At Stour Valley Oak, we use green oak for the construction of all our oak frames, but we often get asked; ‘what is green oak’? So we’ve asked our in-house timber expert and company director Paul Street to explain.
Q: What is green oak?
A: The term ‘green oak’ refers to oak that has been ‘recently’ felled and has not yet dried-out, not it’s colour. Once it’s moisture content is between 20% and 30%, the oak is classified as ’air-dried’.
Q: Why does Stour Valley Oak choose to use green oak?
A: As the oak is still ‘green’ it is softer and much easier to work with. Cutting or working an air-dried piece of oak is substantially more difficult, as the material is much harder and extremely unforgiving on tools.
Q: Does green oak harden as it dries out?
A: Yes. The oak dries-out at approximately 25mm per year. For our oak frames, this mean that the joints become much tighter as the material dries, ensuring the frame becomes even more solid, hence the reason it has been used as a building material for centuries, with many examples of oak-framed buildings surviving for hundreds of years.
Q: What are the benefits of the timber drying out?
A: As a natural product, oak will split, shrink and distort as it dries-out. These ‘imperfections’ provide the character to the building and is often cited as the feature that drives customers towards an oak frame in the first instance. One of the endearing qualities of oak-framed buildings is the unique way each frame ages.
Q: Does green oak shrink?
A: Yes. The shrinkage process is essential to the structural integrity of the frame. As the oak shrinks and dries-out, the joints become tighter and the oak becomes significantly harder, improving its structural capabilities. We use traditional jointing methods, which have been tried and tested for centuries. Effectively, we expect/want our cartlodges to shrink.
Q: Does green oak need treating?
A: No. It naturally hardens as it dries-out and becomes extremely durable. Testament to its natural resilience, it’s often used as the material of choice in harsh external conditions, including in the marine environment.
Q: Does green oak stay the same colour through its life span?
A: No. The oak goes from being honey colour, to silvery-grey as it ages and is exposed to varying weather conditions and UV light. We regularly get compliments on how our buildings look once completed and I think that if we were to ask our customers to provide feedback once the building has aged for a few years, I believe they would like it even more.
Q: Where does the oak that Stour Valley Oak uses come from?
A: The vast majority of our oak comes from a PEFC (Programme for the Endorsement of Forest Certification) accredited sawmill in Brienne-Le-Chateau, France; sourced through our timber merchant.
Q: What are tannins and what is tannin leach?
A: Tannins are naturally occurring and are the collective name given to the acidic chemicals held in solution in the liquid sap. The tannins are still present in dry oak as they are left behind when the water evaporates.
Tannin leach can be prevented with various ‘sealers’ on the market. This however has the negative effect of changing the natural drying process for the oak, and changing its appearance.
There is a limit to the amount of tannin that will leach-out and it can be fairly easily removed. Our advice would be to just leave it to dry naturally, then clean any unsightly marks once this process has finished.