A potential source of regional timber is being overlooked, coppiced Sweet Chestnut. Prolific in the Weald and Kent areas, George Fereday’s HomeGrown House research demonstrates the prospects of coppiced Sweet Chestnut have hardly begun to be plumbed.

Since moving to Kent nearly a decade ago, I have become fascinated with the region’s coppiced landscape. Walking in coppiced woodland, my eyes have been opened to the plethora of benefits coppicing can bring: fast-growth regenerative hardwood, net-gain biodiversity and the support of local woodland industries.

Despite the southern counties of East & West Sussex, Surrey and Kent growing some of the finest quality coppiced Sweet Chestnut in the UK, only a small volume is converted to timber, primarily for fencing applications. Any unmanaged or ‘overstood’ coppiced trees are typically burnt for biomass. Burning coppiced sweet chestnut in this way releases decades of sequestered carbon and is a waste of the tree’s excellent building credentials – natural durability, low sapwood ratio and its propensity to cleave efficiently when green.

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The building prototypes were exhibited publicly in the coppiced woodland from where the trees were felled (Figure 4). This was to emphasise the importance of a holistic understanding of coppice forest management and roundwood processing when designing with this material.

Academic outcomes of the research were published and the project was presented at COP26, the UN Climate Change Conference, Glasgow, 2022.

To explore why local, coppice sweet chestnut is so seldom used in construction, we set up the ‘Home Grown House’ project in 2020/21 with funding from the Higher Education Innovation Funded (HEIF). The project brief was to develop a demonstrator ‘kit of parts’ for constructing buildings from coppiced sweet chestnut timber that align with regional processing capacity and local coppicing skills. To better understand the barriers and opportunities to building with coppice, we ran two focus groups with regional stakeholders in the coppiced sweet chestnut supply chain. Findings from these focus groups helped us define a relevant set of design constraints that aligned with existing opportunities and overcame supply chain barriers, to using this unique hardwood resource in buildings. The project received some industry support from Wood-Mizer, a leading mobile sawmill manufacturer, to test low-waste methods of radially sawing coppiced sweet chestnut poles for our XR beams (Figure 1). The focus group informed brief was used to design speculative floor, wall and roof constructions (Figure 2). All the prototypes were sawn or cleaved from a range of small, medium and large diameter coppiced trees (Figure 3). This meant that the prototypes we developed, had the potential to utilise actively coppiced, as well as unmanaged trees, which have few markets beyond biomass.

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Scaling coppice construction.

Our second research project ‘Home Grown Cabin’ in 2021/22 was funded by the Forestry Commission through the Woods into Management Forestry Innovation Fund and was conducted in partnership with Grown in Britain – an independent, not-for-profit organisation, encouraging more demand for home-grown wood. The aim was to understand barriers and opportunities for using coppiced sweet chestnut in scalable, modular construction, by building on our knowledge from the first phase of research. Central to this project was the development of experimental Structural Insulated Panels (SIPS) made from unseasoned timber, as an example of a modular, scalable engineered timber building component (Figure 5). Simple Vierendeel beams made from half-round coppiced poles, proved a cost-effective method of providing structural stiffness to our SIPS panels, whilst reducing timber wastage associated with saw-milling of small diameter roundwood that can make it uneconomic.

Production of our Vierendeel beams involved cutting a coppiced sweet chestnut pole in half down its length and inverting the two sawn faces to create opposing flanges of a beam. Each pair of half-round lengths were then separated by a vertical ‘web’ of chestnut dowels (Figure 6). The beams provided a lean, repeatable and cost-effective structure with a cavity to insulate for thermal separation between inside and outside of the cabin. Once assembled from pairs of Vierendeel beams, open panels were linked with folded steel brackets to produce an A-frame structure (Figure 7) the SIPS panels were insulated with sheep’s wool, lined with vapour permeable membranes and inter-connected with steel brackets to create a modular A-frame (Figure 8). Each A-frame segment of cabin was then placed in series onto a timber podium and braced together externally with sweet chestnut battens. The cabin was then clad with a cleft sweet chestnut rain screen (Figure 9).

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Preliminary findings from the cabin research are that building designs which accommodate non-straight growth, as well as straight coppiced poles, offer the best prospect of opening new construction markets for coppiced timber. This is because our Vierendeel beams could only be made from the straightest growth poles, representing approximately 10% of coppiced sweet chestnut trees. The remaining 90% of the trees in a typical coppiced woodland are not ‘straight growth’, so finding viable building use as segmented short lengths proved an effective use of this more prevalent material (e.g., exterior cladding, or for the manufacture of dowels).

Our study also suggests sweet chestnut can be used in a green, unseasoned state, if detailed appropriately. Not kiln-drying timber saves significant proportions of embodied energy and carbon whilst supporting a route for green wood-working skills to continue to thrive in new and diversified ways. To build with unseasoned timber, however, requires an understanding of green wood behaviour. The unseasoned sweet chestnut used to stiffen our SIPS were encapsulated with sufficient vapour permeability that the timber could dry in-service without trapping moisture that might degrade the timber over time. Sawn into half-rounds, or cleft into smaller cross-sectional volumes accelerated atmospheric drying of the coppiced sweet chestnut timber prior to construction. Some inherent growth tensions were also released, making the timber less prone to the splitting than unseasoned coppiced poles such as fencing posts. This finding is significant because checks and splits in green timber used for construction, must be factored into structural calculations, and timber drying in-service can effect dimensional stability, as seen in green oak construction.

Highlighting the interconnected nature of active coppice management, local processing capacity and hardwood supply chains, our speculative research has explored the construction possibilities with unseasoned coppiced sweet chestnut in the Southeast of England. By taking a holistic view from forestry to building, this work has highlighted the potential of coppiced sweet chestnut to;

1. Bring neglected woodlands into management and diversify markets for actively managed coppice.

2. Support local, sustainable jobs and coppice related skills.

3. Enhance woodland ecologies.

4. Reduce reliance on imported timber for construction.

5. Divert biomass carbon emissions into long term carbon storage in buildings.

Looking to the future:

The next phase of the research will test up-scaling of coppiced timber from cabins, into housing or community spaces. As part of this work, linear jointing of half-round timber will be developed and destructively tested. Linear jointing of this kind would enable increased length of beams / SIPS and more efficient use of shorter lengths of coppiced timber, taken from non-straight growth coppiced poles. Using a higher proportion of this non-straight growth material would both, help to reduce cost, and tap into a more readily available supply of coppiced roundwood. Upscaling into larger buildings could also help provide routes to market for the high proportion of overgrown, unmanaged sweet chestnut coppice, that’s prevalent in the Southeast. This work may also include an assessment of the biodiversity benefits of coppicing, metrics for which are not currently linked to timber supply. We hope this next phase of research will be in an externally funded collaboration with local social enterprise and/or local councils in the region.

Further –  https://www.londonmet.ac.uk/research/centres-groups-and-units/the-centre-for-creative-arts-cultures-and-engagement-creature/research-projects-/home-grown-house/

George Fereday is Associate Teaching Professor at the School of Art, Architecture & Design, London Metropolitan University. His teaching and research interests include use of natural materials in construction and learning through making. His research featured at the COP26 UN Climate Change Conference, was shortlisted in the Structural Timber Awards 2022, and the  Wood Awards 2023 – a collaboration with the V&A museum, Sylva Foundation and Grown in Britain.

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