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Gridshell at Flimwell
From Building for a Future Autumn 2001
continued from page 1

Here the benefits of the research began to kick in. Fielden Clegg’s rejigged building plan integrated slats as part of the modular gridshell roofing structure. For these, chestnut was cut into short 28mm x 75mm sections and sent off to Newcastle to be remoulded into the 10m lengths, which could then be put together as the gridshell slats for the modular bays - modelled by Brighton based Structural Engineers, Atelier One. The chestnut used was freshly felled greenwood, which meant it was partially dry though retaining high moisture. This meant that less washing and preparation was required before drying, producing a much more stable timber. The suppliers were a local, Kent-based Woodland Trust coppice, and a Sussex-based chestnut forester, Richard Cope. If it hadn’t been used on this project the wood inevitably would have been wastewood. Even If obtaining the supply was relatively straight forward, certain difficulties ensued with the joiners and industrial carpenters inexperienced in working with the hardwood. Indeed what has become clear, is the need to develop the exposure of industrial carpentry and finger-jointers to using the wood - apparently working with it took quite a lot of persuading - as it is a difficult and unknown material for those used to softwood. There is also a need for specific machinery designed for hardwood, as softwood-centred technology is not entirely suitable.

When the wood arrived, it did so in varying condition and quality. The key to ensuring that the chestnut is good enough to be used is through the initial selection process, done by a skilled eye, and experience. The chestnut research consultant, and timber engineer, NIgel Bradon, has developed this to a fine art, identifying where the slope of the grain is too steep, or the proportion of knots too heavy, both of which can lead to failures. Once identified, the potential defects are removed, and visual timber grading is applied for future usage, and then the timber is sawn into 75mm by 25mm sections.
The building, which sits off the edge of the A21, south of Tunbridge Wells, deep in dense Weald woodland, is a compact and attractive mid-sized structure, fitting well amongst the surrounding trees. The large-span modular gridshell carries five bays, the sawn and fingerjointed laths having been bolted together comprising the barrel vaulted arches. The curve of these arches is slightly eccentric, rather than symmetrical. The five bays were constructed on-site by Cowley’s, the timber carpenters. These modular sections could be constructed anywhere, in prefabricated units in a factory, and transported to the prospective site.

Feilden Clegg have declared themselves pleased with how it has performed from the sustainable angle; its durability, and there being no need for chemical preservatives. Beside the chestnut, the building’s main environmental features include its mineral wool insulation, making it a low-energy building. Roof lights provide the upper floor with considerable natural lighting. The aim is to develop a wood-fired heating system, from excess woodwaste thrown out by the site’s separately owned and operated sawmill, which should provide enough heat for all of the prospective buildings. The foundations are concrete - unavoidable because of the Weald’s clayey ground soil.

This said, the principal sustainable innovation is in bringing chestnut into the environmental building equation. The centre demonstrates how chestnut and other hardwoods can be used in mid-sized structures, and as building components, all locally-sourced from across the South East. Where before the only obvious options have been softwoods, and/or tropical timber, this shows that there is a real alternative. Costwise, the structure is coming in at around £500 per square metre, compared with rock-bottom steel and concrete costs of £350. Whilst this is a significant difference, in environmental terms there is investment going into both the regional environment and the local economy. This may not be persuasive for smaller buildings, but as far as mid-sized buildings are concerned it does look potentially competitive. Set against softwoods, and due to the implementation of tougher building regulations, chestnut could well become an attractive option. There is also the wider building usages that the wood can be put to. The centre is looking at furniture, flooring, doors, and window frames (already popular in France) and local wood designers, Tin Tab , are working on a stairwell. With the coming of laminated, or finger jointed hardwoods, the building industry is waking up to a range of very flexible materials.

This shows in Woodland Enterprises’ plans. They are intending to set up production within the Centre’s grounds, and the next proposed building is a factory, which could prefabricate the modular sections and other hardwood building components. As such, it provides a persuasive alternative option to the main new choice being developed for low-grade woods; its use as wood fuel energy. If this is a high volume industry, by contrast the Woodland Centre chestnut initiative employs a value-added approach to a low-value material.

If regional building requirements are considered, both for mid-size and domestic buildings, there seems to be considerable scope for growing a rejuvenated chestnut coppice timber industry. With the arrival of new building regulations, whether you’re mainstream or at the margins of building, the use of wood, already enjoying a national resurgence, will likely increase. Major housing developers are already reintroducing timber, including Wimpey, Westbury, Laing, and Wilcon. But a main difference to previous timber industry growth-spurts is its use in low-rise medium-size buildings, up to 15% average across England, including hotels, nursing homes, community-centres and commercial and leisure centres. Here, large-span structures are clearly an obvious option, and is one of the markets being eyed-up by the Flimwell Woodland Enterprise. The other primary sustainable point is that it is locally sourced wood. This in a context where even though UK grown timber expansion is rapid, at the same time the extra capacity is being swallowed up by buildings’ increased use of timber framing, and is unlikely to affect the balance of 15% home-grown timber market, against imports. Here lies the dilemma. Overall, the Forestry Commission’s vision assumes a continuing dependence in the ratio of imports for the vast majority, that is 85% of the total, including tropical hardwoods, and western red cedar. The double environmental hit this can bring on, is the depletion of hard-pressed tropical forests, and the life-cycle costs of transporting, etc., the timber hundreds to thousands of miles from source to building-site destination.

As far as the South East is concerned two further points accentuate this situation. The Government has charged the region’s local authorities with an ambitious housing programme over the fifteen years; in some plans the need for possibly hundreds of thousands of additional homes in England’s most crowded region, barring London. Set against this is the irony that county by county the South East has twice the average number of trees and forest cover - 20% - compared to the national average of 10%, let alone tree-depleted areas such as Leicestershire with 4/5%.
If David Saunders is right about his back of the envelope calculations - and there is no reason he isn’t - there are 5 tonnes per hectare of chestnut growing each year. Thus, 15 tonnes would be enough to grow another Flimwell. An annual 100,000 tonnes of chestnut could provide the core of many, many, mid-sized and other buildings for the region1, alongside a versatile variety of applications. These are provocative figures to be considered by any other regions - within their contexts - contemplating how to regenerate rural economies. It is early days, but through wood and glue technology, Flimwell has become something of a temple to the chestnut for aficionados of the tree, and something of a laboratory for possible woodland futures.
Oliver Lowenstein

Refs:
1. The building uses 12.5 tonnes of prepared chestnut in the structural gridshell, weatherboarding and joinery. Taking into account the high losses in conversion from small logs, this would be derived from a total of 40 tonnes of coppice roundwood. As chestnut coppice grows at around 8 tonnes per hectare per annum, this amount is equivalent to 1/5 of the production from 1 hectare of 25 year old coppice. Alternatively, the area of woodland required to produce one such building each year is 5 hectares.
David Saunders, County Woodland Officer, East Sussex County Council Tel: 01273 481563 Email: david.saunders@eastsussexcc.gov.uk
Nigel Braden Timber 01825 872150)

Author: Oliver Lowenstein