Rosemoor Cob Shelter In spring of 2011 RHS Rosemoor Gardens commissioned a design for a cob shelter to be constructed in a new area of the Garden that is part of the Peter Buckley Learning Centre. The Pilkington Cob Shelter has been designed by Peter Hall of Van Der Steen Hall Architects and the construction carried out by J & J Sharpe Ltd with Jeremy Sharpe, cob and historic building specialist, as the project manager.
The shelter is seven sided with an open entrance facing north east. The roof is timber and the walls are mass cob on a stone plinth. Formed into the cob walls are four window openings with slate sills. The openings were formed to a tapering profile by using braced plywood shuttering formwork. The cob was built up to the formwork to form the required openings. Inside the shelter is a bench seat with a solid oak top supported on larch frames.
Setting out for the build
Foundations dug
Concrete footings being placed
Stone and block plinth wall
Plinth wall completed
Mixing cob with digger
The first part of the process was to mix the cob. The Rosemoor Garden team supplied suitable sub soil (with appropriate clay content). To this pile of sub soil water and barley straw was added and mixed thoroughly using a digger. Once the pile was mixed it was then covered with tarpaulins and taken from as required to build the walls.
Once the setting out for the footprint of the building was completed trenches were dug to the depth of stable ground and foundations were poured. With groundwork complete the next stage was to build the plinth wall for the cob to bear on.
Cob being placed onto wall
First lift of cob completed with formwork in place for window openings
Wall head protected with breather felt
Paring the face of the cob
Setting out wall plates on the cob
First A frame in position
This base wall that the cob sits on is faced with local stone and built with lime mortar. This plinth wall is 500 mm thick. The cob is placed on the plinth and built up in lifts of about 400mm high. Each lift was allowed to dry out sufficiently to become strong enough to lay the next lift on. The internal face of the plinth wall is rendered with lime mortar. The laying of the cob was achieved in the traditional manner of forking it onto the wall and compacting it as the work proceeded. The compacting was achieved by tamping down the cob by means of stamping along the cob wall with heavy boots. Once the cob was tamped and had dried to a firmer state it was pared back to an even profile using hand tools.
The complicated roof design, which consists of seven pitches, was realised by experienced carpenters, Tim Mills and Jack Bennet. The roof rests on a broad jointed wall plate which slopes from front to back. At the front this plate is supported on two round posts resting on granite pad stones. The remainder of the wall plate is supported by the cob walls. Onto this wall plate two substantial A frame trusses were erected, purlins and a ridge beam were fitted between these trusses. From the front truss two hip rafters were fitted down to the front corners of the wall plate and a purlin installed between these hip rafters. From the apex of the rear truss a further hip was fixed to the back corner of the wall plate. Common rafters were fitted to each pitch of the roof and clad with over lapping wide feather edged boards. All of the timber used is locally milled larch and all the fixings are stainless steel.
Front view of roof structure
Side view of roof structure
Common rafters partially completed
Cladding of roof surface in progress
Roof cladding completed
Start of limecrete floor
The floor inside the shelter is made from Limecrete. This is a more sustainable material than concrete. Limecrete is a mixture of natural hydraulic lime, aggregate and water. For this floor we laid limecrete consisting of yellow chippings and coarse sand from Somerset and Natural Hydraulic Lime 5. It was laid over a compacted sub base blinded with sand and covered with geotextile matting.
A trench was dug around the outside of the shelter, lined with geotextile matting with a perforated pipe laid to a fall and linked into the land drains. Then the trench was filled with large chippings. This is called a French drain. it drains rainwater and ground water from the area and creates an appropriate finish around the building.
In the spring the interior and exterior of the cob wall will be lime plastered.
The final piece to be completed is the planting and landscaping which will be carried out by the gardeners at Rosemoor ready for the opening in May 2012.
Completed roof
Structure and limecrete floor completed and bench installed
