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Last edited 25 Jan 2019
Cross-laminated timber CLT
Timber as a structural element of building is undergoing a renaissance as an effective choice by designers that satisfies the criteria of energy efficient and sustainable buildings. It is renewable and has possibly the lowest carbon content of any commercially available building material. It can be visually pleasing and has the additional qualities of good thermal and acoustic qualities.
Cross laminated engineered timber systems and glulam challenge steel and concrete in relation to spans and strength utilising spruce, pine or larch and stand up to corrosive environments such as swimming pools in a way that cannot be matched. In some instances it has become the first choice for residential projects, schools, hotels, restaurants and supermarkets. Adaptability, weight and speed of erection also give timber an edge over its competitors. The ability to absorb changes and ease of fixing can also give timber an edge over steel and concrete.
PEFC Project Certification offers designers a mechanism for gaining independent verification of the use of certified timber on any project. A residential seven storey building in West London obtained PEFC Project Certification on a structure comprising over 1000 sq m of cross laminated timber.
Cross-laminated timber is manufactured in panels that have an odd number of softwood plank layers laid on top of each other at right angles and glued together under pressure. The panels come in widths of up to 3m, lengths of up to 16m and are typically 50-300mm thick. Walls, floors and roofs can be made out of pre-fabricated panels, reducing construction time and delivering whole-life cost savings (1).
Trees absorb carbon dioxide during their growth and store it until they decay or are burned. This makes timber a highly sustainable material. Furthermore, producing timber building components consumes only 50% of the energy required to produce concrete and 1% of that needed to produce steel (ref 2, p.54).
Provided that timber comes from a certified (preferably local) source and the glue is non-toxic, cross-laminated timber can be a highly sustainable material. Buildings can potentially store tens of tonnes of locked-in carbon inside their structure, reducing the carbon footprint of the whole project.
Unlike masonry, which limits the building’s height and leads to heavy, material-intensive construction, 12-storey buildings are possible with cross-laminated timber, using 135 mm internal wall, 125 mm external wall and 125 mm thick floor panels (3). Furthermore, buildings with increased timber content are generally lighter, which alleviates pressure on foundations and means that savings can be made by reducing their size. In fact, to avoid over-specification of the panels, cross-laminated timber is best applied to large-scale medium- and high-rise projects (ref 2, p.86).
Cross-laminated timber panels are relatively small components that at the end of a building's life can be re-used. This can be an important step towards greater environmental responsibility and a more flexible building stock.
Cross-laminated timber panel building systems allow quick erection on site. This is advantageous not only in rural locations, where workforces can be limited, but in urban areas as well, where it is important to reduce noise and disruption.
However, construction using cross-laminated timber involves certain risks, such as; the difficulty of absorbing late design changes, the necessity to work to tight tolerances and the limited number of suppliers (ref 2, p.14).
 Performance in a fire
Cross-laminated timber outperforms joists and studs by relying on the fire-retarding charring of the panels. The three-layer lamination can deliver a fire rating of F-30, while a five-layer lamination gives a rating of F-60 (ref 4, p.12).
 Structural properties
Preventing the possibility of a progressive collapse can be overcome in cross-laminated timber buildings as the panels can span in two directions, and so can be designed to act as cantilevers when support is removed (ref 4, p.77).
 Thermal mass and acoustic properties
Cross-laminated timber has a significantly higher density than timber frame structures (500 kg/m3) (ref 2, p.14), which not only provides greater thermal mass, but offers acoustic advantages as well. Cross-laminated timber buildings have been shown to exceed statutory requirements (ref 4, p.34).
 Suitability for low energy construction
Cross-laminated timber has relatively good thermal properties (λ = 0.13 W/mK) (5, p.8) and can help in minimising thermal bridges and acting as thermally resistant layers. However, unlike a conventional timber frame, wall build-ups using cross-laminated timber may lead to an increase in the overall thickness of the wall, while substantial amounts of external insulation are likely to necessitate an additional supporting framework.
With a large proportion of manufacturing carried out off-site, overall quality control and precision can be significantly improved. This makes thermal-bridge free and air-tight construction easier to achieve.
Cross-laminated timber panels are inherently air-tight and do not require additional measures (other than the correct detailing of the junctions (5, p.8)). Furthermore, it is relatively easy to cut openings without compromising structural properties. This can be useful for example, for the integration of air handling ducts.
Although the use of cross-laminated timber in the UK is growing, the lack of British or European standards has reduced uptake. A rise in interest is expected once standards are published in the near future (ref 5, p.2).
The biggest producers and exporters of cross-laminated timber are Austria, Switzerland and Germany, where local small-holdings supply timber such as spruce, larch and pine with strength gradings of C16 to C24 (ref 5, p.5). However, importing cross-laminated timber can be expensive and it is believed that UK manufacture is required in order to reduce prices (6).
 Potential for local manufacture
The type and quality of the source timber used in the production of cross-laminated timber is not dissimilar to that available in UK forests. There is already some activity (7) aimed at establishing local manufacture to use up available low-grade timber. By re-engineering the natural product into a homogenous material, improved performance can be achieved, while also optimising the use of resources and minimising waste (ref 2, p.73).
To establish the feasibility of local manufacture, structural testing of cross-laminated timber from local raw materials is currently being carried out by the Wood Products Innovation Gateway at the Edinburgh Napier University (8).
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 External references
- (1) Cross-Laminated Timber: Introduction for Specifiers’, [TRADA Wood Information Sheet, WIS 2/3-61], (TRADA Technology, 2011)
- (2) Robert Hairstans, Off-site and Modern Methods of Timber Construction: a Sustainable Approach, (TRADA Technology, UK, 2010)
- (3) ‘Worked Example - 12-storey Building of Cross-laminated Timber (Eurocode 5)’, (TRADA Technology, 2009)
- (4) Thompson, H. and Waugh, A., Weiss, K., and Wells, M. (eds.), A Process Revealed / Auf dem Holzweg, (Murray & Sorrell FUEL / Thames & Hudson, Belgium, 2009)
- (5) ‘Cross-Laminated Timber: Introduction for Specifiers’, [TRADA Wood Information Sheet, WIS 2/3-61], (TRADA Technology, 2011)
- (6) Designed for Brettstapel - Scottish Housing Expo’, (Brettstapel, 2010), <http://www.brettstapel.org/Brettstapel/Home.html>, [Accessed on: 20 March 2012]
- (7) Binder-Jones - Press Release (Binder-Jones, 2012)
- (8) Edinburgh' Napier University: Wood Products Innovation Gateway (Edinburgh Napier University, 2012), <http://www.napier.ac.uk/ >, [Accessed on: 15 February 2012]
- PEFC: Sustainable Timber: A Guide to Procurement for the Public Sector.
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