News: Should You Use Air-Dried or Green Oak For A Truss: Exploring The Big Differences

28/07/2022

When buying a solid oak truss from Oak Timber Structures, you can choose between air-dried or green oak. Which do you pick?

We’ll answer all your questions throughout this article.

But first, what is the difference between air-dried and green oak?

Green oak is freshly sawn. Whereas, air-dried oak is cut from the log and stored. Storing the oak outside, yet protected from the elements, allows the timber to dry naturally reducing the moisture content.

Now you know the fundamentals. Here's everything you need to know about the differences between air-dried and green oak for a truss.

What Is Green Oak?

We sometimes refer to green oak as fresh sawn oak. Cut between 3 to 6 months after felling, it gets its name because of its freshness. Because it's fresh, green oak has a high moisture content. So most of the natural drying takes place after installation. Such drying is faster with indoor trusses because of the warm environment in which it’s assembled.

What Is Air Dried Oak?

You may hear air-dried oak referred to as seasoned oak. We cut the oak log into standard section sizes and store it in a natural environment to decrease the moisture content. So, air-dried oak has gone through part of the drying process, these beams are then stored for 2-4 years. It’s a complicated task and good storage conditions are essential. Not to mention, without using well-spaced bearers, you can warp the beams.

Personal storage after receiving your structure is equally as important if not moreso than during the initial drying process – more on that in our Information & Knowledge Booklet

Is Kiln Dried Oak The Same as Air Dried Oak?

Although kiln-dried oak is a class of dried oak, there are differences to air-dried oak. Depending on thickness, air-dried oak takes years to dry. Kiln-dried oak is much quicker and uses a kiln to speed up the drying process.

Not to mention the drying percentage of each type of dried oak, typically Air – Dried Oak will contain a moisture content of 30 – 40% whereas Kiln Dried Oak will be between 10 – 20%

A kiln requires employing fossil fuels, so air-dried oak is better for the environment. Also, accelerating drying can cause more stress on the oak. It's for this reason that kiln-dried oak is better for smaller cuts used in flooring, for example.

Additionally, where Kiln Dried Oak is suitable for intricate joinery, Air Dried Oak still has too high a moisture content and is more suited to larger sections. So, where it may be far more stable than Fresh Sawn Green Oak, it will still dry and continue to shrink. Furthermore, Kiln Dried Oak cannot be kilned above 100mm thickness regardless so in beam sections for trusses Air Dried Oak is the only dried option.

Air-Dried Oak vs Green Oak: Appearance

Green oak has a more golden colouration than air-dried oak. In contrast, air-dried oak is darker and exhibits silvery grey hues. However, green oak goes through a colour change and will develop the same silver hue as it dries. Albeit, the colour change occurs at a slower rate when the oak is indoors because of the decreased exposure to sunlight.

As the oak dries, it shrinks. Such shrinkage creates splits, cracks and surface checking. While cracks and splits sound troubling, you needn’t worry, it’s natural and doesn’t affect the durability of good quality oak. More on that in the below section. Besides, these characteristics add to the distinctive and natural aesthetic.

As with the colour change, green oak will develop such splits and cracks over time. Yet, air-dried oak has already developed these characteristics throughout the prolonged drying.

So, there are differences from the beginning. But both air-dried oak and green oak will eventually display similar appearance changes

Air-Dried Oak vs Green Oak: Strength

What’s stronger, air-dried or green oak?

Both are the same species, just with varying moisture content. So, as non-porous hardwood, both are sturdy woods that’ll last centuries.

The bigger question you need to ask when it comes to strength is about the quality of the oak. For any truss, you'll need high-quality. That’s why we only use QPA and QP1 construction grade oak. Simply put, it’s the best you can get.

So why does this matter?

As constructional grades, QPA and QP1 are ideal for trusses regardless of whether the truss is of air-dried or green oak.

As you've learned, green oak shrinks as it dries. It's understandable you’d assume shrinkage is at the detriment of stability. But, it's the opposite and actually shrinkage improves the truss strength.

See, an excellent design executed by skilled artisans makes sure shrinkage improves connections. How? Once joints and pegs are in place, during drying, the shrinking causes joints to fuse together creating a tighter and thus stronger connection.

There's still shrinkage with air-dried oak, but it's much less, and for this reason, you could argue green oak is stronger as it tightens around joinery.

Our Air – Dried oak is initially graded when cut into standard sections to QP1 appearance grade, it is then seasoned for 2-4 years. Due to the physical changes of Air-Dried Oak overtime, it cannot be assigned an appearance grade after the seasoning period. Understandably this is confusing but rest assured whist it cant be graded it is still structurally more than capable of load baring.

Advantages of Green Oak

For starters, green oak is more cost-effective. As air-dried oak requires years of storage, the price reflects the upkeep costs.

A durable timber, green oak is resistant to both weathering and infestation. Hard-wearing, a truss made of green oak will stand the tests of time. As mentioned, a skilled workforce will use the decreasing moisture content to their advantage. The shrinkage of green oak further tightens joints, so the robustness of a green oak truss improves over time.

Once erected, you’ll be in awe of the timeless grandeur of a green oak truss. While green oak serves to add an aged aesthetic, it’s a versatile timber. Nowadays you'll find green oak incorporated into modern builds. So it's not limited to a traditional rustic look. It also looks stylish and contemporary in modern builds.

It's possible to prolong the golden purified finish of green oak using treatment. Something like Osmo UV-Protection Oil does the job. While it won't fully eradicate silvering, a few coats delays the natural greying process up to 12 x more. Yet, you can't revert air-dried oak back to its fresh state.

You do have an option of a planed finish on your structure which will return silver grey UV bleached air dried oak to its natural golden hue

Some customers prefer this as all of the typical effects of aged oak are present such as splits and cracks, yet has a clean and contemporary aesthetic with the uniform golden colouration. In addition there’s far less movement to occur so treating it at this point after some of the drying process has occurred will allow you to maintain your oak structures aesthetic upon arrival for longer.

Advantages of Air Dried Oak

While not eliminated in full, as the drying process has begun, moisture movement is less when using air-dried oak. So you can expect less movement from a truss.

Likewise, the reduced moisture content contributes to another positive of air-dried oak. The decreased moisture has already formed the timbers' rustic appearance. So the unique characteristics of splits and cracks are present. But even air-dried oak isn’t fully dry. New fissures can appear. Anyhow, air-dried beams appear closer to the finished look. So, for the most part, it's a case of what you see is what you get.

Air-dried oak has also already developed its natural silver hue. So you can envisage a more complete aesthetic in your home from the start. You aren't waiting for the truss to develop the aged look years after installation.

In Conclusion

So, should you use air-dried or green oak for a truss?

Did you know that 90% of our customers opt for trusses made from green oak? Safe to say, it's categoric that green oak is the preferred choice.

Why is it the preferred choice?

Here are the key benefits of using green oak for a truss:

• More cost-effective
• More straightforward to work with and install
• Easier to treat to achieve your preferred look and finish
• Tightening of joints as shrinkage occurs improves stability
• Watch as your oak truss age with grace years after instalment

Sure, the price plays a big part. But not only saving you extra cash but there's also deeper reasoning behind customers favouring green oak.

Whatever truss you want, even if you want to ‘go against the grain’ and use air-dried oak, we'll make it to the highest standards. So why not visit our oak truss page, play on the designer and come up with a truss bespoke to your sizes and needs?