26 June 2020
An in-depth look at TRADA's latest off-site timber construction publication
Off-site construction is the factory production and pre-assembly of components, elements or modules before installation into their final location.
Adopting an off-site approach effectively reduces the level of on-site labour intensity and delivery risk by means of pre-manufacture. It is based on the premise of delivering high levels of construction quality efficiently by applying lean practices and design thinking.
Timber and the industrialisation of timber technologies aligns well with this ethos. Timber is the ideal construction material because it is strong in both tension and compression, has a high strength-to-weight ratio, and can be relatively easily worked.
Timber also responds to the need for a more sustainable built environment acting as a prime carbon sink (carbon fixed from atmospheric carbon dioxide by photosynthesis), while being capable of forming high-performing building envelopes through a fabric-first approach – high levels of insulation and airtightness with reduced cold bridging. Timber is a truly sustainable building material and, with good silvicultural practices, trees can also improve land quality and soil fertility. Timber construction therefore conforms to the paradigm of the circular economy by means of using and embracing a design for manufacture and assembly (DfMA) approach when disassembly (DfMA+D) is also considered.
The history of timber in construction
Timber as a construction material, as explained in Chapter 2 of the new edition, has evolved over the centuries and has the qualities to be a major constituent of the clean technology solutions required to sustainably deliver the built environment. Originally, however, the human race employed timber as a building material because it was available in most habitable regions of the world.
The climate and conditions of the region had a bearing on the nature and species of tree growing, which subsequently influenced the end form of construction. Mechanisation, the invention of new timber products and connection methods, as well as approaches to combining timber with other materials, have seen timber being used in an unimaginable array of forms from when it was first conceived as a building material.
More modern market specifics, such as procurement methods and local building regulations, have influenced the end building form and performance requirements, resulting in the need for on-going research and innovation of products and systems to ensure conformity and future-proofing. The re-engineering of timber, particularly the use of adhesive technology, in combination with quality-controlled factory environments, has redefined the boundaries for using timber in buildings. Timber engineered products can now span considerable distances, and mass timber products – such as cross-laminated timber (CLT) – are conquering new heights.
Urbanisation and colonial expansion required efficiency and quality of product, resulting in the first major exploitation of off-site techniques, although there is also evidence of off- site concepts being used by medieval carpenters for quality assurance (QA) purposes. QA, productivity and skills were therefore some of the original drivers in the uptake of off-site methods. However, regulations, automation and digitisation, with the onset of the fourth industrial revolution – digital innovation driven by the Internet of Things (IoT) and Internet of Systems (IoS) – are also influencing the uptake and form of off-site and more industrialised approaches. This is being further intersected with a need to transform the culture of construction towards a more modern and inclusive sector capable of diversification, cooperation and transparency. The convergence of these drivers presents a huge opportunity for off-site and industrialised timber construction.
The revised second edition explains these concepts and provides justification as to why timber is exceptionally well placed to be the material of choice for modern methods of construction (MMC) or pre-manufactured approaches. Given that both of these ideals are about better end products delivered via an efficient process, there is a need to underpin the approach with an alternative business model geared towards whole life value. Customer satisfaction must therefore be considered with respect to not only cost but also enhanced environmental performance, quality of product and programme certainty.
How off-site methods can be used in timber construction
Client and consumer understanding of the wider benefits of off-site approaches are of paramount importance given the need to think of the wider value proposition as opposed to lowest cost, which often results in lowest quality or adversarial contractual terms of engagement leading to higher than predicted costs. Chapter 4 of the new edition explains the rationale for early engagement and partnering at the design stage. In addition, key terminologies are explained throughout the text to present a standardised language for the purposes of a consistent understanding of off-site theory.
Chapter 6 demonstrates the categories and sub-categories of off-site products (panelised, modular/volumetric, hybrid and sub- assemblies/components) using written descriptions, photographs and schematic images. Within the off-site sub-categories, there are details on the varying levels of enhancement relative to manufacturing capability and project context (for example, client requirements, logistical constraints, supply chain arrangements). The new edition also provides the history of these varying systems supplemented by numerous case study examples. There are explanations of technical performance attributes, including advantages and disadvantages.
This should provide readers with an informed perspective of how these systems can be used for a given project. To provide confidence in the information, an internationally wide perspective has been taken, including contributions from other world-renowned experts in the field. The UK has also been included for enhanced contextualisation, acting as a good barometer for off-site and industrialised timber because it is a major importer, manufacturer and pioneer. It has showcased CLT through projects such as Dalston Works and employs timber frame for a large proportion of its new build housing construction, particularly in Scotland where up to 85% of new houses are formed from this off-site technique.
The UK construction market reflects the global trend for more environmentally efficient buildings, with an increasing emphasis on whole life cycle cost. International policies and legislation and devolved regulations, combined with enhanced client and end-user awareness, are necessitating improved fabric performance, as well as the qualified environmental credentials of the component parts. Chapter 4 presents these concepts and includes an explanation of circular economy and designing for circularity via DfMA+D. The use of timber as a sustainable form of construction is assessed and the concept of whole life cycle analysis is presented, including information on ensuring timber is sourced from sustainably managed forests.
Performance and challenges
An inherent part of design is understanding how a material performs. Chapter 4 of the new edition explains the credentials of timber as a structural material, covering its structural, thermal and acoustic performance, both in isolation but also when combined with other materials as an off-site system, sub-assembly or composite component. Chapter 4 also explores the structural design considerations of timber buildings with respect to the European standard codes of practice and regulations for conformity and technical approval. The technical challenges of the industry are also considered (differential settlement, structural stability and disproportionate collapse) and, in particular, issues pertaining to fire performance are explained both theoretically and through case study analysis. This content makes a strong case for how off-site timber construction can create safe and serviceable buildings.
The factory-based approach of off-site construction requires the implementation of industrialised approaches, both at the production and construction phases. Lean theory is critical to successful exploitation and must be applied consistently throughout the process. Chapter 5 provides the tools for applying this theory in both the factory and assembly phases. In addition, the different levels of production automation are categorised, with a view to informing the factory production set-up. Critical to this change of approach is up-skilling throughout, from factory floor to the leadership at the top given the non-traditional business model of off-site construction.
Chapter 5 serves to facilitate the transfer of knowledge required for the decision-making process towards implementation – often incremental as off-site factories scale up to meet demand while evolving and standardising their approach relative to the market they serve. The publication references a multitude of sources, including regulatory documents, research outputs and case studies (Chapter 7 is dedicated to exemplary and international case study content, such as MultiPly) to help readers understand how these principles can be used.
The objective of the revised publication is to demonstrate ways of delivering a more cohesive and socially, economically and environmentally sustainable industry through the application of improved business models using timber. These improved business models – which include partnering arrangements, robust product procurement strategies, material optimisation and system efficiencies – enable the industry to continually evolve and innovate through applied research and development. When these concepts are embraced, timber can readily be seen as the ideal material for a more sustainable and efficiently delivered built environment, deploying a manufacturing-based off-site approach.
About the author
Professor Robert Hairstans, Edinburgh Napier University
This is an extract from the Timber 2020 Industry Yearbook. Download other Yearbook articles here
The second edition of Off-site and industrialised timber construction: Delivering quality and efficiency is available to buy in the TRADA Bookshop. The removal of VAT on ebooks has also allowed us to pass on a 20% saving on all ebook editions. Visit the TRADA Bookshop
31 May 2021