Waste earth forms walls of cob classroom in King's Cross

Continuing our Building with Cob series, we take a look at a cob classroom designed for educational charity Global Generation, which is being built from waste earth sourced near its site in King's Cross.

The bustling heart of London's King's Cross district might be the last place you'd expect to find an unassuming cob building taking shape.

Yet, tucked away on a wedge-shaped site behind Heatherwick Studio's Coal Drops Yard, educational charity Global Generation is currently using the age-old earth-based material to build its own community classroom alongside the railway tracks.

Anyone can "have a go" at cob construction

The rectangular building has been designed by local studio Jan Kattein Architects with engineering firm Expedition Engineering as part of the Triangle Site – the charity's first permanent home, which will also feature an office and a community kitchen.

It is being self-built by Global Generation – a charity focused on engaging the community in environmental projects – alongside a team of volunteers of all ages. The team's decision to use cob was largely to help facilitate a community build project.

"Cob works for community build projects really well," the charity's resident architect Alice Hardy told Dezeen on a walkthrough of the site.

"We wanted to use a regenerative natural material because we are an environmental charity, but also because we wanted to build it with volunteers, and so the cob was a great option," she explained.

"We needed a construction technique that would allow anyone to drop into the site and have a go, and cob is great for that."

Cob is an unfired earth-based construction material made from clay-rich subsoil mixed with water and fibrous materials, such as straw. Sometimes sand and aggregates are added for additional strength.

To build the classroom, the team is using the CobBauge system – a type of composite wall comprising two grades of cob bonded together. A dense version is used for the outer wall, and a more lightweight version for insulation.

CobBauge was developed by an international research team led by scientist Steve Goodhew to help bring the material back into the 21st-century construction toolkit.

For this project, the clay-rich subsoil needed for the CobBauge walls was sourced from waste earth from the construction excavations for Tribeca, a life sciences development a mile away from the site, designed by Bennetts Associates.

For the insulative layer, the team originally planned to use hemp shiv in the mix, as is typical for CobBauge. However, they opted for local straw instead as it could be sourced closer to the site, therefore minimising the project's embodied carbon.

Earth sourced from nearby construction site 

"The clay came from down the road in Kings Cross, from excavations at the Tribeca site," Hardy explained.

"They excavated 20 tonnes of clay, and then it went to a brick factory called HG Matthews about 50 miles away. They processed it, and then we got it delivered back."

For many of its proponents, including design collective Cella, using waste earth from excavation sites in this way is one of the biggest appeals of using cob in contemporary architecture.

"If it cannot be reused on site for backfill or landscaping, the sad reality is that the excavated material, of which a lot of in London may contain clay, is sent to landfill due to the logistics around storing and redistributing," said Hardy.

She added that tests on waste earth's contaminants and properties are required to determine the suitability for use in construction, which can sometimes become a barrier.

"If we could identify large development sites from which vast quantities of excavated clay are coming out, we could then store this clay, process it and redistribute it to projects," Hardy said.

"However, this takes engagement with large-scale developers and contractors. We need the buy-in from all parties and for them to see the value both environmentally and socially in this amazing material."

"Cob is the most recyclable material"

During a tour of the site, the charity's builder Glen McDonald explained that the heavy outer wall of the classroom is the second version to have been constructed, as the first needed to be dismantled.

While that may sound wasteful, the materials were all reused, but sand was added to improve the walls' strength.

"The front section of the wall there, which is our heavy side, had to be taken down due to the wrong quantities of materials being used. We just used cob, no sand," he said.

"We were halfway through, two metres up, and I was saying to all our students, we're going to take this down now," he explained. "We got to the top, and we took it down, we put it over there in a pile, reconstituted it all again with sand and put it back."

McDonald said this offered a valuable lesson to volunteers, who included students from The Bartlett School of Architecture at the time, on how cob "is infinitely recyclable".

"Cob is the most recyclable material," said McDonald."We could just take the roof off, it'll rain and we can grow potatoes out of it," he said.

"In 30 years time, if we decide to build a wall again, we can build the wall up again."

Architect Jan Kattein said that building with cob "is about allowing yourself to experiment, to fail, to learn and to improve" in this way.

"Traditionally, drawings and calculations are produced and design decisions are made in response to known and documented material qualities and well ahead of starting on site," he explained.

"That is not really possible when you build with earth. Design decisions are made in collaboration between the architect, the earth builder and the structural engineer on site."

Once complete, the classroom will accommodate up to 30 students and look out over the site's gardens through a facade of reclaimed windows.

The CobBauge walls are raised on a concrete plinth and, once complete, will be mostly lined with clay plaster, with small portions left exposed. A roof overhang will shelter the walls from driving rain while providing a sheltered space for outdoor activities.

It will sit alongside a community kitchen and office buildings, which Global Generation is now focusing on constructing while the cob walls dry out.

The community kitchen is being built from sweet chestnut wrapped in green timber shakes, while the office is being made from bricks handmade and fired by the charity.

"The design for the Triangle Site is conceived to facilitate engagement and participation and this has informed all design decisions that we have made, from material selection and construction technique to layout, massing and arrangement," said Kattein.

"If we want to really turn the tide on climate change and biodiversity loss, we must be content to work with what we have. Using local earth is not an aesthetic choice, nor simply a pragmatic one, it's a philosophical choice."

The photography is by Global Generation unless otherwise stated.

Building with Cob

This article is part of Dezeen's Building with Cob series investigating modern uses of cob – an ancient building material made from clay-rich soil, straw and water – and exploring whether it can become a mainstream alternative to planet-damaging construction methods.

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