How a changing riverside landscape became a test bed for forward-thinking modular design.
Designed by Knox Bhavan architects, March House was built using a prefabricated OSB cassette system which not only minimized use of steel and concrete but was, impressively, also made without any structural solid timber: the OSB cassettes providing strength, airtightness and insulation.
On a Thames-side site that regularly floods, this modular timber home has been designed for longevity by embracing the agency of the river. The single floor house was constructed using an OSB cassette system, developed through this project by structural engineers Price & Myers and BlokBuild, a digital manufacturing company, together with the design team.
Undertaking this collective step into the unknown relied on great trust between all parties – a trust built on prior collaboration. This groundwork came to fruition with the vision of the client, who sought an environmentally conscious design, integrated within its surroundings.
Nestled within a mature garden, on a strip of the river historically populated with holiday cottages, and which, until the eighties was accessible only by boat, the relationship between March House and its environs is striking. The house sits on a steel tabletop frame, raised high enough to account for the one-hundred-year predicted water level rises. This allows flood water to flow freely underneath the structure and minimizes water displacement. By accommodating rather than resisting the ebbs and flows of the river, this symbiotic approach to design has provided a sustainable, lifelong residence for its occupant, in harmony with the landscape.
March House replaces the client’s original two-storey house, which had suffered from flooding and damp. On a site that currently floods approximately every couple of years, flood resilience was a fundamental consideration to the whole design, leading to the idea of a ‘floating house’ – with the structure raised on steel stilts above the ground.
Project architect, Fergus Knox explains that there were a number of planning requirements to be accommodated, including building eight metres back from the river and maintaining a green corridor along the water’s edge. An initial proposal by Knox Bhavan had been rejected because, as a single-storey building, it had a bigger footprint than the existing house. There were concerns that, despite being on stilts, it could displace water and affect flood flow. This meant the design had to be compacted to reduce the footprint: everything needed to work harder in a smaller space.
The sophistication and precision of the modular design, however, was well suited to this necessarily efficient use of material and space.
A cruciform plan was created, with a long view running through the building, from east to west. Crossing this is a wide living space providing two key views: of the river to the north and Winter Hill to the south. This central area is opened up with a higher ceiling, creating a sense of drama. A spine wall loosely divides this space whilst allowing views around it. On one side of the building, an annex space was created which is able to be closed off, and on the other, the client’s kitchen, bathroom, bedroom and fitness studio.
The client is a committed gardener, and the planting was already well established. The design responded to the garden in a number of ways including with floor-to-ceiling and corner windows which maximise the immersive visual connection to the surrounding natural environment. To the side of the full-height windows, an opening portion with a door was created. Because of the large number of riverside flies, flyscreens were integrated into the joinery of the window frames. “With the house built on stilts, we were worried you’d miss the connection between the planting and the structure” Knox says, “but it was actually even greater because of the amazing views from inside that feel as though you are in the treetops.”
Measurements of the client’s mid-century modern furniture collection were incorporated into the plans, meaning they were able to be fitted snugly into their positions when the house was built. The internal joinery was also matched to her existing joinery, creating seamless lines for the eye to flow through the space. A variety of internal and external wood products were used – such as oak paneling and floorboards, stained larch cladding and Cumaru decking – chosen for their aesthetic sympathy to the landscape and the client’s existing furniture as well as their weather resilience.
An opportunity for collaboration
Describing how the various parties came together to develop the construction system used for March House, Knox explains how two companies the studio work closely with – Price & Myers and BlokBuild – had together been investigating a new prefabricated OSB cassette system.
The March House client came to Knox Bhavan having seen a previous house they had designed, and appreciating the expansive ground floor plan, easily dividable around a central form, that the SIP system they used had enabled. This recognition of the potential of prefabricated modular design on the client’s part presented an exciting opportunity to put into application the prefabricated OSB cassette system that Price & Myers and BlokBuild had been developing.
Accuracy, handleability and strength
The principle of their system is to make hollow objects, filled with insulation, that become the building blocks of the house. “The idea of the cassettes is that they slot easily together”, Knox says. “A digital clone is made of the planned building, and they are cut from OSB boards to very tight tolerances which fit, like a jigsaw puzzle, together.”
The variously shaped cassette components were designed by Price & Myers to make up the floor, walls and roof. Thesewere then cut from sheets of SMARTPLY OSB and assembled, using digital assistance, by BlokBuild at their factory in Hull.
By systematizing and coordinating the templates on the OSB sheets the team were able to reduce wastage on the production line. The accuracy of the digital modelling and cutting process also meant that later, at the installation stage, complex junctions at the eaves and the sills were reliably tight-fitting, resulting in improved thermal bridging performance.
Tim Lucas, a structural engineer at Price & Myers, explains that this was one of the first projects in which they didn’t put any solid timber into the walls. Most cassette systems incorporate timber sticks in the framework, as OSB can lose integral strength if it gets wet. However, if the cassettes are kept dry by waterproof coating, he explains, they can be used without, thus reducing material use and making more space for insulation. A staggered arrangement of cassettes for the walls and floors was also developed, with the boxes overlapping and joined together: notching into each other for strength.
It was important that the wall and roof modules were designed to be small enough to be comfortably handled and delivered in small vehicles. Once onsite, the cassettes were then able to be manually lifted and fixed together using specially designed chunky plywood pegs that aligned the boxes, pulling them tight. The boxes were then screwed in place using nail plates to make them more resilient.
The benefits of the prefabrication system were also embraced early on by the project’s main contractors, Philliam. A narrow access road and limited site space for storage, along with a real risk of flooding, made it difficult to keep large quantities timber on site. Working with components able to be delivered in sequential batches, using mostly small vehicles, had obvious logistical advantages.
With highly accurate parts that could be relatively simply installed, the building’s superstructure and enclosure were able to be erected quickly: within less than a month, the walls and floor were in.
“OSB is a humble material that isn’t properly understood structurally. We are working to understand it better.”
Tim Lucas, Price & Myers.
Refinement and evolution
Lucas notes how March House served as a springboard for evolutions and refinements in the construction process. He recalls issues occurring from the slight protrusion of the nail plates used in March House which have since been overcome by BlokBuild routing slots into the cassettes so that the plates can sit flush. To reduce the size of the floor cassettes, in order that they are also handleable, Price & Myers have gone on to develop a smaller, stressed-skin box for the floor components, with internal struts providing increased strength and flexibility.
As well developing the structural components, Lucas has undertaken research into the whole ecosystem of this approach, considering how it can become a regenerative process that can be locally adopted, adapted and utilized at scale. “I think there is great potential for this OSB system” he says, “for the reuse of waste wood, for application in social housing, for local manufacture.”
By bringing together the ecologically sensitive and pragmatic vision of the client with the reality of the landscape, the innovative ambitions of the designers and manufacturers, and the commitment and skill of the contractors and build teams, March House is not only a remarkable achievement in itself, but an inspiring prototype for future building.
Read Designing Timber issue No 2 here