Despite its ethereal appearance, the garage gains strength from a thin galvanised steel roof. Making, transporting and installing it were BARD's key challenges
‘Windyhill is curiously paradoxical; it combines the traditional and the unprecedented, the casual and precise, the bleak and picturesque,’ said architect and academic Robert Macleod in his 1968 book on Charles Rennie Mackintosh. The 1901 house, built for wealthy merchant William Davidson, is outside the village of Kilmacolm, west of Glasgow, and looks across to the hill that gives it its name. Young Glasgow practice BARD, given a tour by serendipitous invitation of its current owner, left not only having seen a key Mackintosh house, but with the enviable commission to replace its 1980s carport. Practice principal Ruairidh Moir discusses its respectful design in concrete and galvanised steel – ‘more like jewellery-making,’ which sought at every stage to honour the ethos of the master.
Why did the client want to change the original carport?
While the client had invested heavily in the restoration of the house itself, he had also inherited the carport, which wasn’t very sympathetic. It was on the public-facing side of the Category A-listed house, looked untidy, and was top-heavy, interrupting key vistas to it. It took us nine months from being given the commission to arrive at a solution of a concrete ‘basin’ with steel stanchions supporting a galvanised steel roof atop a glass clerestory. The design gained consent quickly as there was lots of pre-app discussion with both planners and the local community.
The 1980s carport roof was an unwieldy structure, sitting on 100mm square steel posts on pad foundations. An old sewer from the road to the house ran under it and we needed to remediate that before establishing new perimeter foundations for our more substantial 34m² structure, which consisted of different height L-shaped precast concrete sections. The largest of our ‘basin’ sections weighs 12t and sits on the foundations with projecting rebar, onto which we placed a steel mesh and then cast a 200mm thick in-situ concrete floor slab. Sections are grouted where they meet at their sides. We were concerned at first that British Standard concrete tolerances of +/- 6mm wouldn’t marry up with the precision of steelwork we were installing, but the fabricator did a great job and got the tolerance down to zero. We cast small channels into the concrete floor to allow water to drain away.
What drove the material choices for the roof?
We were inspired by Mackintosh and really wanted an integrity to the material. We didn’t want zinc and although we did consider using lead or copper, these would have required a substrate, which would increase the roof’s overall thickness.
This mattered because it was important to keep the roof thin, as we wanted to keep the height down in order to optimise views over to the garden’s original Mackintosh dovecote and the house. We also wanted the roof covering to be the structure and so gravitated to using 6mm-thick plate steel. We thought about Corten but thought it too self-conscious; galvanised steel just felt right – it had a material honesty all its own and a quietude that seemed to respect the house it was next to.
The roof design feels highly deliberate
It’s quite elemental; nothing more than two sloped triangular trays of galvanised plate steel that drop 300mm across the length. To derive its intrinsic strength, each roof tray is made of five 1600mm-wide sections with a 105mm upstand. In addition, a discreet 35mm-high fold has been engineered into each galvanised panel, to lend additional stiffness. Each upstand panel is clamped to the next using dome-head bolts and nuts, and then an aluminium capping section runs across the top of adjacent upstands, to close them off.
Each of the two assembled roof sections weighs 3t and bears down on 15 galvanised steel stanchions that sit on bases running around the concrete wall behind the clerestory glass line. The stanchions were partially inspired by Mackintosh’s ‘Tree of Life’ tapered timber columns seen in Windyhill. Made up of two ‘T’ sections of steel, each tapered stanchion is set at 280mm-1990mm, to allow for the roof slope toward the gutter. A flange at the top of each stanchion engages with a roof bolting plate with pre-positioned tabs into which it bolts.
How did the need to galvanise the roof affect the design?
Ideally, we wanted to dip the roof as two sections, but there wasn’t a galvanising bath big enough for that in the UK, so that led to the idea of the connected profile trays. It was all highly considered, to the point of making bolt holes 0.2mm larger to allow for the galvanising dip, which would reduce the overall diameter of each hole. We even told the profilers which side we wanted the hole cut on as there’s a ‘flaming’ effect on the surface that you cut into. The fabricator was a shipwright, and the construction methodology was decided in its workshop in Cumbria, where the roof was assembled and checked for dimensional integrity before being taken apart and sent off to be dipped. It was then brought back to the workshop and reassembled. The sections were loaded on the UK’s only tilting trailer and required an escort to get to Scotland.
How did you install it?
It was craned-in and required a scaffold to be set up within the concrete walls to support the roof while it was being attached to the stanchions. This involved the fabricators gently hammering timber wedges between the roof and scaffold to ensure levels were exactly right. All the stanchions were sitting on jacking bolts so we could jack them up to get them plumbed and levelled in before bolting them on and removing the wedges to allow the stanchions to assume the roof’s full load; the torsion of each bolt was metred too. It was a delicate operation but as the roof arrived on site pre-assembled, it was all done in the best part of a day.
How did you achieve that incredibly fine roof edge detail in a place called Windyhill?
Our engineer, Tom Hay of Pluton Engineering, was fantastic. The blade detail on two edges is supported by triangular fillets on its underside that run out from the perimeter stanchion line to give it the requisite stiffness. One of the blades even holds a 103mm square box gutter, formed from folded steel plate with triangular web fillets on its side, which was lifted into place and just bolted onto the roof’s perimeter steels. There’s also a 150mm by 150mm steel hollow box section running across the opening where the garage doors are. We did our level best to hide it behind the doors when they’re closed, but it does a lot of work to give the roof the overall stiffness it needs. The gutter chains were from a local shipyard and have lovely, time-worn patination – we’ve yet to cast the concrete gulley detail that they’ll drain into.
What about the glass clerestory and mullions?
We looked into etched glass and printed coatings, but the effect seemed too mechanistic, so we consulted glassmakers and decided on a special grisaille hand-applied effect brushed directly onto the glass, followed by an extractive process, after which it’s fired and strengthened. The size of the panes was dictated by the building’s module and the size of the kiln that we could use to fire them. What we love about this hand-applied process is that the glass has a life all its own and creates a golden, iridescent glow inside the garage at the end of the day.
The mullions are a combination of brass and bronze and formed as simple T sections, which the single-glazed, bronze-framed panes merely butt up to on the inside face of the garage. In the roof package there was a hardwood cavity detail to give a softer edge for the glass frames to sit into, while allowing the roof to expand and contract. The sill is a hardwood section and on the inside face there’s a brass detail on the lower inside edge. The glass fits into this and is finished with a nosing and a bead detail, which unifies the glass elements into a readable whole.
How is the roof soffit finished?
On the inside of the roof is a 50mm-thick layer of spray foam insulation and a 15mm-thick plywood soffit painted deep purple. We wanted this surface to be as singular as possible and did think about vitreous enamel panelling but buckling risk and cost put paid to that idea. Plywood allowed us to sink strip LED light channels and diffusers into junctions between boards, mirroring the roof. Colour-wise, white felt too clinical and we didn’t want it to mimic the roof. At one point Mackintosh had the drawing room ceiling at Hill House a deep plum, so we thought we’d try it here.
How do the doors work?
They sit on ball-bearing pivots and are restrained at the top on the box beam. The doors are box section frames with diagonal tension rods used as cross-bracing and set with long sections of the painted glass. The cross-bracing couldn’t be wire as the doors needed additional stiffening during the opening and closing action. They open as two-thirds and a third rather than at the centre, so it's satisfyingly unbalanced, with their pivot 400mm from the wall to further reduce the scale of the doors as they swing out. Had it been half and half, it would have opened very close to the house’s catslide gable.
In all, we are really happy with how it turned out. We felt it wasn’t just David Cairns, our client, that we were working for, but for Mackintosh over his shoulder. For us, it was a labour of love; not only does it have the elemental simplicity we sought, but, with the glasswork we agonised over, an ethereal quality. If you look, the mark of our fingerprints is even preserved on the doors.