Beating Truss Uplift vs Rafter-Framed Roofs

The introduction of the roof truss saved a lot of construction time and costs and eliminated or reduced the need for support posts. Unfortunately, the truss creates a cold climate issue called uplift. When it is part of an insulated attic, the top “chords” stay cold while the bottom ones remain warmer due to insulation around them.

This issue coupled with the normal attic temperature fluctuations leads to moisture differences in the upper and lower portions of the truss. This moisture difference can cause a vertical rise or lift of the bottom truss chord by half an inch or more. If there are interior partition walls attached to the bottom part of the truss, problems will occur in the drywall with constant cracking and worst case, damage to the truss or wall frame. It is impossible to stop truss uplift making it imperative to not only conceal the movement but make it harmless. A great builder will know how to do this.

Lastly, a cold roof is a good roof.

Ice dams cause the greatest damage to homes year after year. Solving the root cause of the ice formation must be done at the design stage. It is almost impossible to fix the problem after the fact; at best, they will only be temporary solutions. When too much heat escapes to the cold roof surface, it causes snow and ice to melt while the temperatures are still below freezing. The roof-heated water then travels down to subfreezing eaves, freezes creating the first part of an ice ridge which, will cause leaks through shingle joints.

These can be prevented by designing a roof that stays cold everywhere during winter.

Easier said than done. Typical hot areas are where the roof slopes down near the top of partition walls. Full insulation amounts are needed along with ventilation channels. More thermal bridging with standard construction produces wall framing that touches the inside and outside surface of the wall. This will create a direct route for heat and cold conduction from the exterior/outside wall through the studs to the interior wall causing a significant heat flow source. During the design and construction phase, is when prevention and minimizing must occur.

Clearly, cold climate construction, insulation, ventilation and heating are very different than warm or hot climates. Radiant barrier plays a role in all climate conditions although in the hot climates it is used extensively it is still very important in cold climates and can greatly increase retained heat.