Articles - Creative Design
Before specifying a timber type for a new project, it’s important to know the difference between the moisture contents. Fresh Sawn, Air Dried and Kiln Dried are all different, but which is best for your project?
Also known as Green Timber, Green Lumber
Fresh sawn timber is exactly what it sounds like – trees that have been recently felled and have had no official drying period before use. The crucial thing to understand is that the moisture content will be at its highest, this can be anywhere up to 80% in some species but will reduce over time to a similar percentage to the environment it is placed in. Once a tree has been felled, it is cut into the desired sections in a sawmill and sent straight to a manufacturing point, ie Logic Manufactured Bespoke, where it’s further processed and manufactured into the final product. This timber has the shortest and most efficient ‘route to market’ – the timescale from forest to factory could be as little as 4 weeks.
Moisture content in this timber will slowly reduce over time until it reaches an equilibrium point with its surroundings. This is referred to as its Equilibrium Moisture Content, or EMC for short, and will vary slightly depending on its final destination. EG a coastal area in winter will have higher atmospheric moisture than a sheltered, south-facing courtyard, so therefore the timber will respond accordingly. Some timbers are more / less prone to movement in the fresh-sawn state. When designing external furniture, care should be taken in choosing the species.
This grade of timber is ideally suited to external applications.
Movement: Due to the high initial moisture content, which will gradually reduce over time, movement will occur as this takes place until it reaches its EMC. As the moisture content reduces, so will the dimension of the timber. Please view insight (why does timber move) for further details. Providing this has been taken into account in the design stage, then this ‘disadvantage’ can be quite small.
Air dried timber, again, is as it sounds. This is the traditional way of drying timber and is simply trees that are cut into long planks of varying thicknesses, stacked together and left outside but under cover. The raw boards start their life as fresh sawn so moisture content is around 80%, and will slowly reduce to the moisture content of the surrounding atmosphere, which in the UK is between 18 – 22%.
The stacks are ’sticked’ which means careful stacking and separating to allow sufficient airflow around each board so the excess moisture can evaporate. The duration of the drying period varies according to the thickness of the planks and species of the timber. As a rule of thumb, each inch of timber takes a year to dry.
EG a 4” plank will take around 4 years to achieve 18 – 22% moisture content. Due to the time involved in the drying process, 4” is the maximum plank thickness, which would machine down to around 80mm finished size.
This grade of timber is ideally suited to external applications.
Again, this is as it sounds. Timber that has been air dried is then stacked in a large sealed oven called a kiln, and warm air, anything up to 65°c is circulated which further reduces the moisture content down to between 8 – 12%. Depending on the species and thickness of the board, this drying period will take days/weeks rather than years. As with air-drying 4” is the maximum plank thickness, which would machine down to around 80mm finished size.
This grade of timber is ideally suited to internal applications but can be used externally in certain circumstances.
Appearance: Over time, all timber regardless of grade – fresh sawn, air-dried or kiln-dried, will weather to a silvery-grey colour.
Moisture content: Normally speaking when referring to percentages, you can’t go above 100%, right? However, when speaking of moisture content in timber we can go way above 100%. So, what do we mean when we say 50% or 200% moisture content? How it is measured is the increased weight in percentage above its dry weight. For example, let’s take a piece of totally dry timber, 0% MC, and it weighs 100kg. If we let the timber absorb a little water and the weight increases to 130kg, that’s 30% MC. Allow the same piece of timber to absorb a lot of water, and the weight increases to 200kg, that’s then 100% MC.
Stability: timber movement is due to the moisture content in the timber fluctuating and varying from its surroundings. Providing the surrounding moisture level is somewhat consistent with that of the timber, then little movement will occur. If fresh-sawn, or even air-dried timber is placed inside a centrally heated building, then it will dry out rapidly and reduce in size which will result in cracking and potentially twist. Likewise, air-dried timber placed outside will absorb moisture and swell slightly.
At the heart of the debate between the grades is down to 3 factors – design, availability and stability, and where they are in priority on the project. If you have a particular design with large sections then fresh sawn would be the best. However, if you need the timber to be more stable and not move, but still in the external environment, then air-dried is probably more suited.
