Roof trusses can categorized into a number of groups, defined by the external profile formed by their top chords (rafters) and bottom chord (ceiling tie). These groups contain tried and tested shapes, solutions to numerous architectural requirements and engineering problems,
This hub looks at a number of the more commonly used truss shapes or profiles, describing their design and some of their applications. This information should allow the reader to make more informed choices, particulalry in the area of roof architecture and interior design,
1. The Common Roof Truss Profile
As its name suggests, the common roof truss profile is the most frequently used. It has a familiar trianguar shape; having two rafters of equal length and pitch joined the appex and connected by a ceiling tie. This triangular shape is inherently rigid.
If we simplify the forces at work for a moment, we can say that the rafters are under compression (from the roof load) and the ceiling tie is under tension (restraining the rafters). In reality things are a little more complex and the truss needs to be subdivided internally into more triangles using webs. These internal triangles help spread the forces (triangulation), reducing the size of rafter and ceiling tie material required.
Webs are found in all truss profiles and the engineering of some of these can become incredibly complex. Common web configurations have been given names such as Fink, Fan and Howe, helping to identify them.
Common Truss Profile: King Post Truss, Queen Post Truss, Fink Truss and Howe Truss
As a genral rule of thumb; the smaller the span of a roof truss then less webs are required. The King Post truss has only a single vertical web and is used over very short spans. Typically you may see a king post truss in your porch, shed or garage.
The Queen Post truss has a fan web configuration: a single vertical web and two diagonals. This truss type is used less frequently because as spans increase, fink truss usually provides an cheaper alternative. It is however perhaps the most used profile when making feature or hand-made, pegged mortice and tenon trusses which will be seen.
The most frequently used web configuration of common roof truss is the Fink roof truss; easily recognised by its W-distribution of internal webs. The fink roof truss is the workhorse of truss design and used in the roofs from the simplest garden shed to industrial complexes.
As common truss spans begin to increase, then the number of webs must increase too. We begin to hybrid web configurations such as the double fan, double fink and modified queen. This complex triangulation of forces allow truss types with configurations spanning upwards of 60 feet without internal support
Last but not least in the common truss category is the Howe truss configuration. This is a special type of truss, most often used to support other trusses and taking the form of multi-ply roof truss girders.
Common Roof Trusses: Raised Collar Trusses
The Raised Collar Truss is not a separate truss profile in its own right, but deserves a mention as a variation of the common truss. By lifting the ceiling tie from the bearing height (wallplate), the designer can produce a vaulted ceiling having a flat middle section and two sloping sides.
As part of the design process the rafter size normally has to be increased to cope with the reduced triangulation and redistribution of loads. The rafter is sometimes deliberately enlarged to allow greater depth for loft insulation.
2. The Mono Truss Profile
As the name suggests, mono trusses have only one rafter, producing essentially a right-angle triangle. They are used to form single pitched and lean-to roofs. Variations are also used in hipped roofs and sometimes two mono-trusses may be used as an engineering solution to a dual pitch roof.
With some exceptions, the web conguration of a mono truss could be described as half of that found in a common truss. Mono truss girders also exist, where multi-ply mono trusses are used to carry the on-coming supported trusses
3. The Double Pitch Profile
A Common truss having rafters at different angles becomes a Double Pitch truss. These may be the result of architectural requirement; dual pitch roofs are used frequently in industrial applications.
Dual Pitch trusses may be also be the result of matching new to old roof lines. Truss manufacturers may make a profile on site, constructing a truss frame against the existing rafters. Although the variation may be very small, on many occasions it will be found that the two sides of the old roof are not at the same angle.
4. Scissor Truss Profile
A variation on the common truss profile, the scissor truss has two bottom chords meeting at an apex and having a ceiling pitch. With variable rafter and ceiling pitch, the scissor truss can provide some interesting roof shapes. It also provides designers with the opportunity to use vaulted ceilings.
There are a number of variations on the scissor profile; including the mono-scissor and hip-scissor trusses.
5. Camber Truss Profile
Like the Scissor truss, the Camber truss also provides designers with the opportunity to include a vaulted ceiling. Here a third horizontal bottom chord provides a flat ceiling area and like the Scissor truss, the other two have a variable ceiling angle. This combination allows for incredible design versatility, although the trusses are generally more expensive than the flat ceiling fink truss alternative.
The Vault profile combines the scissor and camber truss profile, having three bottom chord elements but creating an apexed ceiling.
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6. Attic Truss Profile
Room-in-the-Roof or Attic truss profiles have become increasingly popular, offering the possibility of extra living space in the roof. Attic rooms can significantly increase in the value of a property, whilst providing better value for money than say a built extension.
Attic trusses are extremely versatile. Designers will ofen incorporate roof lights, dormer windows and balcony areas. Walkthroughs allow owners to walk from one area of a roof space to another. Changes in technology now allow Attic trusses to incorporate engineered floor joists, providing greater design opprotunites.
7. Flat and Pitched Flat Truss Profiles
Flat roof trusses are used extensively in the commercial and industrial sectors. The pitched flat truss variation is the more frequently, used to provide some drainage for rainwater and snow melt.
This hub has looked at the a small number of the more frequently used roof truss profiles. There are a great number of roof trusses and they can all be used in combination to form roof structures.
Roof designs, such as those incorporating hips, can become very complex. Bespoke CAD software design systems have made such designs possible and allow designers to keep pushing the boundaries.
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Carolyn on May 12, 2014:
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