Continuing our series on architects’ details and the decisions informing their thinking, Knox Bhavan's Sasha Bhavan and Fergus Knox discuss the prefabricated timber cassette system they developed for a riverbank home, which is now being deployed on other projects
What drove your interest in developing an OSB prefabricated system on a river of all places?
We already had built a lot in timber and had informal conversations with Price & Myers about building off-ground timber structures. We have also worked for years with digital manufacturer BlokBuild, which came over one time and showed us a model of cassette system they wanted to investigate. We felt that March House would be a perfect project to develop the idea.
The client came to us with an exacting brief for a home in the Thames flood plain. We knew it had to be programmed in a particular way due to flood risk and seasonal site access down a narrow lane, which favoured off-site construction. We arrived at the idea of lifting the building off the ground by using the OSB cassette system, mounted up on a galvanised steel sub-frame. In effect, it’s just a few steel pins touching the ground, letting the river flow past or beneath it.
Being American, the client was used to the idea of balloon frame construction and had no prejudices against an OSB system build. Price & Myers pushed the structural and sustainability aspects of the UK-sourced material too. Maybe her being a gardener meant she was keen to push for a lower carbon footprint – but it really resonated for her.
Describe the system you used
We worked with BlokBuild and Price & Myers on the cassette system; we’ve had a longstanding relationship with both for years. We did the initial 2D design, which we turned into a BIM and then a Rhino model. This was then inhabited by BlokBuild’s bespoke script to generate the cassette forms. At March House these were made of two OSB layers separated by fin joists to give a 290mm section, with 250mm of Rockwool insulation sandwiched between.
We used the intermediate form of contract on the project but the OSB cassette design was treated like a contractor-designed portion. It was their responsibility to get the system certified and achieve the necessary warranties on it – the system at March House has a 60-year guarantee.
The beauty of the system is that it was used for wall, floor and roof. We did not have to pour a ground slab, so it’s good for the carbon footprint – we could even insulate soffits of floor cassettes.
What fixes one cassette to another?
A timber locking piece and locator. Once located, they were fixed together with simple nail plates. On March House this method was problematic. Plates would protrude above the OSB face and, once the Tyvek sheet was over it and they were invisible, it became difficult to place fixing batons for the larch cladding as they protruded too. It ended up making the building bigger on every face by about 5mm.
At a later project in Scatterdells Lane in Hertfordshire, BlokBuild decided to rebate the position of the nail plate so it ran flat with the face of the OSB. It also controls exactly where nail plates are fixed, which, when you are using the system to form internal partition walls, allows wiring and pipe runs to be better coordinated as you know the exact position of every plate beforehand.
Doesn’t that mean that you are developing bespoke details rather than a generic system?
Because it’s cut from a flat OSB sheet and a drawing, you can do whatever you need it to do with it. There’s an increase in labour with some elements but if the computer is planning it on a sheet, there isn’t really much cost difference in terms of fabrication.
Scatterdells Lane, for instance, is an evolution. The system we’ve developed there has roof tapers and internal walls and is more complex than March House. We’re starting to use the system architecturally. Eaves provide shading but we’re forming them out of cassettes. It offers more design opportunities and complexities, like an overhang over a doorway.
And what we are looking at with a new project in Bristol is aiming to be more Lego-like with it. We are developing door types, porch types and eaves details where you can mix and match elements. We are working very hard to keep the per square metre costs down. We are pushing for architectural expression but will likely get pushback from BlokBuild in terms of what’s deliverable for the cost. But it is an opportunity to fine-tune the system and can all be achieved without a six-act drama or being too expensive.
Could you imagine it working at four storeys?
It’s as much about fire resistance as anything. At March House there wasn’t any real requirement for it so we just have one layer of insulated plasterboard internally and a pattress lining. But Blokbuild is looking at that at Scatterdells Lane and the Bristol housing project, where we’re having to look at fire separation and means of escape.
It would be nice to make it work but all this is part of an evolving story. Each one of these jobs takes things a bit further. In terms of fire ratings, it's going to be about creating hybrid systems – perhaps teaming it with steel or glulam structure. It’s about playing to the strengths of the various materials to make the cassette system work in housing rather than just one-off homes.
But the OSB system is a cheap and sustainable way to build. Once the steel sub-frame was in, the use of the cassettes meant we had a watertight enclosure in just over three weeks. It means giving the contractor a lot of leeway to develop its methods, which means increased architect input in terms of clash detection and coordination. Ideally, you want to be handing over a building as a single model.
Sasha Bhavan is a senior partner and Fergus Knox is a project architect at Knox Bhavan Architects