For centuries it has been a staple material in building construction, but why is oak a superior choice for roof trusses? Initially the wide availability of oak in the dense woodlands of Britain made it an obvious choice. Large quantities of timber of large size can be produced from just one oak tree, making it ideal for large building projects. A lack of manufacturing processes and ability to produce synthetic construction materials left few alternatives. As its use became more widespread, the natural strengths of oak became apparent and even now few materials can rival its all round suitability.
This has naturally led to oak possessing a reputation as a premium product and a corresponding price tag. However, the longevity and durability of oak more than justify this and the use of cheaper materials is a false economy. Used properly, oak will never need to be replaced and any structure using it will outlast its builders and users by many years. A look around any historic town will yield numerous examples of oak framed buildings hundreds of years old. Roof trusses carry the most significant load in any building and their removal and replacement is costly and labour intensive.
Physical Advantages Of Oak
Oak is famed for its natural hardness and strength and as the green oak used in trusses dries this only improves. This makes the timber used in the members of the upper chord naturally resistant to compression forces. Likewise, the oak beam forming the lower chord is similarly resistant to tension. Combined with the triangular design of the truss this gives an exceptionally sturdy unit that easily bears very heavy loads. Due to the large size of oak trees, it is easy to source long timbers for the main truss members. This avoids the need to combine shorter pieces together, weakening the member at the joins. This is particularly useful for designs such as cruck frames that require large, unbroken timbers. Although oak is a heavy dense material, it has a high strength to weight ratio. Also, trusses are designed to use the minimum amount of material to support large loads. To support the same amount of weight a steel truss would weigh ten times as much as one made from oak. In addition, the seasoned oak pegs used in the tenon and mortice joints are highly resistant to the shearing forces applied.
Oak is one of the most durable temperate hardwoods available, only bettered by a few tropical varieties. As their main advantage is resistance to marine pests, they are rarely, if ever, used in roof trusses. In most other respects, oak is equally durable, cheaper and more widely available. There are three main factors that can damage and weaken structural timber – pests, water and fire. Oak is naturally resistant to all three.
Immunity to Pests
Due to the presence of high levels of tannins in the wood, oak is unpalatable or actively toxic to many insect species. The European climate is also unfavourable to foreign species that could be transported with timber from abroad. Most insect species that feed on oak will only attack live or freshly sawn timber. By the time the trusses are built and installed, they will no longer be a viable food source for most species. Fungus species that affect oak also prefer live trees and the low moisture content of sawn timber makes it largely immune to attack. Any damage done to the live wood will be obvious once the tree is felled and it will not be used in truss construction
Oak has a high level of water resistance, making it naturally weatherproof. Obviously, this is not an issue for internal roof trusses as the envelope of the building protects them. For half timbered buildings with exposed end trusses, gazebos or carports, it is a huge advantage. They will not need to be weatherproofed with messy or toxic chemicals, saving time and money. The only exceptions to this are standing ground water or leaks allowing water to pool on or around timber. This can raise the water content of the wood and leave it prone to dry rot. This can be easily avoided with proper siting of external structures and basic maintenance.
Slow Burn Rate
In a building fire, heat and flames will rise, placing the roof structure at risk. Due to its dense structure, oak burns slowly, creating a layer of charcoal that insulates the wood beneath. This controls the rate of combustion, allowing time to react. Softwoods will burn rapidly, spreading the fire and quickly leading to collapse. Even steel will soften and suddenly collapse well before a similar oak structure is badly affected.
Aesthetic Advantages Of Oak
In buildings with an open roof space or attic conversions, form is just as important as function. The truss itself forms a part of the decoration of the room and oak gives a natural, traditional look to the space. In older buildings, especially those with an existing timber frame, this complements their appearance perfectly. The soft golden colour also gives an impression of warmth and comfort and allows light to flow around the room.
Reduced Environmental Impact
Oak trees absorb carbon while alive and when they are felled this is trapped in the timber. Combined with replanting and sustainable forest management, this makes oak a carbon neutral building material. As a natural material, there are no harmful manufacturing by products and it is easily recycled. Its natural durability means no chemical treatments are needed for preservation or protection. Finally, its longevity reduces the need for replacement and virtually no material is wasted from each tree used.
Hardwoods Group are specialists in the design and manufacture of oak framed buildings. We offer a range of oak roof trusses or a bespoke design service for individual projects. Our oak trusses can be supplied in kit form or assembled and ready for installation. Contact us to discuss your requirements, we will be happy to help.
The Mechanics Of Roof Trusses
Types of Trusses
Roof Truss Design Guide