Information on Trusses

Composition of Trusses

• Trusses are composed of straight members that are pinned together where they meet in nodes in triangular bracing. There are three different kinds of members which make up any truss. The horizontal members are chords or horizontals. The vertical members are diagonals and verticals. These members either act in tension or compression, allowing some restricted movement at the nodes. Triangular bracing is used in many ways in different structures and is geometrically the most efficient way to support roofing and flooring. Architects and engineers design trusses in a variety of ways to provide rigidity and support as well as visual appeal in the finished structure.

Truss Designs

• The variety of truss designs is almost endless. But the two most common designs are the pitched truss which is primarily used in roofs and the flat or parallel truss which is primarily used in floors.

  The pitched truss is constructed in a triangular form which allows roofs to be built at a certain pitch or sloping angle.

  The parallel truss used in flooring has two outside parallel braces and the internal triangles support the sides as they are interconnected and restrict lateral movement. The flooring trusses are primarily designed for strength and support.

The Roofing Truss

• Trusses used in roofing are often designed for aesthetic appeal. The climate is a major consideration as the truss must provide different levels of insulation and protection from air-water vapor. The most common types of roof truss designs are the dropped chord, raised heel, scissor and parallel chord trusses. Other factors which figure in roof truss design are personal preference and practical concerns such as budget and the overall design of the structure.

Raised Heel Truss and the Dropped Chord Truss

• The raised heel truss is the most energy efficient design available. It allows space for ample insulation and creates an efficient moisture vapor barrier. Such a barrier is important to prevent potentially damaging moisture condensation, mold, mildew and dry rot. Because of the expensive manufacturing costs and the additional insulation required, it is the highest priced option, and it is most often used in cold climates.

  The dropped chord truss is similar to a raised heel truss in that it is airtight and allows room for extra insulation. Builders usually choose this option if the interior ceiling is very high and potentially unstable unless well braced, but the extra height, siding, and insulation raise the cost of construction.

Scissor Truss and the Parallel Chord Truss

• Cathedral ceilings often require a scissor truss. Its shape and design eliminate the need for a supporting load bearing beam and wall. This arrangement increases the cost of insulating the attic and increases the labor costs of construction.

  Another choice for cathedral ceilings is the parallel chord truss. It provides the easiest attic insulation construction, but it does require custom designed steel and wood supports for the structure and this drives the cost upwards. In the long run, the thermal effects of the steel braces affect the energy efficiency because the steel absorbs more heat than wooden braces would take in and thus the cooling expense for the structure is higher.