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Roof Trusses

A roof truss is usually purchased as a unit and is fabricated by a company that specializes in roof truss construction, they will use engineering software to determine the size of each of the members based on load requirements. It is a fabrication of assembled rafters and cross support beams. Using roof trusses, as shown in Figure 1, rather than conventional framing allows for longer spans without roof support columns in the building, requires substantially less on-site labor than traditional roof rafter framing and will take heavier loads.

trussed roof
Figure 1 - Conventional trussed roof

How To Design & Build A Roof Truss

There is a disadvantage that comes with the use of roof trusses. The attic space can not be used as living space, because of the cross members used in the fabrication of the truss. As well, it can be difficult to run HVAC duct work through the attic for the same reason.

Construction Of A Conventional Roof Truss (Figure 2):

Figure 1 - Conventional trussed roof

How To Design & Build A Roof Truss

roof truss
Figure 2 - Conventional roof truss construction
  • Chord: The lower member of the truss that supports the ceiling of the room directly below the truss.
  • Cantilever or Overhang: The portion of the truss that extends beyond the exterior building walls.
  • Peak: The highest point of the truss.
  • Slope: Slope is the angle of the roof, its pitch, or how steep the roof is. As an example a roof that rises 3-inches for every horizontal run of 14-inches is called a 3 in 14 inches. The angle of the roof is a major factor in determining the material that can be used to cover the decking of the roof. In general a steeper roof will have a longer the life, primarily because water will drain off much faster.
  • Top Chord: The member of the truss that supports the decking. It is also the member that the decking is fastened to.

Additional information on determining roof pitch

sagging shingled roof
Figure 3 - Sagging shingled roof

It is very important that the roof trusses be engineered for the span of the roof and the roof loading in your geographic area. Incorrect truss sizing and installation can lead to disaster and in the best of circumstances a sagging roof as shown in Figure 3.

Roof Truss Configuration:

Roof trusses can be constructed to suit almost any style, size and configuration of roof, as shown in Figure 4.

roof truss styles and configurations
Figure 4 - Roof truss styles and configurations