Advanced Timber Knowledge

Fresh Sawn vs Seasoned Timber (Stability vs Availability)

Introduction

Using fresh sawn or seasoned timber can make or break the success of street furniture on a project; specifying the wrong option could result in a warped wall top seat which affects the structural stability of the surrounding planter, or a design may be created that is impossible to build due to unavailability of timber in the required sizes. Therefore, it is important to consider the options when specifying timber, particularly Oak, on a project.

What is Fresh Sawn?

Also known as: Green Timber, Green Lumber

Fresh sawn, or green timber, refers to timber that has been recently felled. The crucial thing to understand is that the moisture content will be much higher than Air Dried or Kiln Dried. For example, the moisture content of green Oak is approximately 80%.

What is Seasoned Timber?

Also known as: Wood Drying, Air Dried Timber, Kiln Dried Timber, Dehumidified

Seasoning is the generic name of the process that reduces moisture content of fresh sawn timber. Timber that has been through the seasoning process is known as seasoned.

Seasoning is usually done by air drying or kiln drying – both methods are slightly different. Air drying is the more traditional method which involves simply leaving the timber in a dry place to dry, usually over the course of years. Kiln drying is a newer method and involves applying heat to dry the timber which is a much faster process, typically weeks. However, this the methods are irrelevant for this article because the result for both methods is to achieve timber with a moisture content of around 10% – 20%.

Fresh Sawn Advantages

Fresh sawn timber is more widely available than seasoned timber. For most projects and species this is no problem – for example there is a large supply of seasoned Oak. However, it is practically impossible to obtain seasoned tropical hardwood.
There is no limit to the size of the timber – obviously within the physical size of the tree the timber is from.
Because it does not have to go through the seasoning process, fresh sawn timber is up to 50% less expensive.

Fresh Sawn Disadvantages

As the moisture content is much higher in green timber, the timber is much more likely to move after it has been installed. This is due to the timber naturally having to match its internal moisture content to the humidity level of the environment it is placed in, usually 15-50%. It is this moisture shift, from approx. 80% to 20% which causes the movement.

Seasoned Advantages

The seasoning process reduces the moisture content of the timber down to a level roughly comparable with the atmospheric humidity (in the UK, this is approx. 20% in the winter and 15% in the summer). Therefore, seasoned timber is very consistent and stable – movement is almost non-existent. So why, after seasoning, are you left with a timber that has not warped? Surely you would just be working with a timber that has already warped rather than just warping after installation? The reason why timber will not warp during the drying process is because of the way it is stacked. Each slab of timber is placed on top of each other with small spacers in between to allow air flow between each slab. The combined weight of the timber slabs pressing down on top of each other overcomes the twisting force on the timber as it dries. Therefore, the timber slabs are forced to dry straight and true. On the other hand, the only resistance to timber movement on a bench is several bolts which the timber will overcome.

Seasoned Disadvantages

Not widely available in thicknesses of over 75mm. As drying timber is a slow/energy intensive process, drying timber is not commercially feasible. Therefore, the largest thickness most mainstream timber mills will offer is 75mm. For example, on average, it will take 1 year to dry a 1-inch Oak slab using the air-drying method.
Seasoned timber is up to 50% more expensive than green timber because seasoning is a costly process.
Seasoning can be less environmentally friendly. This does not apply to air drying which is equal to fresh sawn timber in terms of energy use, but kiln drying less environmentally friendly because of the energy used to heat the kiln space.

Conclusion

At the heart of the debate between using fresh sawn or seasoned is if you place higher value on availability or stability. If you need availability particularly in unusual sizes, then fresh sawn is likely to be your best option. However, if you need the timber to be stable and not move, then seasoned is probably more suited to you. Despite this, for external furniture, the answer is very subjective and heavily dependent on the species of the timber and size. Some suggestions below:

1, Iroko and Opepe are not badly affected by movement and warping, therefore more suitable to use as fresh sawn timber. This is because species with a close grain are less likely to move. The grains of these species are closer than almost any other species in the world.

2, Heartwood will not warp as much as sapwood. For more information on these terms, please read this blog.

4, It is unnecessary (and impossible) to use seasoned Oak with a cross sectional area of greater than 150mm.

5, Conversely, it is absolutely recommended to use seasoned Oak if the thickness is less than 80mm.

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