In any building with a pitched roof, roof trusses are the main form of roof support, with wood still being the main choice of construction material. Oak in particular is uniquely suited to this, as oak trusses have exceptional strength and load bearing capabilities for a natural material. Properly installed they will last hundreds of years and require virtually no maintenance. Oak roof materials are surprisingly economical compared to man-made materials and their production has a far lower environmental impact. Air dried or Green oak are the main grades used, further increasing the range of pricing options.
In essence, the basic truss design is that of a long based triangle with a tie beam at the base which sits on top of the opposing walls to support the weight of the roof. Connected to this are two inward sloping rafters which form the truss apex. The point at the top, or apex, supports the ridge beam of the roof. About half way down each rafter sits another beam which runs the length of the roof, known as a purlin. These, along with the ridge beam are the attachment points for the slats which support the roof tiles or similar weatherproof covering. Again, oak purlins and ridge beams are a superior choice due to their longevity and strength. Within the triangle thus formed will be additional posts or braces to further aid in support and weight distribution. The arrangement of these is the basis of the different design and types of trusses available.
Styles of Truss
Traditionally, there are three main designs of trusses used: the king, queen and collar truss. Each performs the role of support equally well, just using different styles of weight distribution. In modern buildings the choice of one particular design over the others is mainly aesthetic.
The king truss is the most traditional design and will be familiar to most people when they picture a roof support. It consists of the triangular design discussed above but with the addition of a vertical post, known as a king post, from the apex to the middle of the tie beam. The tie beam at the base is joined to the base of the rafters to prevent them spreading apart under the weight of the roof they support. The king post then transfers the load to the tie beam itself, allowing the beam to bear some of the weight and reducing strain on the joints. Many people prefer the traditional simplistic design of a king truss as it conveys a feeling of strength and support. When rendered in oak, the sturdy, golden appearance of the wood itself also gives a feeling of warmth and security. If chosen as a feature truss, additional diagonal braces can be added to either side of the king post. These give a more decorative appearance than the rather stark, functional look of a basic king truss. In addition, the extra braces further increase the strength of the truss itself.
Queen trusses take the basic triangular design and add two symmetrical posts from the tie beam to the rafters of the truss apex. Often these are joined to the rafters below the point where they support the purlins to transfer the load to the tie beam. This has a similar effect in weight distribution as the king post but leaves a gap in the middle of the tie beam. This can be helpful in attics used for storage as the gap creates a ‘tunnel’ down the middle of the attic. This allows the owner to move the length of the attic without a king post blocking the way. In a room with an open roof space, queen trusses can give a more decorative, open and airy look. The simpler, open design also allows more light to penetrate the roof space, giving a brighter look to the room. For further strength a collar can also be added to a queen truss. This is a smaller horizontal beam that connects the opposing rafters above the joint with the posts.
Collar trusses are also known as raised tie trusses as the tie beam is not situated at the base of the rafters but further up. This smaller beam is known as a collar, hence the name of this design. This is then complemented by a smaller king post and braces above to maximise strength as the weight bearing nature of a triangular structure is compromised. Since the main function of the tie beam is to prevent the rafters spreading, this design places additional stress on the mortis shoulders where the collar connects to the rafters. However, if this is taken into consideration and the joints are strengthened accordingly, it should pose no problem. This A – frame truss design is particularly useful for attics or rooms where head space is limited as the collar is further up the rafters, maximising space underneath
Half or Mono Trusses
Half, or Mono trusses have a similar design to the above types but are one sided, as if the truss had been halved vertically. They are mainly used for structures with a lean to designs or extensions with sloped roofs. The half-sized design requires additional braces to reinforce the structure of the roof but are a pleasant decorative choice. They are also useful to keep the interior design of a building consistent.
In addition to the truss designs above, there are several more decorative truss styles used for feature trusses. Three are discussed below:
An arched truss shares many design elements with the King and Queen styles, but the tie beam at the base is a curved or arched rather than straight. This gives a more decorative look to the truss but also increases head space below the truss itself. These give a more rustic feel to the design whilst retaining all the strengths of the structure
Also known as a Gambrel truss, this style is used in traditional American barn design. It consists of a wide tie beam, but the rafters have a join two thirds of the way up to give an angled roof shape. A collar then connects the rafters at the join and two king posts connect the collar ends to the tie beam. A small king post connects the apex to the collar and several braces reinforce the space between the tie beam and lower rafters. These types of trusses are a complex design but have a striking appearance rarely seen in British structures.
A Cruck truss replaces the traditional straight primary rafters with curved ones. These are connected about two thirds of the way up by a tie beam, braced underneath by a short, curved beam and internal brace at either side. However, there are many variations on this type of design, varying in complexity. This dramatic design has a gothic, clerical appearance that would be a perfect choice of roof truss for vaulted ceilings for an owner looking to make an impression. It can also be fitted below a traditionally angled roof as the curved arch has the strength to support the roof without a room spanning tie beam.
Hardwoods Group can design and manufacture many types of trusses that you might need. Contact us today to discuss your needs and we will be happy to help realise your vision. As industry leading suppliers of European oak and specialists in the design and construction of bespoke oak framed buildings, we have all the experience and skill you need.