The project team behind Jonathan Tuckey’s innovative Rammed Earth House discuss key aspects of the design and construction process, and explain why rammed earth is a viable material for our net zero future.

Made by hand
Jonathan Tuckey and Emaad Damda, Jonathan Tuckey Design

Making a building out of earth in Britain seems to be simultaneously primitive and trendy. It is widely lauded as a material that is sustainable, beautiful, and readily available, but often dismissed out of hand due to its supposed vulnerability to the elements or simply a shortfall of experience. Yet buildings constructed from rammed earth have existed in the UK and northern Europe for centuries.

Once a rudimentary form of construction – also known as pisé, Stampflehm or tapial – widespread in small domestic and farm buildings, it is now seen as the exception, a special wall type to be used sparingly. Given the combined pressures of climate change, resource scarcity and rising energy costs, it is surely time to reconsider rammed earth as a potential mainstream building material.

The Rammed Earth House in Wiltshire was born from a particular set of circumstances. The client’s brief asked for an understated, unconventional, unmistakeably contemporary, country house that was grounded in environmental sensibility – both in its ‘eco’ approach and its relationship between house and landscape. The house is on the site of a former brickworks – clay-rich with extensive grounds – with a documented history of material being drawn from the ground. Combine this with a team of like-minded designers and builders, with the unwavering support of an environmentally conscious client, and rammed earth seemed like a no-brainer.

Ferme de Cossiat, agricultural building in Saint-Didier d’Aussiat, France, constructed using the pisé method (ph: Philip Heckhausen).

Our work is centred on the belief that re-using existing building stock is the most sustainable approach to the future development of our cities and countryside. The site contained a few small surviving 19th century buildings, somewhat crudely extended in the 1990s. Our approach here sought to strip the buildings back to their original extents and re-use as much demolition waste as possible to form the aggregate for the new walls. With the guidance of our rammed earth specialists, there was a collective push to avoid the use of cement and lime stabilisers, and instead maximise the use of the clay so prevalent on site whilst drawing on the heritage of the site and surrounding area.

The house’s fragmented plan, overlaid between older buildings and existing garden walls, orientates each room to optimise solar gain and creates sweeping views overlooking the surrounding landscape. Moving through the sequence of rooms becomes a journey between thick, carved walls and lightweight timber, from shadow into light and through a curation of art and sculpture that includes the building fabric itself.

A significant portion of the work in making a rammed earth wall is the formwork. Doka panels are assembled with carefully arranged timber-framed boxing. This shuttering, unlike concrete, is struck soon after the final layer of earth is rammed (ph: Nick Ingram).

There is an elemental beauty to rammed earth construction. We found it was practically all-encompassing, its thick walls and monolithic character presenting architectural opportunities at every turn. It is an inhabited wall, enclosing rooms but also carved to form seats and punched to form niches for objects – like the poches of a medieval castle. It can be formed into dramatic multi-storey walls, sweepingly long façades and vast expanses of (heated) earthen flooring. Designed correctly, unstabilised rammed earth can go over 100 metres without any movement joints. Like a potter moulding clay on a wheel, it can be seamlessly retouched. The internal walls here are typically finished with a casein emulsion to prevent excessive dusting, and floors are waterproofed by impregnating the surface with carnauba wax. With the correct technique and treatment, the material can even be formed into baths, basins and worktops.

Rammed earth is not a perfect material and certainly has its limitations. It needs a ‘good hat and boots’, and must be carefully detailed to shed water and spread load. This gives rammed earth buildings a particular tectonic character. On the Rammed Earth House, a brick base ties the different elements together, extending out in places to form plinths to perch on and troughs to collect rainwater. Clay tiles cap the walls to form an expressed drip detail. Traditional examples of earthen architecture often feature a long projecting roof for protection. The more contemporary projects of Martin Rauch and Lehm Ton Erde make use of ‘erosion checks’ – horizontal lines typically of lime mortar repeated at regular intervals. Rain erodes the outermost layer of fine- grained material, leaving the erosion checks to gradually protrude from the surface. This helps control the speed of water running down the wall, much like a speed bump on a road. The erosion slows down and ‘stabilises’ over time as the wall surface becomes harder with the larger aggregate exposed. This gives the building its patina and tactile quality, constantly changing and maturing over time rather than fighting against it.

Aerial view of the site. A brick plinth raises the rammed earth off the ground (ph: Nick Ingram).

There is no denying it is a labour- intensive process. The construction site for the Rammed Earth House has been transformed into part quarry, part workshop, with builders doing everything from extracting clay and crushing aggregate, to creating formwork and ramming layers of earth, to laying bricks and retouching the walls. This makes the building distinctly hand-made, something which is sadly becoming more alien in our industry and would be wonderful to rediscover. It also somehow ties the building to a human scale – earth layers rammed down to 75mm correspond to a course of brick and mortar – making a monolithic wall seem rather domestic. From a distance it is one thing, but from up close it is another. All these qualities help to create architecture that is not only resilient and functional but also joyful and timeless.

The site as a quarry. Stockpiles of clay excavated from the ground, crushed concrete and brick from demolition waste, supplemented by locally-sourced limestone gravel are fed into a forced action mixer with water to create the rammed earth mix (ph: Nick Ingram).

The issue of carbon
Steve Webb, Webb Yates Engineers

We don’t hate bricks per se, but in a world that is exercised by a climate emergency, continuing to use a material that has a high embodied carbon, and is made quite often with lignite (brown coal), seems like a very bad idea. Brick is an iconic building material but, along with steam engines and chimney sweeps, we need to let it go. What are the alternatives?

Rammed earth gives a beautiful finish and natural appearance, and visually reflects the place we are building in. Usually most of the material can be site-won, meaning less transport and less material being removed from site for spoil. The material itself needs to be a suitable mixture of aggregate and clay, and needs to be tested, assessed and trial mixed. If the soil composition is suitable, then the walls can be exposed externally. They are strong enough to bear loads and to form walls in multistorey buildings. Unstabilised walls – where the earth mix does not contain cement or lime – can reach strengths of 1.5-4N/mm2, which are in the same league as 3.5N concrete blocks. Compared to brick, unstabilised rammed earth is much lower in carbon.

Clay from the ground is combined with crushed demolition waste on site, mixed and then rammed in-situ within formwork.

What are the downsides? In this case, a slower build and a heavier building. In poor ground conditions as on this site, rammed earth walls may require concrete strip footings that can seriously reduce the carbon savings of the material itself. If the local soil mix isn’t right, you might have to add cement and then the carbon count can get quite high. The debate around stabilised versus unstabilised rammed earth certainly recurs on these projects. Here, we have had the benefit of specialist input and several months of sampling and testing to arrive at the final earth mix – without cement.

If they are to replace bricks, rammed earth walls need to be outside, but from a building physics perspective it is better to have them inside where they add to the thermal mass, limiting overheating and keeping them dry and protected. There is a perversity in having a very heavy building component insulated from the internal spaces where it would be useful, although constructing a double-skin rammed earth wall in-situ comes with its own complexities. In low-rise buildings, compared to insulated timber studs, external rammed earth might make less sense, but it is a lot less senseless than covering everything in brick!

Deep-revealed openings formed in the thickness of the rammed earth wall (ph: Emma Carroll, Jonathan Tuckey Design).

A light touch on the environment
John Peapell, Sustainable Solutions; Bart Stevens, SGA Consulting

The aim for the Rammed Earth House was to deliver a beautiful home with low embodied energy, low predicted operational energy and as close to net zero carbon emissions as practical. The material choice was founded on the principle of repurposing material available on site – clay and rubble. Notwithstanding the concrete needed in the foundations and floors, the use of rammed earth supplemented by a timber structure and timber window frames, helps tremendously in reducing the embodied energy spent making the building.

The design uses ‘fabric first’ principles to minimise operational energy, in turn minimising the size of the heating system and energy required, and reducing the amount of renewable generation required to offset carbon emissions.

The build targets the AECB standard, which aims to reduce CO2 emissions by 70 per cent compared to the UK average for buildings of each type. All U-values and linear thermal bridges have been designed close to Passivhaus standards, although the airtightness requirement has been loosened due to the building’s complex form.

Earth walls are designed to respond to the effects of weathering and erosion. Trass-lime checks strengthen the corners and slow down the flow of rainwater, and brick tile drop details further help shed water away from the building (ph: Emma Carroll, Jonathan Tuckey Design).

Rammed earth walls are thick, 400mm is typical on this house. An additional 300mm of thermal insulation on the inside of the wall helps achieve a very low U-value, around 0.14 W/m2K on average. A typical wall uses a combination of mineral wool insulation and woodfibre board, finished with clay plaster internally, to create a breathable construction. The thick walls also provide significant solar shading for windows, which are triple glazed.

Rammed earth walls work very well delaying the impact of the solar heating on the external wall. The thermal mass provides a delay of up to 12 hours to the solar gain. Therefore the peak gain occurs at about 2am, benefiting the building by providing passive solar gain into the house when it is coldest outside.

If the maximum effect of thermal mass is to be achieved from these walls, then it is best to have as much rammed earth exposed on the inside as possible. This way the stored heat or coolth would directly benefit the occupants. Whilst most of the rammed earth on the project is external, internal rammed earth walls form an integral part of the main living spaces – the kitchen, the living room and the main entrance – where they provide significant thermal mass. Ground-source heat pumps provide the primary source of heating, while 100-square-metres of photovoltaic slates help to power the house and vehicles.

Calculated erosion. As exterior rammed earth erodes, the fine particles wash out and larger aggregate becomes exposed. This increasingly uneven wall surface causes water to lose speed and the erosion rate decreases sharply. Eventually, after several years, the erosion rate will cease altogether.

Trust in the material
Sami Akkach, Lehm Ton Erde

The challenge of building in the UK with unstabilised rammed earth rests with the lack of knowledge of the technique. From architect to builder, few know what design constraints must be considered, how to assess suitable earthen material, what equipment and infrastructure is required, or possess the know-how to execute a build. There is also a lack of visibility amongst the public. All this translates to little trust in the material. These hurdles are not unique to the UK, however, and must be overcome in most of our projects. They are also hurdles that are man-made, and as such can be changed. Rammed earth is inherently suitable to almost any climate. It only requires the right design and planning to make it appropriate for each site.

One of the difficulties that must be confronted, not only in the UK, is the non-existence of rammed earth specific equipment. Concrete formwork systems can be used but must be adapted to the significantly higher internal pressures. Our ramming tools are pneumatic backfill tampers that have few manufacturers. And mixing must either be done with a frontloader or bobcat, or as was eventually decided at Brickyard, with a souped-up forced-action pan mixer imported from LTE’s own stock.

Like a cake recipe, a rammed earth recipe consists of the ingredients in their proportions and the mixing methodology. The earth mix here had to be developed in a unique way. Earth is not an off-the-shelf product, meaning each local material must first be assessed and that requires an experienced hand. We coordinated the process digitally as far as possible. Setting one essential parameter; to use as much site-won material as possible, several trial recipes were made and tested. Initial compressive test results did not meet the minimum 2.4 MPa standard specified.

Graph from SGA Consulting showing the effect of solar heat gain in a brick house versus a rammed earth house.

Once we were able to hold a vis-à-vis workshop, notwithstanding Brexit and the pandemic, the recipe was refined to meet all necessary parameters, including compressive strength, erosion resistance, workability, colour, and more. Excavated fat clay provided the binder and crushed brick and concrete from demolition work, supplemented by locally-sourced limestone gravel, made up the aggregate. The final mix comprised equal parts of each constituent by volume. Regular lab testing of mix batches continued as work progressed, giving assurance the specified compressive strength was being met.

We are often asked what it costs to build with rammed earth compared to concrete. We would ask, what it will cost us to not build with earth and continue with concrete? To confront the enormous wastefulness, emissions, and resource extraction the construction sector is responsible for, we need to reformulate these questions. We need to set a value for building systems that considers environmental damage. Not to mention creating a resilient British architecture to a contend with the changing climate. Next time the mercury hits 40°C, the Rammed Earth House will be a tranquil oasis.

Credits