Eleanor Brough and Toby Carr
In four of our primary schools we have used different forms of off site timber manufacture to bring benefits to each project.
Sandal Magna Primary School in Wakefield was the first SWA project to use cross laminated timber (CLT), and our work here has informed subsequent work on off-site constructed CLT homes and proposals for an adaptable school. CLT was chosen following a cost benefit analysis with traditional timber frame construction, proving advantageous because of programme savings and the omission of internal linings.
At Sandal Magna, the CLT panel wall and roof panels span between brick cross walls, combining this timber MMC with a more traditional form of brick construction which resonates with the surrounding Victorian terraces. Using this ‘massive timber’ we created large open span spaces with the fair faced spruce finish left visible and services surface mounted, to support a pedagogical approach which enables school children to see how their building is made.
In the hall, the benefits of CLT is most clearly evident. The roof is formed in a single longditudinal 15m span with two 1.5m CLT up stands doubling as both a ridge beam and a dramatic roof light, with the opening braced with CLT fins. The simplicity of the structure and fair faced timber finish create a calm and adaptable space at the heart of the school.
Our project at Mellor Primary School is conceived as a tree house in the woods and a timber structure was a key component of the brief. Off-site fabricated Larch glulam frames form the striking exposed structure both internally and externally. The glulam portal frames were specified to be external grade larch (rather than spruce typically specified for internal use), allowing the same timber to be used inside and out. They were constructed in sections by specialist manufacturer and assembled by the main contractor, ensuring a high quality factory finish.
The project combines a visible timber superstructure with off-site fabricated insulated OSB wall and roof cassettes. These span between the glulam frames, efficiently bracing the timber portal frame. The cassettes are insulated with rock wool and arrived with breather membranes in place. They were installed quickly, minimising work at height to the perimeter of the building and providing a weather protected internal space which could be used as a working area for both the main contractor and for community builders helping to construct Mellor’s Habitat Wall.
The project was Highly Commended at the Wood Awards and was Judge’s Choice and Best Education Project at the 2016 Structural Timber Awards:
Roseacres Primary School in Essex was constructed using off-site fabricated timber Structural Insulated Panels (SIPs). The project had a challenging fast track programme but an easily accessible edge of village site with an existing unoccupied school building which was refurbished in parallel to the extension works. Using an off-site fabricated panel proved invaluable in ensuring the building was delivered on time for the school to open for the new term. Most recently, our client selected a SIP system for the construction of a new library and hall extension on a tight urban site in Camden within an occupied school site. Access to the site for the large scale crane operations and delivery lorries has been tricky, often requiring parking suspensions and highways approval and access restrictions due to craning over existing buildings.
In this case, perhaps a more traditional glulam and timber frame solution where material could be delivered to site in smaller pieces could have had benefits. Both these projects are extensions to existing buildings with some tricky junctions and connections which are not always suited to a panel-based system which may really come into their own on standalone buildings with repeating elements. Although speedy to put up, the SIP system still relies on a seemingly large amount of steelwork making us question if the next step could be to replace this with timber and achieve a more sustainable solution all round. The benefits of off-site timber construction are clear and our experience shows that one solution won’t work for all projects, but there are an increasing number of options. So finding the right approach is key.