Mar 30, 20235 min read

How do you burn timber cladding?

Our most frequently asked question, one that has so many people curious, so, we have written this blog to unravel the process, and explain each step for you. With so many options now in the market, we have covered what you should consider, and be aware of, before purchasing charred timber cladding.

The definitive guide to the Charred Timber Cladding process and what you should be considering before purchasing.

QTD embarked on this journey 10 years ago now and after almost two years of Research and Development using 24 different timber species, we finally launched them into the market, the brand known today as ThermoChar®. So, as the first UK company to do this following extensive R & D we feel justified in having the knowledge and expertise as the UK’s No. 1 experts in this field.

Naturally, there have been a number of other suppliers that have since entered the market with their version of charred timber cladding, however, the price points seem to have had more focus on this expensive, time consuming and risky process instead of the quality of the finished product and how it performs in our variable UK climate.

Charring the surface of timber cladding is actually no different to painting the surface in terms of the timber species being used and its preparation beforehand.

The Japanese method (Yakisugi) back in the day of standing boards upright over a fire to char the face and then hosing them down, although very effective, was a bit ‘rough and ready’, and not very practical for the more refined customers of today looking to create that contemporary design for their dream property.

One of the many reasons Charred cladding became popular, was because people were fed up with having painted boards that started to peel and flake off after a relatively short period of time. So, the thought of having a black finish that didn’t do this was obviously more appealing, (no pun intended).

However, the reason the paint peeled off was because the paint was being applied to timber that was either not properly clean or was cheap softwood. Timber is very accurately priced, and you tend to get what you have paid for, which reminds me of the saying ‘Buy cheap, buy twice’.

Softwood at the lower price scale is fast grown and is popular for use in fencing, garden buildings and some cladding products, and therefore has a far more open cell structure which allows them to absorb and expel moisture at fairly high levels. This means that they will naturally expand and contract with the local climatic conditions. Paint therefore will have a hard job also expanding and contracting at the same rate leading to it opening up allowing the moisture to penetrate and eventually peel off.

This also applies to a Heavy Charred surface which is inherently brittle by nature and therefore cannot flex with a softwood timber that expands and contracts and will consequently start to break down as the weather gets behind the surface skin.

The way to avoid this is simple, you only heavily char a timber that is very stable and has a low moisture content and minimal to no resin and sugars within its makeup.

This why, here at QTD, we only Heavily char a select number thermally modified timbers along with the other various finishes that are applied.

Heavily Charring Natural Larch, Pine, D. Fir, Cedar, or Oak all contain a higher moisture content at 12% or more, along with natural resins and sugars, that under intense heat can either distort in width known as cupping, or have the surface disrupted by the resins and sugars being pulled to the surface and compromising the final finish.

Every board that is charred reacts differently depending on the grain structure. Some will burn very quickly, and others can take longer, and this will be the case when charring the same species of timber as no two boards will be the same. This is the beauty of natural timber over manufactured composite, and this is why we char every board meticulously by hand, making sure the final finish and pattern is as consistent as possible with no adverse effects to the variety of cladding profiles on offer with this product.

Here at QTD, we do not use a furnace machine to pass the boards through as the finish cannot be regulated and controlled in the same way that can be achieved by a manual visual process. The machine process is obviously quicker and can only really be of benefit if the charring is then all brushed off to leave it in parts of the grain and then finished with a pigmented colour or oil to protect it from the weather.

What's next for the charred market?...

Another process that QTD experimented with a few years ago was Heat Embossing a pattern into the surface and then painting it black. This process uses a pre-etched drum roller that presses the pattern into the boards surface, aided by a gas flame or electricity. Care has to be taken not to end up with a repeat pattern on every board like you get with composite boards that imitate the timber grain as previously mentioned, this surface although initially more robust, will still rely on a sound painted surface that again will only be as good as the timber its applied to. If the timber is a softwood pine for example, then it will not have sufficient stability and the timber movement will have an adverse effect on the final finish.

We thought we would share with you some examples:

These examples had 6 months exposure show what can happen to a Hot embossed pattern board if the hot embossed pattern is not applied to very stable timber.

This is why QTD only use ThermoWood’s for their various charred finishes.

After Hot Embossing this image clearly shows the distortion of KD Siberian Larch with a Moisture Content of 12% after 6 months exposure.

After Hot Embossing this image clearly shows the distortion of KD Douglas Fir with relief grooves in the underside with a Moisture Content of 12% after 6 months exposure.

Due to the movement in the softwood larch and Douglas Fir this is what happens to the Hot embossed surface that also has a flexible black paint applied. The Douglas Fir (Bottom two boards) fared slightly better than the Larch. (Top two boards). After just 6 months of external exposure on our test rig.

So, in summary...

Any aesthetic surface finish applied to cladding or decking boards that are being used externally, will only be as good as the timber species it’s being applied to. The reason QTD only use ThermoWood species for charring is because they only have between 6%-7% moisture content, the cell structure is closed and all of the resins, sugars and natural impurities have all been removed during the ThermoWood process, providing you with the most stable and durable timber product in which you can successfully apply a wide range of finishes too that have the best possible chance of standing up to what the UK climate throws at us.

The image below is of our Thermally Modified Heavy Charred Ash in Ayra profile installed 8 years ago on the west face of our office and it has not been cleaned or maintained and has rain splashes from a recent downpour.