AT – Ask the Expert with Martin Milner, STA Technical Consultant


1. What are the main benefits of using structural timber on construction projects?

As we see sustainable construction climb higher up the priority list, timber has inevitably grown in popularity; widely recognised in terms of its versatility and environmental benefits. In Scotland, over 85% of homes are being built in timber-frame, with market share in the UK, currently at 23%, steadily rising.

Timber represents the UK’s greatest opportunity in achieving net zero by 2050 due to its carbon saving benefits. Whilst the speed of construction can significantly reduce build programmes using timber frames, it also addresses some of the pressures on skill shortages in the industry where with timber frame, time on-site is greatly alleviated. A pressing issue when there are more than 45,000 builder vacancies in the UK alone, according to Build UK. 

2. Are there any disadvantages?

On site storage requires additional care and attention. Once the engineered timber has been delivered to site, it is important to ensure it is stored correctly and away from any moisture ingress risk.  We would always advise a thorough programme of protection, moisture inspections, ventilation and allocated drying out periods (if necessary) to avoid any issues. 

The STA has produced a guide to moisture management and durability, which outlines the steps required to develop an effective moisture management strategy.

3. What are the structural properties of timber?

Structural timber can be used for all parts of a building structure, as well as external structures. As with all materials it comes down to the design and how it is handled and used.  Applied correctly, using the correct grade and moisture content, timber provides structural properties in strength and rigidity that last indefinitely.

4. What are the main structural timber systems available? (SIPS, CLT, timber frame etc)

There are two types of timber frame systems – Open panel systems are panels that are structurally engineered to serve as the load-bearing inner leaf of the external wall. Closed panel systems are pre-insulated and can be supplied with fitted windows and internal service zone battens for easy installation and construction. These systems are typically used in residential applications for both single family homes and multiple occupancy developments.

Structural Insulated Panels (SIPs) – An advanced method of construction using composite panel techniques whereby an insulating foam core is sandwiched between two structural facings, normally oriented strand board (OSB). Similarly used in residential applications where energy efficiency is high on the agenda.

Cross Laminated Timber (CLT) consists of perpendicular alternating laminations of softwood, creating a solid panel.  CLT is a build method used for walls, floors and roofs, with quick and easy fitting on site. More regularly used in large complex commercial buildings where the aesthetics are seen to improve well-being.

5.  Are timber structures suitable for all types of building?

The use of timber should always be appropriately detailed and included where it fits best for the application design. We would always advocate ‘right material, right job’ approach.

6. How high is it possible to build using structural timber?

The height very much depends on the system and application.  The market for lightweight timber systems is up to four storeys, beyond which a complex fire assessment is required.  The possibilities for increasing the use of CLT and Glulam (glued laminated timber) can be demonstrated by the Mjøstårnet in Norway, which has been verified as the world’s tallest timber building at 84.4 metres high using CLT. When looking at tall buildings the design and engineering teams should consider a right material – right application approach. Early engagement of your timber system provider will always ensure the best possible outcome.

7. What can be done to ensure the fire performance/integrity of timber structures?

The key is that all buildings must be designed, detailed, manufactured, assembled and erected with care and attention, complying with the functional protocols of the Building Regulations for fire safety requirements, as a minimum standard.

The STA invests in a continuous programme of testing to support the construction industry, carrying out fire performance research to provide test-based evidence of behaviour. All of which is available from the STA.

8. What is the recommended/permitted moisture content for structural timber?

This is a complex circumstance, but in simple terms the moisture level of timber should not go above 20% in common softwoods. Typically, housing structures would, when correctly designed, not rise above 18% and more commonly between 14% and 10% depending on the location.

9. How do timber structures compare to steel and concrete ones in terms of cost?

A study carried out by Rider Levett Bicknall for the housing sector concluded price parity with load-bearing masonry when all construction factors were taken into the costing model. More complex buildings, such as those designed and engineered in mass timber systems, may or may not be subject to the same cost parity due to design features.

10. What is the life-expectancy of timber structures?

Around the world there are examples of timber buildings that are hundreds of years old. Providing structural timber is correctly designed for the building, there is no reason why a modern timber structure could not last for hundreds of years also.

11. Can timber structures be repurposed, reused and/or recycled at the end of their life?

The current practice when demolishing a timber-based structure is for the timber to be repurposed for biomass for energy production. The timber industry recognises that the volume of material being used in this way could serve the climate change agenda better if it were repurposed in other ways, such as for the wood-board industry or wood furniture industry, for example. Furthermore, steps are being taken in the design phase of timber buildings where the structural elements can be repurposed in other buildings.

12. Is structural timber suitable for retrofit projects?

There are several systems on the market that essentially wrap a building in a lightweight timber system – they are fully designed and engineered solutions, which contribute to improving energy conservation.  As an incredibly versatile material, timber systems have the potential to be used on more dramatic refurb projects, where it works well with existing facades.

The Gramophone Works in London is an exemplar refurbishment project using engineered structural timber, converting a large post-industrial site, presenting numerous logistical and technical challenges, into a striking new landmark commercial development.

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