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A cob and timber roundhouse shelter in a beautiful garden

environment.

The Pilkington Shelter is a new building, built using ancient traditional techniques and materials. It fits well into the stunning environment cultivated by the Royal Horticultural Society. The cob and timber frame shelter provides shelter for visitors from the rain and a cool haven to retreat to when temperatures are high. It uses wood and earth obtained from the site.

IntroductionTruro

Exeter

Great Torrington

Pilkington Cob Shelter

Key dates :Planning permission : 2011.Construction : 2011-2012.

Photo © Jan and Jerry Sharpe - J&J Sharpe

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The Pilkington Shelter is set in the grounds of the RHS gardens, Rosemoor in Torrington. The land was given to the Royal Horticultural Society in 1988 by Lady Anne Palmer, who had a great love of horticulture, which led her to many exotic places around the world, discovering and collecting new plants for her collection. The grounds covered 40 acres, including pasture land and her original garden. The grounds have now been transformed into an array of stunning formal and informal gardens, and Bicentenary Arboretum; it is used for information, training, events and most of all, pleasure. All of the new buildings on the site have been designed not only for their function,

Contextbut with great care to ensure that they blend in well with the environment. Thus, when the client was choosing what to build as a new shelter for the public to use, they agreed on a design which not only suited its purpose, but fitted in with its environment. It is aesthetically pleasing, with its timber roof, soft lines and earth hues, blending in with the woodland behind. This natural, elemental design was the work of Peter Hall, of Van Der Steen Hall Architects, of Chagford, Devon, and was influenced by the principles of Rudolf Steiner, an Austrian philosopher, reformist, educationalist, architect and esoteric.

Drawing © Van Der Steen Hall Architects

French-British collective Libnam3

The Pilkington Shelter is 5m high and has a floor space of 25 m² and, in keeping with the design principles, was built using sustainable resources, most of which were acquired from site. The timber was sourced from their woodland, and soil/clay obtained from the excavation of the foundations and the gardens was used for the cob. A local contractor and heritage build specialist, Jerry Sharpe, of J&J Sharpe, was hired to build and act as project manager for the project. Local carpenters were hired to build the roof, thus, reducing costs and minimizing the environmental impact on site and through travel and haulage.The single storey cob building stands on strip foundations, which were dug down to reach firm ground to ensure

Building overview

stability. These were then filled with reinforced concrete with a stone plinth built on top. The stone plinth is 500mm wide and was built using local stone and lime mortar. The walls are made out of mass cob, which is set on top of the plinth with four tapered windows. The windows were formed using forms made from plywood to act as the shuttering. On completion of the walls, the forms were removed and slate sills were put in.The interior of the shelter kept to the original principles of the design. The floor was formed using limecrete. Limecrete is a material made from mixing natural hydraulic lime, aggregate and water. The limecrete for this project was made from yellow chippings, course sand from Somerset and natural hydraulic lime five. To provide a suitable base for this, the surface of the floor was first compacted, then levelled out,

Photo © Jan and Jerry Sharpe - J&J Sharpe

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(blinded) with sand and covered with a geotextile matting membrane. The limecrete was then mixed by machine, barrowed onto the spot and levelled.The stunning roof design is made up of seven panels, with seven different pitches. Local carpenters Tim Mills and Jack Bennett, undertook this complicated task, using local larch to form the structure. The roof sits on wide wooden wall plates which sit directly on the cob walls. Sitting on the wall plates are two A frame trusses, (shown in the picture above), which were joined with purlins and a ridge beam. From the trusses, hip rafters then formed the roof framework to the wall plates. The roof was then finished with larch boards.The interior and exterior walls were

finished with lime plaster. Lime plaster is often used in conjunction with cob as it allows the walls ‘to breath’ allowing the transfer of moisture which is essential to the integrity and longevity of the material.Finally, a French drain was constructed around the building. This was formed by digging a trench around the outside of the shelter. The trench was lined with a membrane of geotextile matting which stops the earth from blocking the drainage system. A perforated pipe surrounded by clean chippings which allows the free flow of water, takes the water away from the area of the shelter to nearby land-drains.

Photo © Jan and Jerry Sharpe - J&J Sharpe

French-British collective Libnam5

Technical focus : Use of Cob as a viable, sustainable building materialUsing cob construction for the Pilkington Shelter at Rosemoor was fundamental to the initial design principles. It harmonizes with nature and its environment by the use of earth elements from the site, and by using predominantly hand working building methods and local companies for everything from the architects to the contractors, the cob helped to ensure a low impact build. As well as fitting in with the design principles and being a low impact building material, cob is a local traditional building material used widely in the South West of England.

There are many cob houses and barns in the Devon area, some which date back 500 - 600 years.The Gardens and foundations of the building supplied the soil, which had a suitable clay content to allow for this. In order to make cob the soil was mixed with chopped barley straw and water. This can be mixed by hand, but in this case, because of the quantities needed and time constraints, a digger was used to mix the material. Time is a critical issue when working with cob, as adverse weather damages the cob. This is because the cob takes a much longer time to ‘go off’ compared to cement, and during this time extra water will damage the structural integrity of the material. For most of the year, Devon has one of the highest average rainfalls in the country, so time is of the essence. The use of a digger reduces the number

Photo © Jan and Jerry Sharpe - J&J Sharpe

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of people needed and the time taken to mix the material and speeds up the process.Cob also needs to be built in stages called lifts. This is where a section is built and allowed to dry out. This enables the material to develop structural strength as it ‘goes off’, which is the time that it takes to dry sufficiently before the next lift can be added. A variety of factors affect the time it takes, including sunlight, humidity, wind, size of wall and mixture. In the case of the shelter at

Stakeholders Architects : Peter Hall Dip Arch, RIBA AABC of Van Der Steen HallContractor : J&J Sharpe LtdCarpenters : Tim Mills and Jack Bennett

CostsThe project cost a total of £40,000 which was funded by a donation to the RHS.

Rosemoor, the contractor decided that lifts of 400 mm, set on a 500 mm plinth would be appropriate. Although the mixing and moving of the cob material was done mechanically, the mix was forked and compacted in the traditional manner by tamping, which is stamping the mix down as they walked along the wall. Each lift was allowed to dry and become firmer, then the edges were pared back (cut back), to the desired shape using hand tools.

Drawing © Van Der Steen Hall Architects

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