The Structural Timber Association (STA) represents the UK’s structural timber sector and supply chain. Andrew Orriss manages its STA Assure Quality Assurance programme, which sits at the heart of the STA and seeks to drive up quality standards across the industry.

Here, Building Design sat down with Andrew to unearth some insight into specifying for structural timber. 

What are the main structural timber systems used for construction and how do they differ?

It is important to distinguish between the two main structural timber technologies – lightweight timber systems and mass timber systems.

Lightweight timber systems include timber frame and SIPS and are the preferred systems for low and medium-rise buildings. This can be seen by the popularity in Scandinavia when it comes to the number of timber-framed homes – and even as close as Scotland, with 85% of homes being timber-frame. These countries have fully embraced timber as a truly renewable construction material. Comparatively, in the UK timber accounts for around 23% of the market share, however I am pleased to say that this is beginning to grow steadily.

Offsite timber frame construction allows for a faster build and to a higher sustainability standard compared with traditional construction methods. There are two types, open and closed panel systems and to simplify, the difference lies in the factory value added to the closed panel timber frame.

Open panel systems are structurally engineered panels that form the inside load-bearing leaf of the external wall with studs, rails, sheathing on one face, and a breather membrane.

Whereas closed panel systems, as a minimum, are ready insulated. They may also include fitted windows and internal service zone battens for ease of installation and construction.

The other main lightweight timber system is Structural Insulated Panels (SIPs) and can be used for walls and roofs. This advanced method of construction uses composite panel techniques whereby an insulating foam core is sandwiched between two structural facings, normally oriented strand board (OSB).

Like timber-frame, SIPs are manufactured under factory settings to fit any building design, delivering a lightweight build system that is incredibly durable, versatile and energy-efficient, whilst also being quick to erect.

Mass timber systems include cross-laminated timber (CLT), and glue-laminated timber (Glulam), which are typically used in complex and larger buildings. Mass timber presents an excellent opportunity to develop projects that deliver significant sustainability benefits and are known to enhance both the living and working environment, as the volume of timber used in the manufacture of these systems is high. Purposely left visible, the timber creates a tactile and sensory environment, also minimising the need for additional internal finishes.

What to look for in a credible supplier of timber systems including timber procurement?

Our main goal at the STA is to promote the use of structural timber in construction, as timber presents our best opportunity for meeting the UK’s net zero commitments by 2050. The quality and standards of timber construction must be second to none and therefore, accreditation is crucial in providing investors and insurers with evidence that companies are held to high standards.

This is where the role of quality and compliance schemes, such as STA Assure, is crucial, as is independent acknowledgement from building warranty providers. STA Assure is our Quality Assurance Scheme that provides confidence in the use of structural timber and determines member competency and compliance. It is also recognised by the UK’s main warranty providers.

Plus it’s worth noting that for members to attain STA Assure accreditation, the installers that they contract with must have successfully completed the STA’s Timber Frame Competency Award Scheme (TFCAS), an invigilated test to ensure competency and knowledge.

A further consideration is to ensure that your timber system supplier is only using renewably sourced timber from sustainably managed forests, certified by the Forest Stewardship Council (FSC) or Programme for the Endorsement of Forest Certification (PEFC).

What are the sustainability benefits of specifying structural timber?

The increased use of structural timber presents a significant opportunity to transition towards a more sustainable and carbon-efficient method of construction.

Environmentally, the advantages of building with structural timber are vast. Firstly, it possesses the lowest embodied carbon of any building material, and as trees sequester and store carbon from the atmosphere, timber offers a carbon-negative impact throughout its lifetime.

It also provides specifiers with a truly renewable alternative to traditional construction materials. In properly managed and maintained farmed forests, typically for every tree cut, a further five are planted.

In terms of the energy consumption of a building in use too, it is easier to super insulate a timber frame/SIPs home, which will significantly reduce the energy required to heat the home – a positive for both the environment and the pocket of the home and building owners.

In addition to reducing carbon emissions significantly, the use of timber in construction also offers other advantages, particularly in offsite timber frame construction. Where pre-manufactured value (PMV) is under consideration, a timber frame building can represent between 30% and 55% of the building fabricated offsite.

What are the main considerations for specifiers when working with timber construction? Does timber construction have constraints on architecture?

By and large the considerations when specifying timber are no different to other forms of construction – only in high-rise complex buildings or where regulations dictate.

The importance of architects making early engagement with contractors and other partners cannot be over stated. To ensure the construction of an efficient timber building, material choice should be made at the early stage of design and timber technology suppliers will be able to advise on the best route to take. This material-first approach should be best practice across the industry. 

For specifiers, the benefits of partnering with our members is vast from having access to a wide range of technical documents and a technical helpline for specification guidance, to a solid bank of research and testing to support all design considerations and visions. 

What regulations need to be adhered to when working with a timber design?

Like all technology used in a building’s construction, structural timber will be subject to compliance with the different building regulations. For building regulations in England and Wales, structural timber systems are required to conform to Approved Document Part L conservation of energy, Part B fire safety and resistance to sound and Part O for overheating.

Whilst, the Scottish Building Standards, typically look at Section 1 Structure (EC – Mechanical resistance and stability), Section 2 Fire (EC – Safety in case of fire), Section 5 Noise (EC – Protection against noise) and Section 6 Energy (EC – Energy, economy and heat retention).

How do specifiers ensure best practice?

Best practice can be achieved by making early engagement with suppliers and to design with a material in mind. Specifiers should ensure that they select a construction method and material that meets their client’s commitments to sustainability, building performance requirements and programme.  

Partnering with an STA member will help you meet all the above criteria, as all members are audited by our STA Assure scheme. The robust assessment process ensures that design, production, site assembly processes and quality controls are in line with customers’ expectations for consistent, high levels of quality.

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