UK grown Oak is usually seen as an aspirational choice for external furniture. The slow growth of Oak produces a dense wood, giving it its Class 2 durability rating and long lifespan. More than anything else, it has a beautiful grain detail and colour. Most hardwood of a similar durability and strength is grown in Africa or Indonesia, whereas the best quality Oak is grown here in the UK, creating huge carbon emission savings on transport.
However, despite these notable advantages, Oak is perceived as being expensive and prone to timber movement.
To find out in more detail about how Oak is suitable for seating, continue reading. However, if you just want the quick answer, UK grown Oak is absolutely suitable for outdoor seating. The main problem is movement within the timber. This can be avoided, it just requires some prior knowledge to know what is best to specify. For large sections of Oak (anything above 150mm x 150mm), you do not need to worry about movement so therefore you can use fresh sawn, or green Oak. For smaller sections (less than 150mm square), fresh sawn Oak is not suitable because it will move – the grain pattern is too uneven. However, for smaller sections kiln and air-dried Oak is fine to use without a risk of movement.
Timber movement in general is a bane for anyone working with timber. It is caused by the timber essentially drying out as soon as it has been felled. It continues to dry until it reaches an equilibrium with the surrounding environment. Movement occurs when one part of the timber dries faster than another, creating deformities in the surface.
Movement is only relevant to fresh sawn, or green timber. This is because the timber has not had time to equalise its moisture content with the surrounding humidity. As soon as the timber has dried to meet to the surrounding humidity level, it is known as equilibrium moisture content (EMC). In the summer, the surrounding humidity is approx. 15% and in winter, anything up to 20%. At this point any timber, not just Oak, will not noticeably move any further.
Oak is especially susceptible to movement because of two reasons. NB this part of the blog only refers to fresh sawn (green) Oak. Kiln and Air dried is not susceptible because the moisture content has been removed during the drying process.
Firstly, Oak has a high moisture content relative to other timbers. 18 months after an Oak tree is felled, you can still expect a moisture content of 60%. Therefore, more moisture leaves the timber as it achieves EMC which makes movement much worse.
Secondly, the beauty of UK grown Oak is its undoing. Its twisted grain, full of character, is widely admired but this is partly what makes it so susceptible to movement. The twisted grain is caused by the gnarly, bending trunk which is characteristic of Oak trees. However, this encourages movement because the grain influences the way the moisture leaves the timber. On a tree with a straight grain, for example Douglas Fir, the moisture will leave the timber evenly whereas the twisted grain of Oak will not allow for even draining.
Like all timber species, tannins will leach out of Oak as it adapts to its environment. Oak is no more susceptible to tannins than any other timber, and there is nothing to stop tannins leaching out – it is a natural process. Tannins are acidic chemicals held within the sap. After the timber is felled the sap solidifies, so the tannins within the timber are stable. Rainwater will destabilise the tannins and wash them out.
It is a fallacy that UK grown Oak is more expensive than equivalent tropical hardwoods, such as Iroko. Especially in recent years, Oak has become increasingly cost effective to specify.
For this section of the blog, consider European Oak to include UK Oak – there are only small differences between the two.
European Oak is far better suited to external use than American Oak. This is because American Oak tends to move more easily because the cells are wider which means it is much easier for moisture to leave or enter the timber. Secondly, the colour of the timber is less consistent.
European Oak is practically identical to UK Oak. However, the only small differences can be found in the grain. European Oak as a slightly straighter grain whereas the grain of UK Oak is more twisted and interlocking giving it more character.
Oak is a fantastic option for external seating. It looks great, is exceptionally strong and is environmentally friendly (long lifespan and small transportation distance). All in a cost-effective package.
While is it true that Oak is susceptible to movement, this risk can be managed. For starters, green Oak timbers with an end section greater than 150mm will rarely noticeably warp – because it is so large, moisture is released at a much slower rate essentially enabling the timber to be classed as stable.
It is not advised to use green Oak in sections smaller than 150mm, because the moisture content will change rapidly causing extensive movement.
However, air and kiln-dried UK grown Oak in sections smaller than 150mm is well suited to use in outdoor furniture, as the moisture content has already left the timber. Just bear in mind that the dimensions of air and kiln-dried timber are limited to a thickness of 75m in most cases. Length and width are not limited.