Why Modified Wood?

Posted on 25/03/2017

Modified wood is undergoing something of an evolution. The range of products available has dramatically increased in recent years, and more specifiers are using the product in new applications.

Modified wood’s rise in popularity can be credited to improvements in key areas such as durability, stability, sustainability and cost - making them more attractive to the modern market.

To give some background, modified wood is wood that has been heated at elevated temperatures or treated with a chemical, biological or physical agent to enhance its performance, without using a biocide. The three principal processes for modifying wood are usually classified as thermal modification, chemical modification or impregnation. Although most modified wood products are manufactured elsewhere in Europe, many are widely available from UK suppliers.

Thanks to new technology and the scaling up of modification processes, the timber is more durable than ever before. It is now also possible to supply this material in dimensions suitable for commercial applications.
We recently worked with TRADA for a revision to their Wood Information Sheet on modified wood. First published in 2010, the information sheet highlights the increasing range of modified woods coming to market and the need to make specifiers aware of the properties and suitability for different end uses.

As part of this process, the properties of many modified wood products have been examined using existing standards and test methods developed for unmodified woods. Although long term performance in service is less well understood than for unmodified woods, in tests, modified woods have often been shown to outperform reference materials, such as preservative treated wood or hardwoods valued for specific properties (such as durable or low movement species). Given these findings, it is easy to see why increasing numbers of specifiers are turning to modified wood.

Why modify?

One of the main drivers for modification is to improve the decay resistance of low durability timber species, which normally require preservative treatment to achieve acceptable service lives.

Decay fungi are considered the greatest risk to the service life of timber in the UK, so testing the resistance of modified woods against these is where we receive the most enquiries. Most modification processes enhance the durability of wood against decay fungi, offering an alternative to preservative treatment. Testing has shown that some modified woods can provide service lives equal to or longer than preservative-treated wood or some naturally durable timber species.
While improving durability, wood modification can also result in a number of other beneficial changes to wood properties, including improved dimensional stability, improved aesthetics and increased hardness.
These changes have dramatically improved both the performance of wood products and their range of applications, and open up a new realm of possibilities for modified wood. Different types of wood can now be used in applications where they were previously unsuitable. For example, a number of non-durable British grown timber can now be specified for cladding, thanks to the emergence of modified woods such as Brimstone.  Aesthetically, all of the Brimstone species are relatively free of knots (equivalent to BS1186-3 Class 1, similar to No. 2 clear, and better than Western red cedar) providing an alternative to the more knotty thermally modified softwoods.

Equally, where durable and stable materials are required – for example in cladding and decking – there used to be very few economically viable options apart from western red cedar and Siberian larch. However, thanks to developments, there are now many more materials which can offer stability and durability at a reasonable price. Thermowood® (from MetsäWood), for example, is a thermally modified non-durable hardwood which has been made applicable for this purpose. It is now one of the most popular choices in the UK for exterior cladding, thanks to its excellent properties and profile range.


In testing, improved stability is demonstrated using all types of modification process. Since the cell structure of modified wood products absorb less water, the equilibrium moisture content of these products is also reduced. Improved dimensional stability reduces movement in service and the risk of cupping in cladding, floor and decking boards; it also reduces splitting or checking around fixings over time; and there is likely to be less differential movement between members at joints, which can result in a better quality finish. Tests to evaluate movement have shown that some modified woods outperform even timbers considered to be very stable, such as teak and idigbo.

Modified woods typically attain a ‘small’ movement classification. Comparative tests have been conducted using a number of modified woods against commonly used softwoods. These tests set out to establish the comparative swelling or shrinkage of modified woods when exposed to high and low humidity, and to investigate the relative risk of distortion when wetted.

Modified wood outperforms the three commonly used softwoods (Siberian larch, redwood and western red cedar) across the board when it comes to swelling and shrinkage.

While durability and stability are often the drivers for modified wood, sustainability is an additional key benefit. There is clearly less need for chemical treatment – the chemicals for chemical and impregnation modification processes are often derived from natural materials and are not biocides. However, there is also an opportunity to specify home grown timber which minimises transportation, and it is also possible to modify sustainably sourced tropical hardwoods.

Modified woods are suitable for a wide range of applications, but their properties differ significantly from unmodified wood, so specifiers need to be aware of their properties and suitability for different end uses. It is always worth checking with individual product suppliers for confirmation of how these products can be expected to perform in their intended end use.

While the opportunities for modified wood are increasing, it is crucial to investigate the properties for each individual end use prior to specification. While some modification techniques improve certain wood properties, other properties may remain unchanged or be negatively affected. For example, while modified wood can be more durable, the process can sometimes weaken the timber, limiting its use in structural applications.

Nevertheless, with an increasing number of modified woods being used for glulam, the possibilities remain open. We look forward to seeing what the next Wood Information Sheet revision will bring!