Advanced Timber Knowledge

Understanding moisture in timber, what’s the best for me…?

When designing and/or specifying timber for any project, it’s important to understand a little about moisture content of timber and how it works. There are 3 levels of moisture content in timber which we’ll discuss in this article:

Fresh Sawn – typical moisture content <80%

Air-dried – typical moisture content 18 – 22%

Kiln-dried – typical moisture content 8 – 12%

Fresh sawn timber – what is it?

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 saw mill 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’ – 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 specie. 

This grade of timber is ideally suited to external applications.

Fresh Sawn Advantages:

  1. Very large cross sections available – this is only limited by the physical size of the tree… and maybe transport if you go really, really big!
  2. Low cost. This is due to the very short, and very limited processing required, so no extra costs incurred.
  3. Environmentally friendly – as there is a very short timescale from forest to client, and very little energy required to process it, this grade would have the smallest carbon footprint of the 3 grades.

Fresh Sawn disadvantages:

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 really quite small. 

Air Dried Timber – what is it?

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 air flow around each board so the excess moisture can evaporate.  The duration of the drying period varies according to the thickness of the planks and specie 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 is 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.

Air dried advantages

  1. Stable. Movement that occurs during the drying period will be machined out as part of the manufacturing process. EG, a 100mm plank of rough timber that has slightly twisted will be machined down to 80mm. Providing the environment the timber is placed in has a similar level of moisture, then there will be virtually no further movement.

Air dried disadvantages

  1. Limited sizes available. As mentioned above, the maximum plank thickness is 4” (finished dimension 80mm) but widths can be up to 300mm.
  2.  Cost. Due to the time involved and space required for drying, there is a cost associated with this.

Kiln dried timber – what is it?

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 specie and thickness of 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.

Kiln dried advantages

  1. Stable. As with air dried timber, movement that occurs during the drying period will be machined out as part of the manufacturing process. EG, a 100mm plank of rough timber that has slightly twisted will be machined down to 80mm. Providing the environment the timber is placed in has a similar level of moisture, then there will be virtually no further movement.

Kiln dried disadvantages

  1. Limited sizes available. As mentioned above, the maximum plank thickness is 4” (finished dimension 80mm) but widths can be up to 300mm.
  2. Cost. Due to the time involved, space required, and energy consumed in the kiln drying, this grade is the most expensive.

Other Considerations / points to note

Appearance: Over time, all timber regardless of grade – fresh sawn, air-dried or kiln-dried, will weather to a silvery-grey colour. 

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 central 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.

Conclusion

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.

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