Certification Knowledge
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, varied grain detail, large and small knots, 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, then yes, UK grown oak is suitable for outdoor seating.
Due to the moisture content in all fresh sawn timber, movement will occur when it transitions from its fresh sawn state (high moisture %) to its equilibrium moisture content, or EMC for short (low moisture %). When timber is going through this process, as a general rule, timber with straight grain will move less than that with varied. This is due to varied ‘parts’ drying faster than each another, creating deformities of some sort or another.
How do we go from here as we know that UK grown oak has these beautiful, varied grain pattens…?
Fresh sawn oak can be used in large sections (anything above 150mm x 150mm), as there is usually sufficient grain structure to equalise any dominant grain so the noticeable movement will be minimal. However, with smaller sections of fresh sawn oak, there will be more chance of a dominant grain, or knots, to overpower and cause the timber to twist, etc. So, for small sections, we have to use seasoned oak.
We have a separate blog (air dried / kiln dried) which goes into the details of this so will just briefly refer to this here. Seasoned timber has had its moisture content reduced to a point where there will be very little further movement in the environment it is designed to be in. However, what must be understood that it’s the fluctuation in the moisture content of the timber that causes the movement. Another point to be aware of is, due to the drying processes, seasoned timber rarely is available in anything thicker than 80mm finished size.
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 drying.
Tannins are the name given to the acidic chemicals held in solution in the sap of the tree, full title – Quertannic Acid. All timber species contain such tannins, it’s just some have more than others – Oak and Chestnut have the highest levels.
Once a tree has been felled and planked ready for further processing, any water that comes in contact with the timber will dilute these acids and cause them to ‘leach out’ – looks a bit like creosote. It is a natural process and will gradually reduce as the timber dries out. With seasoned timber, due to the fact that it’s been ‘dried out’, leaching is very minimum. These tannins are of no detriment to the timber itself but does react with other materials, raw steel in particular, so careful consideration should be given to each design.
Well, compared to what? As timber is a natural product, prices can vary due to availability which does apply to any specie. 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.
There are over 500 different species of Oak, each with some differing characteristics; commercially we would only be looking at a small handful. Interesting fact, there are more oak trees within the UK than any other tree. In considering the different characteristics of Oak, as there are so many species, to keep it simple we’ll just consider the country its grown in, or more importantly, the latitude. Typically, the colder the climate the slower the tree will grow, which in turn will produce a denser timber with more varied grain structure. The warmer the climate, the quicker the growth, tends to be straighter grain, but less dense.
In addition to this, plantation grown Oak with differ from original growth Oak. Plantation grown trees are typically straighter grained with more open grain structure, whereas original growth has more character and denser grain formation.
Oak is a fantastic option for external seating. It looks great, is exceptionally strong and is environmentally friendly (long lifespan and small transportation distance from forest to factory), there are just a few considerations to take in. Overall, a great cost-effective package.
https://www.iwood.co.uk/timber-knowledge-library/oak/european-vs-american-oak/
https://www.woodengatecompany.co.uk/why-do-wooden-gates-bend-and-warp/
https://www.intouch-quality.com/blog/wood-warping-and-how-to-prevent-it
