Confused by what ‘seasoned’ means in structural Oak? You’re not alone.
We’ve all been there. Thankfully, it’s not very complicated. Having said that it’s not an exact science, but this could be as much as you really need to know.
By way of example…
Above are some spectacular looking beam sections. 42 to be exact. All short Oak sections made by crosscutting (we used a chainsaw) through the centre of some 11m long 500x500mm seasoned Oak beams.
This was a unusual job and it was some years back but you can see why we remember it so well.
On the outside the original beams were fully weathered, having dried out in the open for more than 10 years. The photo’s show clearly that the silvering was only surface deep. It;s easy to imagine that a light plane or strong sanding would strip back the grey weathered layer and bring back the warm, soft honey colour of dried Oak.
As we didn’t imagine we’d ever repeat this job we took the opportunity to do a bit of an experiment. We put a moisture meter across every cut and found conclusively that whilst the first 25-30mm of thickness was air dry at 17-20ish% moisture content any deeper was a pretty consistent 32%.
This is seasoned structural timber… settled, stable, strong, still workable & beautifully fit for the purpose. If you’re restoring Oak timber frames or working up glazed curtain walling in Oak frames this is probably what you’re working with.
At EWT we use an aging formula to help apply a bit of logic to the otherwise ephemeral qualities of structural Oak beams. It goes like this…
Anything up to 6 months from it’s sawn date is categorised as FRESH SAWN (FS).
Anything 7-24 months from the sawn date we categorise it as PART SEASONED (PS)
Anything above 24 months, i.e. 25 months or more from the sawn date, we categorise as SEMI SEASONED (SS).
In this way the age of the sawn structural section corresponds to a level of drying / seasoning.
For engineering calculations our structural timber is technically ‘wet graded’ as, due to section size, it cannot reach the moisture content required for ‘dry graded’ structural timber.
In case you’re thinking we’ve forgotten one, there is no FULLY SEASONED solid structural timber.
We learned a long time ago that the knowledge passed down, and eventually tested, regarding the drying of hardwoods is simply true. Hardwoods naturally air dry up to 1 1/2 deep, that’s it.
Luckily there is a lot of timber framing experience out there and a lot of ancient timber frame buildings that we can learn from.
Here’s some more info if you’re feeling a bit woodgeek-y…
See our Structural Timber stock
Read more about Seasoned Structural Timber
Find a timber frame builder
Read more about Oak
Find out more about ancient timber framed buildings at the Weald & Downland Museum