Structural facade in stone

On going project - Structural facade

Extract from Architect Journal


Amin Taha’s Groupwork has started building a 10-storey block of flats in Hampstead, north London, with load-bearing lava stone

The scheme, which contains 22 flats as well as lower-level office space, won planning permission in 2016. However, developer LineaHomes only carried out demolition of the existing building at 317 Finchley Road before selling the site .

Now the 3000m2 building will finally be built, using approximately 725 tonnes of basalt from Mount Etna, in Sicily. Basalt for use as a structural material is produced from slowly erupting, rather than exploding, volcanoes.

The Groupwork building will deploy the volcanic rock across 596 columns and lintels. According to Taha, the project will have around 80 per cent less embodied carbon than a steel frame clad in stone, and 55 per cent less embodied carbon than a concrete structure clad in stone.

Groupwork selected basalt after testing its fire performance against that of limestone at the British Research Establishment. It found that a basalt exoskeleton required roughly half the amount of stone as a limestone exoskeleton due to its superior resistance to internal fracture during a fire.

Taha said the cost of stone construction was around 15 to 25 per cent less than cladding steel frames or concrete frames, when adding up quotes from specialist subcontractors. He urged other architects to use stone as a means of lowering their carbon footprint.

However, in a mini-guide to using stone (below), the Groupwork founder also warned that design-and-build contractors sometimes manipulate tender prices for stone buildings upwards – so they can instead push to use concrete and steel, where they spend less money on subcontractors.

According to Taha, the Finchley Road project is the ‘first 10-storey, loadbearing self-finished stone structure without steel reinforcement or secondary structure since the last stone cathedrals’.

How does stone compare to manufactured timber in terms of embodied carbon?

It would be a false dichotomy to say we need to choose between stone or timber, or for that matter aim to eliminate steel and concrete altogether. It’s more a matter of understanding the structural, fire, cost, embodied carbon, textural/tactile and visual properties of all materials and making an intelligent choice.

If there were no regulations against timber at all and we had a ready supply of sustainable forests close to manufacturers and sites, then almost all building could be carbon negative. Yet timber cannot perform the same structurally as stone without becoming more and more engineered and increasing its embodied carbon beyond stone on some structures – nor would it be would be much good below ground.

We also do not have an unlimited timber supply: 1ha of farmed forest will provide around 10m3 of structural timber after 20-30 years. A three-bed timber-framed house will need 12.2m3 of sawn timber, 4.1 m3 of OSB and 5.5m3 of timber waste for woodfibre production – 21.8m3 overall.

We could still build with negative embodied carbon (not a net zero structure that we currently aim for) if we combine stone and timber – 1m3 of stone literally being that from the ground we stand on, the cooled magma round the earth’s molten iron core being the crust that is the land, hills and mountains we inhabit.

We then use it like termites, just moving one bit of ground from here to there without the need for heavy processing and energy use.

Imagine: we currently crush, burn and chemically mix limestone to make cement for concrete that then has 40 per cent of the strength of its original strength, needing steel to reinforce it. Why do we use concrete then? Like other Second World War innovations, such as processed foods made with lots of sugar and fat for longevity and energy, we’ve made them very cheap and convenient with parallel problems such as obesity from its ubiquity.

Source your timber carefully and mix the overall superstructure with stone, such as an exoskeleton and the overall result will have a negative embodied carbon footprint – as negative as a concrete frame equivalent is positive. The thought experiment leading to construction going from a net contributor of atmospheric carbon to a carbon sequestration industry. Furthermore, it’s cheaper: see both EightAssociates (sustainability engineers) and Jackson Coles (QS) reports.

Architects : Groupwork 

Engineers : Webb Yates

Manager : Ernest Park