Cross Laminated Timber: What are the benefits of CLT construction?

Proctor and Shaw Architects report on the recently published Zinc House in a 4 part series charting the realisation of this new build dwelling, starting with the benefits of cross laminated timber panel systems in new build house design.

The decision was taken early in the design process to accommodate a pre-fabricated timber structure for the Zinc House. Whilst carrying a higher up-front cost when measured against steel frame or masonry alternatives (around 20% in this case), overall benefits were considered holistically so that design, cost and sustainability ambitions could be all met together.  Rich and robust self-finishing interior surfaces desired by the client, simple and quick to erect construction techniques and excellent environmental performance made Cross Laminated Timber, or CLT,  a logical choice for the structure of this new build house.

The structural design for the timber element of the design was developed with specialist engineers at CLT fabricator KLH, and coordinated with separate engineering design for the concrete foundations.  The CLT engineering fees and procurement costs  were helpfully separated to allow flexibility later in the tendering process (as it turned out the same provider were selected for both).  The benefits of the panelised timber system were explored in the early design, where impressive cantilevers for the entrance porch and glazed flying corner could be effortlessly achieved by simply employing the structural depth of the wall panels themselves, perforated with windows as required, to act as deep beams where otherwise more complicated steel beams, columns and footings would have been required.  Simple cardboard modelling provided a surprisingly simple and effective tool for testing what CLT was capable of as the planar characteristics of both seem to translate well in scale.   These models and simple sketches were translated into 3D digital files that could be analysed thoroughly for structural performance.

Key CLT design details for this project developed prior to fabrication included:

  • Roof to wall panel junctions that were carefully offset by 25mm to allow plasterboard wall linings to be accommodated on site in order to achieve a neat flush internal eaves finish.
  • A stepped concrete slab edge detail was required to provide fixing grounds for the wall panel support brackets whilst maintaining vertical clearance from external finish levels.
  • CNC cut recessed were formed in the factory to allow flush plasterboard to timber internal wall finishes.
  • Flush finished recessed linear LED light fittings and cableways were recessed in the surface of the CLT in the factory to specified dimensions and locations.
  • A large span corner opening to accommodate folding glass doors at ground floor was achieved through utilising the upper storey wall panels as storey high cantilever beams.
  • Projecting window boxes were formed on the orchard side of the house to allow framed views of the countryside.

Once the design was complete, detailed fabrication drawings were approved (and the product paid for!) 12 weeks before the structure was required on site. The prohibitive costs of delay and storage of completed elements in the factory, did require that the preparatory works on site necessary to allow the seamless erection of the structure on arrival, including piling, DPM installation and concrete ground slab works, had to be carefully programmed and completed ‘just in time’.

Construction: 

CLT panels were cut in the KLH factory in Austria and brought to site on a single conventional flat bed lorry.

A lifting plan was agreed with the contractor following a pre-construction site visit to assess constraints on the ground, which included poor ground conditions due to adverse weather, tight vehicular access into the site and the over-sailing branches of a nearby pear tree. Once addressed, a mobile crane erected the factory cut panels in less than 3 days, secured in place  by a team of expert installers.

Only one steel angle was required to help support a large span internal opening. All other structuring, including the open vault soffits and bi-folding door cantilever overhangs, are achieved entirely in the load-bearing panelised cross laminated timber system.

The methodology of erection was extremely simple and well considered. Aside from the cross laminated timber panels themselves, there were only 3 types of screw (of varying lengths) and 2 types of metal brackets (timber to concrete, timber to timber) used throughout. An erection ‘manual’ was prepared at the technical design stage that uniquely numbered each panel (119 in total, the largest being 12m long by 2.2m wide, the smallest a single stair tread) and sequenced the assembly of each component.

Once the panels had been craned and fixed in place, the joints were all sealed with a waterproof tape to ensure an air tight structural envelope (the window openings of course were sealed at a later date.) The CLT installer completed their works and tarpaulins were fixed in place ready to allow the roofing contractors to begin their work.

The success of the CLT structure in this project was down largely to a commitment to the system early in the design process. Whilst requiring a leap of faith from the client, this paved the way for the benefits of the system to be well exploited in the realised design, from its structural behaviour to CNC factory cutting possibilities.  Constraints around site access must be considered at that earliest stage to ensure its viability, and cost and time benefits of quick erection probably need to be somewhat of a requirement to make the overall economics stack up. With those in place, the added benefits of performance can be exploited; strength and robustness, an authentic and rich interior expression, and very good acoustic and air tightness qualities all combining to create a unique sense of home, built to last.