Simon Conder Associates drops a prefabricated home onto a former warehouse at London Fields
A two-storey building, designed by Simon Conder Associates, has been added above a three-storey Victorian warehouse in London Fields, east London. The extension, providing a home for the designers who own and work in the original building below, was prefabricated to keep the budget low and avoid disruption to their business and neighbouring properties.
The 255-square-metre project employs a robust architectural language that responds to the industrial character of the original building, respecting it without attempting to mimic. The externally expressed, loadbearing, planed larch glulam frame is infilled with sawn horizontal weatherboarding, finished in a matt black preservative stain. The timber columns are set out on the grid of the existing window openings below, and the external walls between the frame members are lightweight, super-insulated panels, reflecting the limited loadings that could be supported on the original nominal foundations.
Openings on the north elevation were minimised to reduce heat loss and overlooking of adjacent properties, while the large windows on the south and east elevations maximise sunlight and solar gain and exploit the views. An open-plan top floor accommodates the main living spaces, which open onto a planted loggia at the eastern end, while the floor below contains two bedrooms, two bathrooms, a studio and utility room. The new building can be entered either through the design studios below or via a new dedicated lift at the rear of the building, which ensures that the accommodation is fully accessible.
The structural frame and lift shaft were made from European larch PEFC glulam with highly insulated timber cassette panels for the external walls, roof and internal floors, all of which were prefabricated in South Yorkshire. The structural frame arrived on site with pre-fitted concealed galvanised connection plates that allowed for rapid construction.
The existing structure consisted of timber floorboards on joists spanning between steel beams, that in turn spanned onto brickwork walls of at least 13.5 inches thick and incorporating ten 18-inch thick piers. The existing stair core, located at the centre of the building, was flanked by 215mm block walls. The nominal foundations drove the structural strategy for the lightweight roof extension. Historical data indicated that the building would have been designed to accommodate at least 168lb/ft², equivalent to 8.0kN/m, suggesting that the original use imposed far greater loads than those of a design studio.
The roof extension consists of timber portal frames connected to a perimeter ring beam at the existing roof level that is tied down to the gable and party wall below with reinforced concrete downstand walls. In addition, a horizontal steel truss frame is cast into the ring beam to transfer wind loads to the masonry walls. The portal frame structure comprises 300mm x 140mm wide glulam columns supporting 300mm x 140mm wide glulam beams. The beam-to-column connections were formed using steel plates installed in slots formed in the timber beams and columns. The design involved an assessment of progressive collapse, as the new steel frame at the existing roof level needed to potentially accommodate the weight of the new structure in the event of collapse due to accidental loading such as from a fire.
The extension was conceived as a highly insulated, lightweight structure to minimise heat loss using a ‘fabric first’ approach. Openings on the north elevation were minimised to further reduce heat loss, while the openings on the east and south elevations were increased to maximise both sunlight and passive solar gain. The double-glazed units have a U-value of 1.46 Wm²K, while external shading on the east and south elevations helps prevent overheating.
A simulation by environmental engineer XCO2 confirmed that the typical construction junction details achieved psi-values below the relevant Accredited Construction Detail (ACD) values within SAP 2012. Ventilation is largely based on natural stack and cross ventilation, while trickle vents within the windows allow for background ventilation during winter months. The prefabrication method reduced construction time, material waste and carbon dioxide emissions during construction. The structural frame was constructed from European larch PEFC glulam sections with Scandinavian sawn FSC weatherboarding as infill cladding.
Simon Conder Associates