Trade Secrets: Truth, materials and CLT

Updated: Jun 4, 2019

With 6a architect's award-winning design making the front pages this month, the architectural mantra "truth to materials" is being given a good dusting off. Rather than conjuring up visions of cutting edge design, the phrase is more likely to be associated with memories of smoke-filled lecture halls and the soporific thunk of a slide projector. But innovative applications of traditional materials as well as the emergence of relatively new materials are providing designers with an entire spectrum of unexplored possibilities. The truth is out there and it is suddenly hip again.

© Feneley Studio

CLT is one of these new kids on the block - a bit like timber version 2.0, CLT comprises planks of wood glued together in layers to form large, structural panels that can be shaped as needed and then bolted together very quickly. The result is a panellised structural system that further reduces the need for wet trades and moves a step closer to the nirvana of zero defect, industrial, off-site manufacture of buildings. Whilst CLT is gaining traction as a quick and competitive means of constructing a building's structural frame, with one or two notable exceptions it is less widely known for its use as a finished material - until now.

Enter boutique practice Feneley Studio and their design for a new-build house on Thornfield Road in London's Shepherd's Bush.

Previous Harris Calnan projects such as the Brick House, the House at Highgate Cemetery and Latimer Road could all be described as case studies in truth to specific materials such as brick, glass or concrete. But together with CLT fabrication specialists Constructional Timber and structural engineers Engenuiti, Feneley Studio have produced a design that broadens this palette further and brings the full potential of CLT, both as a structural material and fine finish, into the domain of the bespoke, single-family home.

So what is the truth behind this wünder-material? To find out, Harris Calnan talked to Simon Feneley (SF) of Feneley Studio , Andrew King (AK) from Constructional Timber and Nicholas Nearchou (NN) from consulting engineers Engenuiti:

1. When did you first think of CLT as a possible solution for the project at Thornfield Road: was it something that you knew you were going to use at the outset, or was it simply a case of being an elegant solution to the needs of the design?

SF: We had worked with CLT previously, but this was a first in a domestic situation. The design itself is a response to the restricted boundaries and airspace we were able to build within. The resulting complex geometries called for an efficient structural system and CLT seemed the obvious choice, to achieve accurate setting out and keep structural lines slender and efficient. Wall panels act as tapered beams around the staircase for example, and the CLT allowed for minimal wall and roof zones. The result allows soffits and walls to be oiled as a final finish, something the client was interested in also. The exposed CLT looks wonderful against the plaster and dark brickwork internally.

© Feneley Studio 2. Industry forecast reports suggest that timber frame systems such as CLT are becoming more popular and that market share is growing. Already in Scotland, 70% of new build houses are constructed using some form of timber framing. Is this a trend that you expect to see in London?

AK: With the push for more houses to be built and land being limited in London, off-site / modular construction is at a forefront which is something that the timber industry has embraced. Modular construction, such as CLT, both speeds up the construction time but is also a lighter method of building compared to traditional building materials.

3. In the past, government regulations (particularly those with respect to fire resistance) as much as the available technology limited the scope of timber as a structural material - what aspects of CLT have moved the goalposts and have government regulations caught up yet?

NN: A number of key stakeholders, including manufacturers, architects, engineers and trade organisations have been working tirelessly to educate both the government and the market on CLT and have made several important recent contributions:

  • The STA has undertaken fire-during-construction testing and produced standard architectural details for CLT to improve knowledge of appropriate detailing;

  • TRADA has produced the NSTS (National Structural Timber Specification) document standardise the specification and procurement of structural timber in the UK;

  • Eurocodes are being updated to codify the structural design of CLT;

  • BIM has enabled further automation of design to manufacture integrating different structural materials and architectural finishes.

In parallel with all of the above, the GLA is currently re-writing its planning policy for housing to reduce construction traffic into London, this will require a solution which reduces the amount of deliveries to site. The use of lighter weight, e.g. cross-laminated timber panelised off-site solutions, is therefore key to this, as the amount of deliveries is greatly reduced when compared to a volumetric or traditional building solution.

4. Once you knew that you were going to be using CLT, did you have to make any changes in your approach to designing the project and did this result in any kind of learning curve? If so, what were the major issues that you had to overcome?

SF: We worked closely with our engineers, specialists in engineered timber, to study the complexities of the house. The particular site location and vehicular access to the site restricted panel sizes, and the presence of party wall chimney breasts called for greater tolerances than the material itself could offer. The sequence of panel erection also informed fixing details and how certain panels would be visible to exposed wall panels. In some locations spruce cover-panels have been incorporated to cover end-grain, for example where secondary steelwork is necessary to support floor edges.

5. In your experience of working with designers to develop CLT solutions, what are the most common misconceptions that designers have when they approach CLT for the first time?

AK: With CLT coming to site as a finished product the design team must be aware that all services have to be co-ordinated to enable a design freeze before the CLT panels are put into production. The most significant knock-on effect can be with respect to timing as it can take up to 8 weeks before any materials arrive on site.

© Feneley Studio

6. Has the experience of working with CLT on-site changed your view of its limitations or of its potential at all and has that experience changed your views of how you might use it again?

SF: Having witnesses the installation at first hand, a clearer understanding of its erection and fixings has highlighted areas that require great attention to detail. The erection time is incredibly quick, and protection of the panels essential to avoid staining or damage whilst following activities catch up, for example the bricks production was delayed. It’s a wonderful system that we’re keen to use again in the future.

© Feneley Studio

If you would like to find out more about CLT, or if you are keen to develop a project of your own, please make use of the additional resources listed below or contact anyone of our contributors by clicking on the following links:

Feneley Studio

Constructional Timber


Additional CLT resources:

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