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Up on the roof: prefabricated rooftop house by Simon Conder

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Simon Conder Associates has designed a two-storey 255m² house which sits on top of a three-storey Victorian warehouse in London

This house forms a new home for the owners of a design firm, which has its studio in the warehouse conversion below. It was devised as a prefabricated, two-storey roof extension, both to keep the budget down and to minimise disruption to their design business and neighbouring properties. 

The extension, clad in black-stained timber weatherboarding, consists of timber portal frames comprising 300mm x 140mm glulam columns supporting 300mm x 140mm glulam beams. They are connected to a new perimeter ring beam at existing roof level. 

The project uses a robust architectural language which is site specific and responds to the industrial character of the original building in a way that respects the existing without attempting to copy it.

Sca black house 07

Sca black house 07

Source: Paul Smoothy

Architects’ view

This architectural language consists of an externally expressed, load-bearing, planed larch glulam frame with an infill of sawn horizontal weatherboarding. The timber columns are set out on the grid of the existing window openings below to form a visual link between new and old. Between the frame members the external walls 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 those on the south and east elevations were increased to maximise sunlight and solar gain and exploit the views.

An open plan top floor accommodates the main living spaces, which at the eastern end open out into a planted loggia structure, while the floor below contains two bedrooms, two bathrooms, a studio and a utility room.

The new building is finished in a matt black preservative stain and can be accessed either through the design studios below or via a new dedicated lift on the rear of the building. This ensures that the new accommodation is fully accessible. 

Sca black house 24 new residential floor plans

Sca black house 24 new residential floor plans

Source: Simon Conder Associates

New residential floor plans

The lightweight timber framed ‘house’ is supported by a brick clad reinforced concrete ring beam which is cast onto the top of the existing external walls. The ring beam is braced by a horizontal steel frame, with timber joists spanning between the steels to form the lower floor of the house. Warmcel recycled paper insulation fills the void between the existing warehouse roof and the joists.

European larch PEFC glulam columns and beams are connected by recessed galvanised mild steel plates to form the structural frame of the house. Super-insulated external wall panels span behind the columns and are sealed with a Solitex breather membrane. Between the columns, the walls are clad in Scandinavian FSC feather-edged sawn redwood weatherboarding, finished in a matt black Falun preservative stain. The Velfac windows are black polyester powder-coated aluminium with timber side hung casements.

Super-insulated roof panels sit between the larch glulam roof beams. The Prelasti EPDM single-ply roofing membrane is laid to fall and dressed under and lapped over the larch glulam coping beam.

The internal envelope of the house is lined with a Visqueen vapour barrier to minimise air loss. Two layers of fire-rated plasterboard line the external walls and lower floor ceiling to provide 60-minute fire protection. Timber battens create a dedicated service zone behind the finished plasterboard wall surface.

An American white oak floor runs through the entire house below which is laid with underfloor heating.

James Carefoot, project architect, Simon Conder Associates

Sca black house 28 external fabric detail annotated + clean 1

Sca black house 28 external fabric detail annotated + clean 1

Source: Simon Conder Associates

External fabric detail

Project data

Start on site July 2015
Completion October 2016
Gross internal floor area 255m²
Procurement route Traditional
Form of contract JCT Intermediate Building Contract 
Construction cost Undisclosed
Architect Simon Conder Associates
Structural engineer Fluid Structures
Environmental engineer XCO2 Energy
Quantity surveyor Listers
Contractor Famella
Timber prefabrication sub-contractor Constructional Timber
CDM: principal designer Orsa
Approved building inspector Assent Building Control
CAD software ArchiCAD
Estimated annual CO2 emissions 3,638kg/m²

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