ceilings repainted. This solution will be difficult to accomplish and will be costly.
2. Install crown molding around all the second floor ceilings, nailing the trim only to the ceilings. This procedure would cover the cracks and maintain a good looking ceiling corner. Note: When installing the crown molding in this manner, remember to paint the trim and drywall in the winter months so that there is no paint stripe at the bottom of the trim when the trusses lift next winter.
3. Change the way we insulate attics. Insulate against the roof plywood. This will create a conditioned attic so the temperature and humidity will be better controlled. In homes where the attic is part of the conditioned space, typically where expanded foam is sprayed against the roof sheathing, truss uplift problems are less likely.
4. Truss Backing Angle: Trim-Tex has a product called "Truss Backing Angle" which is installed before the drywall is hung which helps prevent truss uplift.
The idea is not really new; Trim-Tex is just trying to offer a better alternative that is also available at the numerous suppliers that carry their products. The backing angle is made out of vinyl, which is plenty strong enough. It just needs to hold the drywall edge in place along the top edge of the wall. It is attached the entire length of the walls being treated. This means that the drywall is backed for its entire length and can easily be fastened anywhere along the edge as long as the fasteners don’t go through the angle and into the truss. The screws start very easily in the vinyl engineered extrusions which speeds up production.
truss backing angle:
Eliminates inside ceiling corner cracking due to truss uplift at interior partition walls. The rigid PVC Framing Angle keeps the inside corner stationary during truss uplift.
For more information and to request a sample of Truss Backing Angle visit the Trim-Tex product page here.