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Explainer: Delignification a.k.a Defibrosis a.k.a. "Hairy Timber"


If you have ever been into the roof space of an older house - to hide Christmas presents perhaps, or investigate the source of that disturbing scuttling sound - you may have noticed something odd about the timber above your head: it appears to be growing hair.

Rub your hand over it and it comes off in mysterious fluffy brown clumps. What could it be? Surely it's not supposed to be like that. Is there some sort of insect eating your timber? Moisture? A beaver on the loose?

In fact, what you are seeing is the breakdown of lignin in a process known as delignification.

What is Lignin?

First mentioned by the Swiss botanist Augustin Pyramus de Candolle in 1813, lignin is an organic polymer which makes up approximately 20-35% of the mass of wood.

Lignin is found in the cell wall of the plant and fulfils two important roles. It enables the vascular conduction of water inside a plant, and it helps give the plant rigidity and strength.

This rigidity and strength is exactly what makes wood such a wonderful building material. Lighter than steel, lovely to touch, and self-replicating to boot, wood ticks all the boxes. Furthermore, Western Australia is home to some excellent native construction hardwood. Until pine became the industry standard a couple of decades ago our wonderful Jarrah and Karri were the go-to material for all Perth homes.

Alas, there is a downside: the potential for delignification, especially on older tiled roofs. Roof tiles allow air in to a roof space, along with particles and pollution which appear to gradually cause the breakdown of lignin in the timber battens. As the lignin degrades, the fibres of the timber start to fray, and this is what is responsible for the "hair".

Karri tile battens showing evidence of delignification at this coastal property

Causes of Delignification

While delignification can be caused by fungi or bacteria, it tends to be more common in properties by major roads, factories and the sea, suggesting that airborne pollutants are at least partly responsible. The photo above, for example, was taken at a property in North Beach approximately 30 metres from the sea.

Larger roof timbers such as rafters and beams tend to be less susceptible to delignification, and so are usually not a cause for concern. Being more massive, they can also withstand some surface delignification without losing their structural integrity. But tile battens, typically only around 25mm thick, are more vulnerable to the effects of delignification and can often become dangerously weak.

This weakness is not rare or theoretical. Every week we pull old battens off roofs, and the furry ones come off easily, often snapping in to several smaller pieces. I have seen people inadvertantly put their feet straight through battens weakened by delignification. In very bad cases there is only one remedy:

Yes, you know where this is going - roof replacement.

Steel battens are installed as standard on a tile-to-steel reroof

The Solution

We use steel battens as standard on all our new roofs, which means delignification is never an issue. We also install an insulated anticon blanket underneath the roof sheets, protecting it from pollution and dust. Finally, the roof sheets form a much more sealed environment than roof tiles, allowing less ingress of airborne salts or pollutants.

If you are worried your roof timbers may be suffering from delignification, we are happy to provide a free inspection, including a detailed written quote and photos of every area of concern. For the final word, we are also happy to arrange an inspection from a licensed builder. Click here to book your free reroof quote today.


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