24 October 2019
Member Case Study: The Crannog
If we are happy to design a pier, which is exposed to the wildest of elements, in greenheart – then why not a building?
The ocean is a forceful and all-consuming mass, which patiently wears down our coasts, eating through rock. Yet along our coastlines we have built many piers – a quintessentially British structure, long-visited for decades and alluring across generations – which are typically constructed using timber. When one imagines a pier, it is a long structure reaching out over the ocean, held up above the water by a lattice of beams and columns spaced equally below. This structure, which needs to withstand wave after wave, is generally made of hardwood.
Of course, the love affair between timber and piers is more than the structure. The iconic Brighton Palace Pier, first opened in 1899, contains 85 miles’ worth of planking, while Hastings Pier, winner of the 2017 RIBA Stirling Prize, has lovingly incorporated scorched timber boards from the remains of the original deck and used them to create new external seating and furniture. Structural engineer Ramboll consciously selected timber for the renovation of Hastings Pier due to its high strength and low density properties.
Why then do we not use hardwoods, the most durable of timbers, in our buildings?
The most obvious barrier to use is cost. Hardwoods are generally more expensive than their softwood counterparts; however, in this case, a higher price is indicative of an enhanced product. Hardwoods are slower to grow and generally denser, two characteristics which increase their price, but these characteristics also contribute to timbers which possess enhanced durability and strength – with few exceptions. Overall, this results in a wide variety of hardwood timber which are long lasting and extremely tough, requiring minimal maintenance. The majority of hardwoods are also available with FSC or PEFC certification. So, with all these excellent qualities in mind, why aren’t we using them more?
Another barrier is a lack of familiarity. Take greenheart, for example; it is D70 strength-rated, class 1 timber. British Standards provide the rules around natural durability without preservative treatments, but there are many examples from railways and harbours of greenheart lasting well over 100 years. Certainly, it comes into its own when a design necessitates a longer span in order to create unobstructed, flexible spaces; its higher strength classification makes it a natural contender to materials like steel in these situations. In addition, just one cubic metre of greenheart stores 1.16 tonnes of CO2 – which, when used for construction, is stored for the anticipated lifetime of the building.
But why would you choose greenheart, a remarkably dense and heavy material many contractors are not fully confident with, when it would be easier and less daunting to incorporate steel?
This lack of familiarity can largely be solved through guidelines and The magazine for TRADA members 11 Member Case Study instruction, such as those offered through TRADA membership. Just a sample of our resources include the Online Book Hardwoods in construction, the Wood Information Sheet Structural use of hardwoods, and our independent technical helpline (available 9:00 – 17:00 GMT Monday to Friday) – all for free for TRADA members. In addition, we also have an entire Wood Species database dedicated to helping you choose the most suitable wood species for your project.
However, this hesitancy is also dismantled by inspiring project teams – like those behind the Crannog, a legacy building set in Mallaig, Scotland.
The concept for the Crannog – a contemporary reconstruction of an ancient Scottish roundhouse, traditionally built on the sea-edge or erected over lochs on stilts – started when Nevis Estates funded a renovation to replace a disused village hall. Crannogs have taken many forms and construction generally used what was easily available, and the majority utilised timber – thus, while the design engineers of the Crannog had steel work in the underbuilding and connections, the local contractor – whose background lies in boat building and wild location construction – sought to address all of the building’s structural requirements using hardwoods. Like those in the past, who built crannogs using local materials, the contractor recognised that introducing steelwork to their remote location would cause additional difficulties in terms of time and cost. Steel cannot be adapted on site, which would have restrained the project. However, the Crannog, set on the shoreline, would have to endure the strong coastal winds and salt spray.
The local contractor was already familiar with the remarkable strength and durability properties of greenheart as a marine timber for piers; at their encouragement, the design engineers adapted the flitch beams, connection plates and angle brackets from steel – as a result, the only steel used in the building is a few bolts. Post and beam techniques were combined with boat-building methods. For example, the local contractor pioneered the steaming of greenheart on-site into crannog curves while steam lamination facilitated building the greenheart ringbeam roof without centre supports. Balau was used for the trusses, and home-grown larch and cedar were used for the cladding. The hardwoods were supplied by TRADA member Gilmour & Aitken undressed, and prepared within a workshop only 100 yards away from the site of the Crannog itself. Here the local contractor designed and drew the timber joints; once perfected, each element was manufactured using only a drill, hand tools and a chainsaw.
‘The Crannog has curved greenheart strips which we as the timber supplier have never seen before,’ shared Roderick Aitken, Gilmour & Aitken, ‘And these were achieved by using fishing boat steaming boxes, typically used for larch, oak or other boat building timbers – with which the local contractor has good experience. The Crannog is a true example of craftsmanship and demonstrates the ability of engineering hardwood timbers when they are fully explored.’
As a result of the project, 13 local staff were trained in heavy timber joinery, and many more walked away instilled with possibilities of all that timber can achieve.
Client: Nevis Estates