Reprinted with permission of Wood for Good, for more info visit: www.woodforgood.com
Engineered timber takes wood’s natural properties and boosts them to create elements that compete structurally with concrete or steel. Designing with their strengths and properties in mind will lead to a whole new generation of super-sustainable buildings.
What is engineered timber?
By combining veneers, strands or particles of wood, and playing to the strengths of the new material created, engineered timber makes the most of a natural and sustainable product.
“It’s about optimising the use of the material,” explains Professor Robert Hairstans, director of the Centre for Advanced Timber Technology (CATT) at the New Model Institute for Technology and Engineering (NMITE), seconded from Napier University. “So, for example, a glulam beam could have lower grade timber in the middle of the section and higher grade at the top and bottom where the beam is doing more work as a result of compression or tension stresses.”
Whereas sawn timber products are limited by the size of the tree they come from and are subject to variations and defects, engineered timber products are not. Their dimensions, properties and quality have improved uniformity.
The engineered wood product family is quite an extended one according to the Structural Timber Association, which puts open web joists, structurally insulated panels (SIPS) and even oriented strand board (OSB) and chipboards in the group. This article will concentrate on engineered timber that is used most often for large structural elements in the UK: glued laminated timber (glulam), cross laminated timber (CLT) and laminated veneer lumber (LVL) – and is challenging the carbon intensive materials which are still currently used in most cases despite the climate crisis.
Glulam is perhaps the most familiar of the structural engineered wood products in the UK, having been utilised for decades. First used in Europe in the early 1890s, it is manufactured by gluing layers of timber laminations together, parallel to each other, most often spruce or pine. More durable species such as larch or Douglas fir may also be used, depending on the application. If glulam is to be used outside, hardwoods such as oak or sweet chestnut would be a good choice – though clearly more expensive than the softwood.
Used for large structural elements such as beams, columns and portal frames, one of glulam’s strengths is that it can be manufactured in different shapes and configurations. Glulam beams can be made up to 50m long and 4.5m wide and are often curved, such as those used for a house in Blackdown Hills in Devon, which was featured on Channel 4’s Grand Designs. Buckland Timber used British grown larch to create a glulam rib framework which was overclad in timber and glass.
Cross-laminated timber (CLT)
Similar to glulam, cross-laminated timber (CLT) is manufactured by gluing layers of timber together. Unlike glulam, the layers are oriented perpendicular to one another – as the name suggests.
Built up symmetrically around a central layer, CLT panels consist of an odd number of layers, from three upwards and can be made up of differing thicknesses. They can be up to 22m long and 3.5m wide; generally, floor panels come in 2.4m widths and wall panels 2.95m in the UK.
Often made from spruce or pine, CLT originated in Europe in the 1970s and is widely used there, particularly in Germany and Austria. Since 2015, it has been recognised in design codes and standards in the US.
CLT panels can be used for walls, roofs, floors and beams. It comes into its own in medium-to-high-rise buildings. Although the UK boasts some of the world’s largest CLT buildings in terms of volume of material utilisation – the 10-storey residential Dalston Works in Hackney, designed by Waugh Thistleton for example – changes to fire regulations are currently limiting its use at such heights. Consequently, it has been used more in the commercial, leisure and education sectors than in residential in the UK.
The Black & White Building in Shoreditch, also designed by Waugh Thistleton Architects, has a CLT core and slabs, combined with Baubuche columns and beams, a type of hardwood laminated veneer lumber (LVL). At six storeys, the building is London’s tallest engineered timber office.
Laminated veneer lumber (LVL)
Laminated veneer lumber (LVL) is manufactured by peeling thin veneers from softwood logs and then bonding them together, normally with their axes running parallel to each other. Sometimes 20% of perpendicular veneers are added so that it does not warp and twist. Due to the way it is made, LVL can be created from smaller logs and trees, making the most of forest resources.
It was first used in the early 1940s to make aircraft parts, such as propellers, in World War II and then used more widely as a construction product from the 1970s. The LVL market in Europe has historically been dominated by Finland and Sweden, with imports to the UK also coming from Germany and North America. It can be used in a wide range of applications including beams, columns, trusses, structural decking and I-joist flanges.
Sister products to LVL are laminated strand lumber (LSL) and parallel strand lumber (PSL) often referred to as structural composite lumber. However, these have become less widely available in the UK, due to the preference for LVL, according to the Structural Timber Association.
LVL can be a good alternative to CLT for lower-storey housing because slimmer, lighter panels can be used. For instance, Kiss House, which delivers pre-manufactured homes predominantly for self-build, chose LVL for its homes. The lighter elements can be lifted by forklift rather than cranes and is a more efficient use of timber.
What about hybrid structures?
It’s difficult to define exactly what a hybrid structure is. “Most buildings are hybrids,” says Hairstans. “They may have concrete foundations, high levels of timber products, with steel connections. It’s really about how you use materials most appropriately.”
Combinations of engineered timber are commonplace. “You often see hybrid systems where the glulam is used for longer load span conditions and the CLT for slabs,” says Hairstans. “It’s about putting the right products in the right place, relative to the conditions.”
In the Delamere Forest Centre in Cheshire, designed by Design Group Chester, glulam downstand beams span between internal columns and support the CLT roof panels. Longer beams in the centre’s café span between the perimeter walls and taper upwards to give a sense of height.
Hybrid systems which combine timber with steel or concrete may be used for taller buildings or more complex designs, with offsite-manufactured engineered timber elements used to help reduce time spent on site. For instance, Hawkins/Brown’s 10-storey residential building, The Cube, combined CLT with a steel frame and a concrete core.
Hybrid structures are often a good solution when reimagining existing buildings. Adding an engineered timber frame to an existing concrete one can hugely boost floor space without requiring additional foundations. One such case was the Gramophone Works in Kensal Rise, which grew from two to six storeys, courtesy of CLT, glulam and a design by Studio RHE.
Why use engineered timber?
Perhaps the most obvious reason for the uptick in the use of engineered timber is its environmental credentials. Compared to concrete or steel frames, engineered timber frames will significantly reduce a building’s carbon footprint. Designing engineered timber buildings so that they can be easily disassembled, and the timber reused, makes the carbon footprint as low as possible.
“There’s definitely an upward trajectory in terms of the use of timber in the round,” says Hairstans. “There’s going to be a move to more circularity and the whole life performance of the asset. It’s not about just the lowest cost which often leads to the lowest quality forms of delivery.”
A shift to more home-grown products would make engineered timber an even more sustainable choice. The Construction Scotland Innovation Centre’s (CSIC’s) led Transforming Timber programme in partnership with Ecosystems Technologies and Edinburgh Napier University is showcasing how to do this.
Unlike sawn timber, engineered timber can be treated for moisture resistance, helping to manage the risk of environmental factors. In moist and potentially corrosive environments, such as swimming pools, glulam is often a more efficient choice than steel.
From an aesthetic and wellbeing perspective, leaving timber elements exposed delivers benefits due to biophilia. Work and educational environments that incorporate wood have been shown to lower heart rates and reduce stress.
One limitation of many engineered timber products is that they cannot be used externally since most are made of softwood. Exceptions to that rule would be glulam made from a durable species, such as larch.
Recent changes to fire regulations in the UK have constrained the use of engineered timber in taller buildings. The ban on the use of combustible materials in the external walls of residential buildings over 18m high means that timber systems, such as CLT, will need separation between the structure and outer face of a building over six storeys high.
Timber is the future
As more architects and designers become au fait with the capabilities and strengths of engineered timber products, Hairstans sees a shift in approach for tomorrow’s buildings:
“We have a good pallet of parts now, a vast array of products and systems that can be manufactured using timber,” he says. “What we hope to see is a change in the design thinking approach so that we think about the materials we want to use upfront, rather than designing something and thinking about how timber could be used later.”
Hairstans is certainly putting his money where his mouth is: he recently built a small extension to his house using CLT, glulam and nail-laminated timber, all made from home-grown timber.