What are the different types of roof truss design?

A roof truss provides support for a roof.

There are a seemingly countless number of types of roof truss styles. In fact, since custom roof trusses can be made as per the needs or desires of a particular homeowner, the possibilities are truly endless. Most homes, however, typically employ one of four main types of roof truss design: high jump, drop rope, scissor, and parallel. Each of these types offers distinct advantages and disadvantages depending on the application.

A high-rise roof truss separates, or lifts, the upper ropes, or upper edges of the truss, from the lower rope. This results in extra space in the eaves which in turn provides enough space for full depth insulation in key areas above the ceiling. This is especially useful for homes that are more likely to suffer from frost. This design also helps create an airtight vapor barrier that reduces the likelihood of condensation forming, helping to eliminate dry rot and mold. The disadvantages of this style are primarily financial due to the requirement for more materials needed to complete the overall final structure.

Dropped rope trusses provide a secondary rope below the bottom rope. This type of roof truss design helps reduce lift and provides extra space for full depth insulation within the exterior walls. It also creates a positive airtight vapor barrier. The disadvantages of this type of roof truss project are also primarily financial. The use of taller nails, more blocking and more siding increases construction costs.

In a scissor roof truss, the bottom rope is made of two pieces that are not parallel to the ground. Instead, the lower chord is defined at angles, similar to the upper chord. The sloping nature of the lower chord allows the interior of the house to have a vaulted or cathedral ceiling. When used in conjunction with a high heel design, a full depth of insulation is also possible. This design also helps eliminate the need for a load-bearing wall in the middle of the structure. The main disadvantage of scissor scissors is that the design allows limited access to install insulation.

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Parallel Rope Roof Trusses, also known as flat trusses, utilize two cords parallel to each other. Like all roof trusses, these top and bottom strings are joined by an interconnected web of materials. Steel and other metallic materials are often used to help reinforce the structure, especially at joints or knots. The main advantage of the parallel rope roof truss design is that it provides space for easy installation of large amounts of insulation. Disadvantages include increased costs due to the steel clamps required, as well as the added challenge of insulating the additional space between strings.

Within each of these four main categories, there are several styles designed to suit individual situations. The distinguishing factors between the types are the number of boards used for the web and the distance the bottom span can reach. Types include: king post, queen post, Fink, Howe, fan, modified queen post, double Fink, double Howe, hip, scissors, monopitch, curved, dual pitch, gambrel, Polynesian, attic, rope, stub, inverted, nas back, studio, cathedral, inclined plane and flat. While ready-made trusses typically fall into one of these standard types of trusses, custom roof trusses can also be created to meet the specific needs of each.

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