The structural design was informed by close collaboration with the architect, Wellspring Architecture, and The School for Natural Building, who specialise in straw bale buildings. We also drew on existing research and spoke directly with engineers and researchers involved in previous projects to understand where established approaches worked well, and where their limits lay. That background helped us assess which aspects of the design could be carried across directly, and where new solutions were needed. Even with the available precedents, this building has several features that made it unusual for a straw bale building. It had a reasonably large-span roof, which we were keen to keep as open as possible. It is an education building and so was subject to greater levels of scrutiny. It also had several trees and shrinkable soil, limiting the foundation options.
The brief to prioritise natural and low-carbon materials set clear constraints on the structural strategy, influencing material selection, load paths, and construction sequencing. Rather than selecting a typical structural system and adapting it, we had to work the other way round, developing solutions around the materials available.
We learned that working with straw as a loadbearing material demands care and precision, as well as a willingness to step outside the usual approach. Early on, we reviewed previous research into straw bale construction and spoke directly with engineers and architects who had worked on similar projects. We also talked through traditional construction techniques and applied engineering principles to them to understand their reasoning and benefits. Through workshops and discussions, we were able to unpick the principles of designing with straw bales and apply them to our project.
All the external walls were built entirely from loadbearing straw, which has a short growing cycle and sequesters carbon that would not otherwise be stored. It was also sourced from a nearby farm, making it an ultra-local regenerative material. To design with straw, we had to become familiar with the traditional techniques used to compress and stake together the bales, and the importance of a rigid timber box beam at the top and bottom of the walls to evenly distribute loads.
The stacked bales are typically rendered on the outside with lime render and plastered on the inside with clay, both around 20–25mm thick. This keeps the construction water-resistant and breathable, yet airtight, without any additional membranes, layers, or cavities. Timber window surrounds were thermally broken with strips of wood fibre insulation, a level of detailing that exceeds most typical projects.
When designing with straw bales, a key constraint is the size and makeup of the bales themselves. Typically, they are 900mm × 450mm × 300mm high, and the straw is relatively consistent across the UK, though there is some variation. Our source was already known when we started design, so the size and specification were set. The straw itself was a constant and so could not be made thicker or stronger to provide additional load resistance, but we discovered that the render used to finish and stabilise the walls could be fine-tuned to add strength. We quickly realised that using clay plaster led to a significant reduction in the overall load capacity of the wall when compared to lime plaster. This was because the thickness and strength of the coatings were directly correlated with the wall test results. While the straw bales provided some inherent strength, this was limited, so having a strong coating on the sides created a composite structure, much like SIPs, where the outer layers dominate the overall wall properties beyond a certain point.
Due to the applied loads, we specified lime render inside and out, with the thickness and strength set in line with what the supplier advised they were confident could be consistently achieved. The client then funded a bespoke testing programme using several different samples provided for free by the lime supplier before the project was tendered. The tests showed an improved strength, and we adjusted the render thickness accordingly. This reduced risk and provided confidence in a system that could be repeated and refined on future projects.
This way of designing, by adapting to the materials available, is the opposite of current typical design practice. It forces designers to really understand the materials they’re using and tailor solutions to those materials. This is the basis for truly regenerative design, where we build with what we have rather than ordering it from a catalogue, and is something we have developed at Webb Yates to allow the engineering of structures made from a wide range of materials.
What sets this project apart from previous straw bale buildings is its scale and public nature as a working school, rather than a small private house or demonstrator project. It provides a modern, highly usable space by combining traditional techniques with modern design analysis, research, testing, and construction methods. Through it, we learned how to design and detail straw projects, and hope it can serve as a precedent for others working with similar materials.
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