Oak behaviour in structural use can present some challenges to the user and should be considered when used in construction. Fresh sawn oak timber has been used in areas where its growth is prevalent for as long as buildings have been made. This is mainly due to its dense structure and inherent strength, giving oak exceptional load bearing capabilities for a natural material.
Green oak, rather than air or kiln dried, is used for construction as it is easy to source in large sizes and easy to cut and shape. For structures such as large wood support beams this makes oak a natural choice. Dried timber is cut to size before drying and the standard lengths available may be inappropriate for the project in hand. It is also very hard and consequently more difficult to work with than green oak. However, as green oak dries it is subject to physical changes and these should be considered before and during use.
Factors in oak behaviour
Freshly cut oak will have a water content of 60 – 80% depending on the season it is felled. Once the tree is felled, it stops taking in water from the environment and begins to dry. As the outer layers of sapwood contain the highest proportion of water, they will experience the highest level of water loss and shrinkage. Due to this, timber will always shrink tangentially, that is, around the heart of the log. This quickly results in the appearance of drying fissures, or splits, along the length of the wood. There are several methods to compensate for this and each have different effects, mainly on the finished beam structure.
Most common is ‘boxed heart’, where the beam is cut with the heart of the long at the centre of it length. This will result in more fissures, but they will be more evenly distributed and the beam will remain more straight and square. ‘Halved’ cut beams are the second most common, where the wood is cut down the line of the heart. As these dry, one side contracts more than the other, leading to a flattened curve across the section on one side.
‘Quartered’ beams are less common, as it requires a larger log to produce enough usable timber to be cost effective. As only one corner of the beam contains the heart, these are subjected to far less fissuring. The downside of this is the resulting diamond shaped profile. Quartered beams are less readily available and hard to find in longer sizes, making them unsuitable for support beams and similar large pieces. They can also be much more expensive than other beams.
It is also possible to bore out the centre of beams to reduce shrinkage but this can also reduce strength. This makes them unsuitable for roof or truss beams that have to support large loads. Historically, timber framers would fix large moulds along the beam so fissures would only form on inside edges, but this is rarely done now. As shrinkage occurs along the length of the beam, it does not affect the strength, despite appearances. As long as the design of the structure takes shrinkage into account, the differences are mainly cosmetic.
Just as oak can lose water, it can absorb it, albeit to a far lesser extent than other wood. This can cause swelling, known as movement, which can affect the stability of the structure Oak is classed as a ‘medium movement’ timber, meaning it is less susceptible than many other timbers. For frames within the building envelope, this is rarely a consideration, as internal humidity levels are usually low and consistent. However, for structures exposed to the elements, the surrounding humidity can fluctuate greatly with the seasons. As a result, air dried oak should be used for external structures. During the drying process it is exposed to the elements and as a result it is in balance with external humidity levels. This will cause a minimum of movement, even in wet weather. This is also a good reason to use European oak, as it has grown in a similar climate to where it is used. As a result, it is already more ‘in tune’ with local environmental factors.
All wooden beams, even oak, will experience what is known as ‘creep deflection’. Over time under a constant load the timber will bend, or ‘deflect’ away from the load. This will be visible as a slight downward bend in the beam. This is less common in dried beams than with green oak and affects shorter beams less noticeably. The higher moisture content of green oak gives it a higher level of elasticity, allowing it to bend under pressure. When the timber dries and hardens this bend will remain. This will be more evident in green oak beams exposed to the elements as repeated wetting and drying exacerbate the effect. The effect can be reduced by using dried oak where possible. With green oak beams, a slight bend can already be evident and installing them ‘bow up’ will lessen the final deflection. For floor beams, correct spacing and sensible placement of heavy fixtures above will lessen this.
When using metal fixings, it is advisable to use those with little or no iron content. Oak is a naturally acidic wood and combined with the tannins it contains, this can react with iron. This can cause unsightly blue – black staining to the wood and corrosion to metal components. For joins such as beam to beam connections, this can cause dangerous weakening of load bearing joints. If green oak is used externally, for example as cladding, it will exude tannins as it dries. These can be washed off by rainfall and cause black staining to surfaces underneath. They can also corrode iron based fittings so stainless steel or non-ferrous metals should be used nearby.
Reaction to fire
Due to its density, oak is remarkably resistant to fire, even when dried. Once the outer layer has charred, the charcoal itself acts as an insulant, slowing the transfer of heat to the core. This slows the rate of combustion and the transfer of heat to the surrounding structure that causes fire to spread. This gives firefighters more time to react and the slow, consistent burn rate makes spreading easier to predict. An oak structure using oak pegs at the joints will fare better in a fire than one with metal bolts. Steel is a better conductor of heat and steel bolts will melt and weaken well before oak pegs are affected.
Hardwoods group can supply air dried, kiln dried or green oak for any project you have in mind. Whatever your oak needs, contact us to speak to our experts and we will be glad to help.