Casting off

Origin story: The Rock House

Gort Scott's Joe Mac Mahon on a project that started with a walk in the woods

We were invited to enter the competition for this private house in 2012. It was the last plot on a development in Whistler, and it was quite a challenging site, on a rocky outcrop overlooking Alta Lake.

We really wanted to win the project so Jay [Gort] and Fiona [Scott] took a gamble and flew out to Canada. Getting to know the site like that was probably what made the difference to us winning the competition. The essence of the design stems from that first visit and the experience of walking up through this rocky promontory.

You work your way through the woodland, and there’s a series of changing horizons, as different views reveal themselves. There’s Whistler Mountain behind you, and Blackcomb Peak and Wedge Mountain through the trees. It culminates at the top of the rock where you see Rainbow and Sproatt mountains in front of you across the lake. The design of the home was trying to replicate that journey.

The prow of the rock was absolutely sacrosanct, so we cut down into the ground behind that, excavating by hand. We like to quote Frank Lloyd Wright: “No house should ever be on a hill. It should be of the hill. Hill and house should live together with each the happier for it.” Obviously, the best material to achieve that with was concrete. From an early stage we had the idea of a concrete base that would then hold the living spaces floating above. These would be clad in black-painted cedar – a way of connecting it to the surrounding forest.

We were really lucky to find a local contractor who loved working with concrete. We worked very closely with them from early on to design the formwork for the whole building. The house has a very complicated geometry so we had to carefully plan out every junction and joint line – we basically drew every single board, so we could get it spot on. We figured out that the most efficient way to set out the building was in increments of 3½ inches, an off-the-shelf size for the cedar planks. (We had to learn how to work in Imperial measurements – a first for me.)

This follows through to the stairs, which are 7 inches, and all the way to the tops of the chimneys. We also had to plan all the light fittings and switches at a very early stage as the electrical conduits are cast into the concrete. It was important that the tone of the concrete fitted in with the rock, so we lightened it with titanium dioxide.

Internally, we decided to strike it in such a way that it would leave strands of the cedar in the concrete. We did a test where a little bit of cedar was left behind and we really liked it. It’s not very noticeable at first, but at certain angles you just get this golden shimmer. It’s especially nice when it catches the light. The only concrete interior that isn’t textured is the double-height basement gallery space, which acts as a hall connecting various parts of the building. Here, we used phenolic ply formwork to give a very smooth finish.

All of the walls and the slab were cast at once, with a self-compacting mix pumped from below. When we started working on the project, low-energy design wasn’t high on the agenda in this part of the world, partly because they had such cheap hydroelectric power. But together with the client, we wanted to push it to be as sustainable as we could achieve. We designed the building to Passivhaus principles, but with natural ventilation. Big overhangs shade the windows in summer and a geothermal field under the house supplies hot water and cooling.

The concrete structural system has 4 inches of rigid insulation between an 8 inch inner structural leaf and 5½ inch outer leaf. Thermally it’s brilliant, it really helps to balance the house – which is a challenge in a climate that gets very, very hot in summer and very cold in winter. On a hot day, when you go down to the lower levels it really feels cooler, like you’re in the belly of the rock. 

Joe Mac Mahon is an associate at Gort Scott

Photos Rory Gardiner