Trained Eye

 
 
 

Truss Uplift

Question:

We have a two storey home in the Lindenwoods section of Winnipeg , which was built in 2001. Every winter, in various locations on the second storey, gaps develop where the ceiling meets the walls. In some locations they are half an inch wide where the ceiling drywall is separated from the drywall on the wall. In the spring/summer the gaps disappear. I initially thought that this was part of the settling process, but have been told by a colleague that it is a symptom called rafter lift. I questioned A&S about the process for repairing rafter lift, and was advised that there is nothing that can be done.

Are you familiar with this condition? Can it be repaired? If so can you recommend a repair company?

Answer:

Yes, I have heard of this condition but have only observed it a few times. The term commonly used is “Truss Uplift”, and I have never confirmed that this was truly what I saw, on those few occasions. This phenomenon is fairly well documented, although the exact cause is subject to some speculation. I will elaborate on this further and will recommend one possible solution.

Truss uplift is a fairly recently discovered occurrence in newer homes that have pre-manufactured web trusses installed for the roof system. The effects are just as you describe, a noticeable gap between the partition wall drywall and the ceiling drywall underneath the insulated ceiling. This gap will open and close seasonally with changes in exterior temperature and other environmental conditions. This effect may be worse on homes with a wide span between parallel exterior walls, but may be seen on smaller homes, as well. The unusual thing is that there may be no predicting which houses will be subject to this happening. Many identical houses built at the same time will have no visible signs of truss uplift, while other ones will be like yours.

There is some debate about the cause of truss uplift and the actual mechanisms are not fully known, but it is believed to be due to the design of the trusses. Modern trusses are designed to incorporate triangular webs that are built with normal dimensional lumber, held together by metal plates. These triangles create very strong structural components, using the natural properties of the wood while minimizing the overall weight of the individual trusses.

The most widely accepted current theory for truss uplift is that the temperature and humidity changes in the attic during the winter months affect different sections of the truss, differently. The area of the truss above the insulation level will be subject to different temperature and humidity levels than the bottom chord, buried deep in the insulation. This difference may cause the upper cords of the truss to expand or contract in the cool winter, while the bottom cords remain fairly static. This may results in an upward bowing of the bottom chord, as it is securely connected to the top cords. The result of this differential movement is that the centre section of the bottom chords move upward, and gaps appear at the top of the interior walls, where they join the ceiling. Consequently, the ceiling drywall, which is attached to the bottom cords, is picked up by the trusses and pulled away from the wall sheathing. The result is gaps in these junctions, often see as large as five to eight centimeters.

Home builders that have had problems with this issue have found one possible solution, but it may only be done at time of construction. Some new builders are attaching an additional top plate to the partition walls and securing the ceiling drywall to this plate, rather than the trusses near the centre partition walls. This will not prevent the seasonal movement related to truss uplift, but will allow the ceiling drywall to move with the wall, rather than the ceiling. Gypsum-based drywall is slightly flexible, and may be able to withstand small amounts of movement, if installed in this way. This will not help you in your home, but there is one other possible solution.

Truss uplift is not considered to be a significant structural concern. It is primarily a cosmetic concern, although nobody wants large gaps to be seen at the top of their walls, especially in relatively new homes. The simple solution to your problem is to install mouldings at the junction of the walls and ceilings in the offending areas. The key is to secure these mouldings to the ceilings, rather than the walls, which is the typical method of installation. This will allow these small trim boards to move up and down with the trusses while covering up the gaps. Ensure that these mouldings are deep enough to cover gaps at their widest point and do not caulk or secure them to the wall sheathing.

While not a significant structural issue, truss uplift can have the appearance that there is a major defect in the construction of your home. This is simply not the case. While the true cause of this phenomenon has not been fully researched, there is fairly consistent consensus in the building science community that it is primarily a cosmetic issue.

 

Contact

trainedeye@iname.com

204-291-5358

P.O. Box 69021
#110-2025 Corydon Ave
Winnipeg, MB
R3P 2G9