What are modified timbers?

Like glulam, modified timbers have become increasingly popular for woodworking projects. The demand for modified timbers is on the rise, but what are they and can they be used for glulam in the same way as hard or softwoods?

How are modified woods made?

Modified timbers are created by taking a species of timber and changing its known properties to improve certain characteristics. This is typically done to make it more durable and dimensionally stable.

They are modified in two ways – essentially they are either modified thermally or chemically.

Thermally modified wood:

Wood which is thermally modified is timber treated using temperature to change the internal structure of the wood. Incredibly high temperatures are used to heat the wood, but Oxygen is removed from the wood so it doesn’t burn. No chemicals are used in the process, only heat and steam.

The most commonly used timbers for this are Ash and Pine, and it is believed the process was used by the Vikings to protect their vessels from saltwater.

Brimstone is an example of thermally modified timber. British grown woods: ash, poplar, and sycamore, are transformed into durable timber for outdoor use. One of the many advantages of Brimstone is it’s locally made, as well as locally grown (see our recent project here using locally grown timber).

While we haven’t made any Brimstone glulam yet, we are planning to and will share how we get on when we do.

Brimstone timber cladding, and the thermal modification unit at Vastern Timber

Chemically modified wood:

Chemically modified wood is where timber is treated using chemicals to strengthen and modify the timber.

There are several different ways it can be modified chemically – it can be bathed in heated oil with chemicals, placed in a high nitrogen atmosphere or put through an acetylation process (pickled in vinegar), which is where an acetyl group is added to the timber’s compound structure to increase durability.

An example of chemically altered wood is Accoya, and we have been working recently with some Accoya for a Swimming Pool Cover in the Middle East in conjunction with Outhounds, who are supplying them with a retractable roof system.

CGI and draft design of the Accoya Pool Cover, Squire & Partners

Why use modified timber?

Modified timbers have much greater durability than traditional untreated hardwood. Hardwoods are typically durable (see our hardwood blog here) but do not accept the treatment of preservatives very well (pressure treatments).

Modifying the timber can be a way of turning cheaper non-durable timber into very durable timber. Typically, the costs for modified timber can be comparable with hardwoods such as Oak, but they will usually have a guaranteed lifespan externally. Also, in the case of Accoya and Brimstone, the stability is better as the moisture content is so low there aren’t the usual movements associated with drying out.

Block gluing of accoya glulam in progress

When are modified woods not advisable?

As with all timbers, there are upsides and downsides! The disadvantage of modified timbers is the costs are usually high.

When it comes to glulam manufacture, we have to make sure lamination of modified woods is done very carefully. Modification of the timber means it does not behave in the same way as standard timber; for example when we are making Accoya glulam (as pictured above) we need to use a different glue called resorcinol, instead of the MUF glue we usually use.