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Which timbers are best suited to charring? And which ones should you avoid?

Deciphering which timbers are best suited to charring which can also provide longevity becomes an important factor to consider when looking for external cladding in the UK. This whittles down to the constantly changing climate and high moisture levels which we experience in the UK.

Generally, all-natural timbers carry a moisture level of around 18%, and also contain natural resins and sugars. If these timbers are kiln-dried, their moisture level is pulled down to around 12%, helping to stabilize the material. Historically, the UK has used chemically treated softwood and Cedar for cladding. In order to prevent the natural greying effect and black knots that these timbers give off over time, the material is usually painted. However, over time it became obvious that the paint would flake and peel off, and many people wanted to avoid the expense of stripping it back and repainting.

Why did the paint peel/flake off?

The predominant factor causing the paint to peel and flake off of all-natural timbers was not down to the actual paint applied. Instead, it was due to the timber that the coating was applied to. Lower cost Knotty Pines, Larch and some Cedars are prone to large degrees of movement. Such as, swelling, shrinking, cupping, and splitting. This is because the timber takes on moisture and later tries to expel this, causing the timber to move. Consequently, the movement effects the painted skin as it cannot accommodate the same movement, leading to surface cracking and eventually causing a breakdown of the surface material.

9 years ago, QTD Ltd were one of the first in the UK to embark on an R&D Programme to discover and understand the benefits of charring different timber surfaces. Uncovering the ranging finishes and whether this would be a better and different alternative to painting. We tested 15 different species of timber. Some looked attractive with a ‘Snake-Skin’ texture and we wanted to further test whether these new finishes would withstand the typical UK winters and summers that we experience.

Naturally, deciphering what impacts a variety of seasons would have on the timbers took some time. Whilst we were waiting for strong and confident results, other competitors decided to go straight to the market with their freshly charred timbers. With Larch being a popular choice due to its low cost.


Eventually, our findings expressed that all of the timbers that had been charred in their all-natural state had suffered varying amounts of degradation. From board movement (swelling and cupping) to the peeling and flaking away of the charring itself. Exposing the natural timber underneath.

We also found that a heavy charred surface is the only surface that naturally becomes resistant to UV degradation. This is due to the thicker carbon layer created by the charring process. This layer does not have flexibility and is more delicate than other charred surfaces. If the timber in this case, shrinks or expands with the varying weather conditions, then it will crack. Permitting rain to penetrate beneath the surface, causing damage to the board.

Positively, the timber’s that did not replicate the same signs of fatigue as the charred all-natural timbers, were those that had been thermally treated. Since the thermal process removes the natural resins, sugars and impurities that are normally present in all timbers. These thermally modified timbers cured at 7% moisture content, effectively creating a very stable moisture resistant board. In turn, providing high durability and resistance to movement throughout the board.

Larch and Cedar

There are three very good reasons as to why we do not char Larch and Cedar as well as other all-natural timbers that are to be used for external cladding.

  1. The moisture content of these timbers is generally high between 15-20%. Consequently, when you apply a high degree of heat to one surface rapidly, the board surface will break, causing the board to cup. In contrast, the moisture content in ThermoWood is only 7% therefore no distortion will occur during charring.

  2. Due to the presence of natural tannin, resin, sugars, and impurities the intense heat applied during the charring process bubbles these natural properties to the surface causing disruption to surface finish. None of these are present in ThermoWood’s.

  3. The action of charring the surface of timber, naturally makes that surface become brittle and therefore more delicate. Applying this charred surface to a timber that is prone to expansion like Larch and Oak, Kebony and Cedar means that the brittle charred surface will crack under the movement of the timber and over time deterioration of the surface will be inevitable.

So, if you are looking for a beautiful, low maintenance and durable charred finish, we would advise you to consider our ThermoWood’s instead of cheaper all-natural timbers like Larch and Cedar which will, in the long run, cause inevitable rotting, warping, movement, flaking or peeling. The moral of the story is: don’t buy charred larch or cedar unless you intend on replacing it some years later.

(Here is what your charred cladding SHOULD look like).

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