For oak timber structures with an open ceiling design, decorative roof trusses are an eye catching and elegant finishing touch. While oak already benefits from an attractive texture and soft, warm colouration, a feature truss can enhance this still further. An open truss design can transform a small room into a large airy space, allowing light to penetrate the whole room. All oak roof truss designs derive their strength from the inherent rigidity of triangular structures. Even the most complex roof trusses are formed from a series of interconnected triangles. However, the ways in which these are combined can lead to some amazing and inventive results.
King Post Truss
By far the most common British style, King Post Trusses are the most recognisable roof truss design. The most basic form consists of two rafters and a beam connected by pegged tenon and mortice joints. This is completed by a king post connecting the apex of the triangle and the centre of the beam. Often this will incorporate two straight or curved diagonal braces. Further posts and braces can be added symmetrically to create designs such as the Howe and Double Howe.
Queen Post Truss
Queen post trusses replace the king post with two smaller posts from the tie beam to the slopes of the rafters. A collar beam connecting the tops of the post increases strength by incorporating another triangle into the shape. The addition of a top post and diagonal braces further strengthens the truss and enhances the design. Queen Post Trusses are particularly useful for attic spaces where access to the whole length is required. The square space in the middle acts as a tunnel from one end to the other.
Collar Truss (Raised Tie Truss)
Collar trusses lack the traditional long tie beam connecting the base of the upper chord rafters. Instead a shorter beam, known as a collar, is situated further up the rafters. This is the ‘raised tie beam’ that gives the truss its name and is surmounted by a shorter king post and diagonal braces As a normal tie beam prevents spreading of the rafters at the base, the mortis shoulders at the base must be strengthened. Alternatively, two short beams and posts can be added to the base of the rafters to form a Hammerbeam truss (The roof of Westminster Hall is a great example of a hammerbeam roof). For particularly low ceilinged rooms, an exposed ceiling and collar truss can increase the height and lighting significantly.
The Fink Truss design is more common to America and relatively uncommon in British oak framed buildings. The rarest and simplest design consists of a tie beam and rafters with two diagonal braces from the apex to the beam. More usually, two more braces are added to form a ‘W’ shape within the truss. Further members can be added for a more complex, decorative effect. An inverted version is sometimes used as a particularly strong Mono Truss to support a one sided roof structure.
Coffer Trusses are a variation on a Raised Tie beam truss, incorporating two diagonal beams from the rafter bases to the collar. This forms a design somewhere between a Collar Truss and a Scissor Truss, sometimes known as a Raised Tie Scissor. These beams prevent spreading of the rafters in a similar way to a Hammerbeam truss.
The basic Scissor Truss design is named after its resemblance to an open pair of scissors, with two crossed diagonal beams and a small king post. In some variants, these beams meet in the middle but do not extend all the way to the rafters. The space above can incorporate various combinations of posts and braces, to give styles such as the Howe Scissors or Modified Queen scissors. For a design such as an open ceiling in a living room this provides all the necessary strength with a more novel design.
Parallel Scissor Truss
This style foregoes a tie beam or collar completely, using a set of small beams and posts in a stepped structure similar to a Hammerbeam Truss. However, this is complemented by a pair of diagonal beams enclosing these steps for an exceptionally rigid and strong frame.
Gambrel Truss (Barn Truss)
The Gambrel Truss is another design more common to America than Europe, although it has been historically used in Britain. It is also known as a Barn Truss as it was traditionally used to maximise loft space in barns while using fewer materials. In countries where high rainfall or snowfall is common, the angled, steep roof design allows greater runoff than a two sided roof. The internal posts and braces subdivide the truss into triangles to cope with tension and compression more effectively.
Polynesian Truss (Duo Pitch Truss)
As the name suggests, this style, also known as a Dual Pitched Truss is mainly confined to tropical countries. In practical terms, it is designed with a steep pitch in the centre to deal with the heavy rainfall of tropical storms. This is combined with the shallow pitch lower down to provide shade for upper story verandas. Admittedly, neither of these situations would be a practical concern in Britain. However, as a bespoke oak truss design it is certainly a bold and eye catching choice.
Bowstring trusses are traditionally an industrial design and rarely seen in domestic structures. Traditionally they would be constructed from steel or iron, as it is easier and quicker to cast the curved roof from metal. While it would be impossible to make this from one piece of oak, it is possible to use several shorter beams. Once joined at the correct angle, these can then be planed or cut to form the curve. A long tie beam and internal members form a lattice of connected triangles to strengthen the whole. While the amount of material needed will put this beyond most budgets, they are occasionally found in exhibition centres or performance halls.
Barrel Vault Truss
As part of an exposed beam ceiling construction, it is hard to imagine a more impressive sight than a Barrel Vault Truss. This design is rarely seen in oak as it must be designed carefully and the amount of material can be costly. The central arch exerts an outward thrust so the vault must be buttressed along its whole length. This can also be offset with stainless steel tie rods and fixings, but again this can be prohibitively expensive. If this can be factored in, the resulting vaulted oak roof is a unique and impressive feature.
Hardwoods Group specialise in the design and manufacture of bespoke oak roof trusses. If you have been inspired to include a decorative oak truss in your latest project, contact us. We will be happy to discuss your requirements and work with you to make them a reality.
Why Is Oak A Superior Choice For Roof Trusses?
The Mechanics Of Roof Trusses
Ten Amazing Architectural Truss Designs