Harrow Arts Centre’s new Greenhill Building combines a low-carbon CLT structure with a no-nonsense fibre-cement cladding that pays tribute to its site’s agricultural legacy, explain its architects Chris Dyson and Mathew Witts
The Greenhill Building at Harrow Arts Centre (HAC), by Chris Dyson Architects, is a fully cross-laminated timber (CLT) two-storey events space in north-west London. This method, which was more typical in education and commercial buildings when it was proposed in 2019, was chosen as an affordable, low-carbon alternative to structural steel. The entire main build, lightweight and fabricated off site, was completed in under nine months.
The building replaces a crumbling Portakabin to the rear of the arts centre’s grand Grade II-listed Elliott Hall. It provides the spaces for yoga classes, art clubs and celebrations which the local community asked for during consultations, and supports HAC’s financial sustainability ambitions. The design is inspired by the site’s legacy of agricultural buildings, which informed the L-shaped plan around a ‘yard’ and the red corrugated facade, made from a fibre cement more often found on farms.
Agricultural fibre-cement cladding
The building is clad in Eternit’s standard corrugated fibre-cement sheets, which were cut to shape and installed on site. Panels overlap by 150mm, with visible joints masking the seams beneath. There are two sizes specified: a standard height/width, and wider version.
We used a long panel on all the corners to stretch to the centre of the windows, then standardised panels everywhere else. In that way, it looks as if every structural bay is divided by two and the joint is visible at the centre line of each bay. It’s a straightforward material to apply (and remove for repairs), which is why it’s so popular for farm buildings.
Installation required no specialist teams or equipment. Our carpenters fitted the panels directly, screwing them onto battens with great care and, despite it being such a basic material, with careful attention to detail. This included dipping the silver screw heads in paint matched with the cladding.
The facade system layers CLT structure, insulation, timber fire break, and a waterproof membrane beneath the corrugated exterior, and the junctions are capped with powder-coated aluminium to provide durability and a more polished edge.
Windows to optimise natural ventilation
The building was designed for a high degree of flexibility. Although we started with larger windows on the first floor, we reduced them to enable the main upstairs studios to be subdivided along the central beam if needed. Each space would then have a north-facing rooflight and an openable window, avoiding the need for mechanical ventilation.
To bring extra daylight into the building, we designed a large window to follow the profile of the staircase, using only two irregular-shaped panels. The project was won through a competition, but planning and delivery coincided with the Covid-19 pandemic when material costs rose significantly and the design team had to be pragmatic and inventive to keep the building within the council’s budget.
The windows are standard, off-the-shelf products (Westcoast System aluminium/timber composite), while the rooflights are Velux. Because these were integral to the whole-building natural ventilation strategy, they weren’t value engineered.
Standardised drainage components
The roof is coated in grey Sarnafil membrane, and collects and channels water into a spitter above the gutters that drains straight into a new tank we created to address a longstanding problem with flooding on the site.
Downpipes, like the facade’s other metal components, were of particular interest to Harrow’s design and conservation officers. At first they asked for all external metalwork to be white, the colour used across the rest of the HAC campus, but we felt the cladding of our building, being agricultural and bolder in colour, did not suit this more Victorian, domestic treatment. We were also aware the cladding would fade over time so we wanted a neutral colour to complement the facade as it weathered – as a result, we chose anthracite grey.
We also wanted all the metal components to match each other. The facade had to be cost-effective, so restricting ourselves to a handful of colours and textures gave us more control over its final appearance. To standardise the project’s metal components, we specified downpipes of PPC-coated aluminium, the same material as the parapet cappings and the external faces of the windows.
Low-carbon CLT envelope
The form of the building is a pure expression of its design and structure. The choice to construct it entirely from CLT was driven by cost efficiency and the low-carbon environmental strategy.
The structure was prefabricated to precise specifications and rapidly assembled inside a cube of scaffolding – it went from a concrete slab to an enclosed structure in around two weeks. This accelerated timeline cut construction costs by minimising on-site labour, as a conventional brick and steel frame building would have added five months.
Yet we had to find savings elsewhere. Working with engineers and fabricators, we managed to reduce the amount of timber by 25 per cent by adjusting dimensions of beams and heights. To cut costs further, we used non-visual grade timber, which hasn’t had its knots and stains smoothed out. Outside, this is covered in cladding, but internally, we revealed it to show the life of the wood.