Architects Kaner Olette explain how green oak craftsmanship and timber detailing helped restore a beautiful listed water mill after a devastating accident laid waste to it.
On the night of February 7 2018, a fire broke out at Bere Mill, destroying much of its house and surrounding buildings. But there was to be a second act to the story. As well as being a private residence, the gardens of the Grade II listed mill were opened periodically to the public. Therefore it was in the wider interest to find a way to restore the historic structures as sympathetically as possible.
The green oak frame in the West Barn supports timber stud walls above the pre-existing masonry.
Architects Kaner Olette were asked to redesign and rebuild the mill (the building above the water channel, or ‘mill race’, which powers the mill machinery), and the west barn. They already had a good relationship with Bere Mill’s owner. They had previously designed a breakfast room extension to the main house and a photography studio within the barn – the latter sadly lost to the fire.
Rethinking the layout
The client was keen to re-think the barn’s layout and uses to bring some positive benefits. A three-storey open gallery space was planned at the south end – partially public when the gardens are open for visitors and partially private for parties or cinema use.
At the north end a series of smaller spaces incorporate a Japanese cedar tub, plant room, kitchen, toilets and a painting studio. On the top level would be a small self contained apartment for visitors.
A green oak frame to provide the structure
Early on, a new green oak frame was decided upon which would run through the mill and barn. Vertical circulation would be provided via a bespoke metal staircase and bridges.
The green oak frame informed the internal materials within the barn itself: traditional hand-cut joinery, oak sarking boards and hemp lime plaster for the retained masonry.
The frame was prefabricated and supplied by the Green Oak Carpentry Company – experts in their field and three times Gold winners at the Wood Awards. The frame had to be constructed to very tight tolerances, but also had to work within an irregularly shaped building.
“It had to take into account some of the details – the walls were far from being all straight and true,” explains Bart Smith, Project Architect.
He was very impressed to see Green Oak Carpentry at work in their workshop.
“You are trying to fit a rectangular frame into a non-rectangular building.”
“At opposite corners it is very tight. The walls are bending in 3 dimensions as well – some lean out quite dramatically.”
Working with green oak: expect some movement
Kaner Olette had worked with green oak before but never as extensively as this. They cite movement and some cracking as issues that you should anticipate. Natural plasters, they suggest, can accommodate some movement.
The original frame started at first floor and was built off the masonry walls. Due to the fire damage the structural engineer required a free standing frame with occasional ties back to the masonry. Working closely with Green Oak Carpentry, these ties were carefully concealed.
It’s also important to consider the impact of any direct glazing when it comes to movement. There is a glazed screen at the top section of the barn, and the detailing was carefully worked out to allow for this.
Hemp plaster being applied to the existing masonry walls.
A mix of 3D modelling and gut feeling
Green Oak Carpentry use a lot of historic techniques to make the connections, but in a 21st century context. To ensure those tight tolerances, they developed a complete 3D CAD model of the frame which Kaner Olette had to rigorously cross check against the digital survey of the remaining building fabric.
But, as Michael Kaner, Project Director, points out, medieval oak barns weren’t calculated in the same way as modern structures. So Green Oak’s engineer had to rely, to an extent, on experience and gut feeling in certain areas.
“It was a really interesting mix of hi-tech and rule of thumb,” Michael remembers. “There was an element of ‘we need a bit more strength here, let’s put in a strut and see how that performs.’”
Rules of thumb were clearly successful. The frame went in quickly and unproblematically.
Good shoes for a frame to stand on
The site also slopes, with quite a dramatic level change from one end to the other. This was useful in some ways, as insulation could be more easily added where required at floor level – raising the floor where it was lower. But the oak frame required a level surface.
“We introduced plinths throughout,” Bart explains. “Like a contemporary version of the saddle stone that the oak columns would have originally stood on. They are steel shoes, each one a different height, encased in concrete.”
Keeping up appearances
During fabrication, transportation and installation the finish of the oak frame does get marked. The team and the client decided to lightly sandblast elements of it within the main barn, but left it raw in the mill race.
“There was also a short period where the oak was exposed to the elements, which led to some acidic tannin leaching,” Bart recalls. “That’s something to be aware of. It stained some of the existing brickwork, but you can easily clean it off, if dealt with quickly.”
The timber frame is assembled.
The sprinkler system
The installation of sprinkler systems throughout the west barn and in an area of the mill race was considered a priority. It was actually quite hard to find an installer, given the increased demand for sprinkler systems within the housing and commercial market at the time. Also, the scheme would need special care in its detail due to the sensitivity of the listed building.
“A ‘wet system’ was installed rather than the less intense mist system,” Bart tells us. “It’s more typically found in commercial properties. There needed to be rigorous co-ordination and setting out to get the pipework around the structure – there aren’t many voids.”
Sprinkler installers prefer to have at least a half-metre ceiling void to use. So it was decided to keep the pipework visible with a series of galvanised pipes that are exposed. Those in the unheated mill building required thermally insulated lagging.
“The pipes generally follow the purlin lines, so sit coherently with the structure,” Michael states. “Historically, there always was a lot of mill machinery on display – pumps and generators for instance. So it was felt that the exposed pipes were appropriate for the context.”
The sprinkler system also allowed more flexibility to the layout, permitting the design to feature an open staircase from the second floor apartment in liaison with more traditional fire detection and alarm systems. The system tests itself automatically each week.
Valuable learning
Kaner Olette evidently learned a great deal from this project. The care and attention they brought to it comes across very clearly in their account. The project as a whole has won an Architects Journal Retrofit Award and has been recently shortlisted for a RIBA South regional award. More importantly, they have helped give an old building a deserved second life.