Contemporary South African architects, engineers, wood scientists, builders and manufacturers are embracing cross-laminated timber (CLT) construction while grappling with a critical challenge: preserving the wood effectively while adhering to stringent building standards.

Willie Conradie believes surface-applied supplemental wood preservatives are key to unlocking CLT's potential without compromising safety.

Willie is the Technical Director of Timberlife, a specialist wood preservation company he co-founded 44 years ago. He is an approachable scientist, researcher, inventor, innovator, patent-holder, entrepreneur, and astute business owner whose reputation in the wood preservation industry reaches far beyond South African shores.

In this Q&A, we discuss the industry-led solution to balancing engineered timber innovation and wood preservation: getting the Registrar to accept and approve the treatment of CLT with registered surface-applied supplemental wood preservatives.

Willie, first, what is CLT?

Cross-laminated timber (CLT) is an engineered wood product made by glueing together layers of sawn timber boards, with each layer oriented perpendicular to adjacent layers. The panels are large, usually between 2.4m and 4m high and up to 12m long, and the thickness depends on the number of layers.

Structures, including homes, offices, tourist lodges and multi-storey buildings, are prefabricated in a factory and speedily assembled at the construction site.

Why is preservative treatment compulsory in South Africa?

Construction timber that isn't naturally durable must be treated to extend its service life by preventing termites, wood borer and fungal decay. Even when used inside a building and fully protected from the weather and moisture that may cause fungal decay, certain wood-destroying insects can still attack untreated timber.

In the coastal regions of South Africa, softwood timbers like pine and spruce used in building structures can be severely damaged by the European house borer (Hylotrupes bajulus) and the West-Indian drywood termite (Cryptotermes brevis). The sapwood of eucalypt hardwood timbers is very susceptible to attack by the powder post beetle (Lyctus brunneus).

What is the dilemma with CLT?

The question is, can CLT panels treated with surface-applied supplemental wood preservatives be certified for compliance? It is not specified as an approved primary impregnation method, process and wood preservative in the SANS 10005 and SANS 1288 standards for preservative-treated timber.

Why is conventional preservative impregnation of CLT a problem?

Because the assembled panels are too large for conventional preservative impregnation plants. Although treating solid timber boards before assembly into CLT products is possible, it is very costly and time-consuming. It may also require pre-assembly machining that removes some of the already-treated timber and exposes untreated areas. Also, the process produces preservative-treated waste that may require specialised disposal measures due to environmental concerns.

Is there an alternative method?

Yes, there is. The alternative is surface-applied supplemental wood preservative treatment. It is brushed or sprayed onto the outer surface of the timber as an unbroken envelope.

What is a supplemental wood preservative?

It is a wood preservative formulated and registered with the Registrar at the Department of Agriculture, Land Reform and Rural Development (DALRRD) in terms of the Fertilisers, Farm Feeds, Agricultural Remedies and Stock Remedies Act (Act 36 of 1947). It is specifically used as a secondary surface-applied supplemental treatmentto protect timber against potential external attack and structural damage caused by wood-destroying insects such as wood borers and termites.

Can surface-applied treatment of CLT with currently approved wood preservatives be done?

The short answer is no, not in South Africa. The SANS 10005 and SANS 1288 standards specify wood preservative treatment requirements and do not include surface or envelope treatments as approved application methods.

Also, all the listed wood preservatives have only been registered and approved for application by primary industrial impregnation processes. It cannot be used for surface treatment of timber.

However, treating CLT with surface-applied supplemental wood preservatives in New Zealand is an alternative option when used under defined low-risk exposure conditions. An option mentioned is the surface treatment of CLT with a glycol-based borate wood preservative formulation that contains at least 20% boric acid equivalent (BAE), applied by brush or roller as two coats to all surfaces.

Has there been any word from the BRCS about the proposal?

Through the SA Wood Preservers Association (SAWPA), the industry has submitted a proposal to the NRCS Technical Advisor: National Building Regulations to enable compliance certification of suitable surface-applied treatments by registered Pest Control Operators (PCOs) with a list of requirements.

A PCO is registered in terms of Act 36 of 1947, Section 3(2) (c) and is qualified to identify wood-destroying pests and, for commercial purposes, apply either registered pesticides or manual solutions to eradicate or prevent attack by such pests and to issue a Certificate of Clearance to the client.

To answer your question, SAWPA is still waiting for feedback from the NRCS Technical Advisor.

Tell us more about the way forward for our mass timber industry.

The good news is there is a way forward. Wood preservatives in South Africa have been registered with the Registrar as a secondary surface-applied supplemental treatmentto protect timber against structural damage caused by wood-destroying insects such as wood borers and termites.

Two such wood preservatives from TimberLife are CTX 108 and GlyBor 300. Glybor 300, is used by Mass Timber Technologies and XLam. It is a glycol-based borate wood-preserving solution. Interestingly, it meets the criteria and requirements specified in the Draft New Zealand Standard. GlyBor 300 contains a minimum of 30% (m/v) boric acid equivalent (BAE), exceeding the minimum active ingredient content requirement by 50%.