House in a Garden, London

Introduction

Tucked behind two rows of tall 1840s Victorian villas in Notting Hill, London, is a modern single-storey house with a truly dramatic roof, its exposed timber frame rising in an elegant double curve to a glazed oculus. The house stands on a site formerly occupied by a dilapidated bungalow, hidden away and reached only by a narrow passage between the villas. But the bungalow could be seen by the architect Gianni Botsford, from the window of his upper floor apartment and in 2010, intrigued by its possibilities for development, he bought it.

It was a project which came with almost overwhelming constraints: severe constrictions on access and, in particular, the need to achieve privacy and light on a site overlooked on three sides by tall houses, while avoiding intrusion into the views and outlook of close neighbours. It meant that the new house had to be single storey, with extra accommodation achieved by creating two basement floors. To control and distribute daylight down to these two lower levels, they are flanked by light wells and gardens, which have been modelled using solar analysis tools.

With such restricted access, the construction of the basement floors was especially complex; the piling to the perimeter was followed by the casting of the concrete floor slabs from the top downwards.

The ground floor is an open plan pavilion, a living, dining and kitchen space, glazed on both sides and sheltered by the curved roof, its exposed timbers rising to the oculus, which is positioned to receive maximum sunlight throughout the day. From the living pavilion, a staircase descends to the basement containing a large master bedroom and bathroom with a smaller guest bedroom and bathroom, each lit by adjacent lightwells. The lower basement houses a ten metre swimming pool and a large flexible space which could be used as a gallery or cinema. Daylight from the lightwells is amplified by the use of light and reflecting surfaces such as copper and marble.

Timber is used throughout the project. Botsford describes the concept as ‘a diagram of timber -lined “internal” spaces (living rooms and bedrooms) adjacent to marble lined “external” spaces (wet areas, pools, and courtyards) juxtaposed with stainless steel and copper lined doors and cabinetry to provide an intensely rich experience of materials and atmospheres tempered by light’. Externally, the roof is lined with copper sheet which in time will oxidise to green patina.

The use of timber was a fundamental choice in the design; it forms the structure of the main roof and is used as linings to walls, floors and ceilings, as well as for a floating staircase, all of which are made of Douglas fir. As Botsford explains: ‘We found that timber was not only the best material to build the roof, but also the best way to visually describe the complexity of the roof form itself while also creating a warm and comfortable domestic environment.’

All timber is sustainably resourced. The Douglas fir floor, wall and ceiling linings used throughout the project are PEFC-certified. The slender glulam elements of the roof structure are all of PEFC-certified European spruce glulam.

The roof is a unique timber structure, both in its double-curvature geometry and its use of slender glulam elements. The roof contract was let to a German company, Züblin Timber, who subcontracted the timber structure to a timber workshop in the Dolomites in Italy. The roof shape was fabricated in eight sections and, when completed, craned into place over the surrounding five-storey buildings. ‘The fit had to be perfect,’ Botsford says. ‘We had to think through every inch of the house as a completed project from early on.’

The roof structure is covered with a double layer of 6mm birch plywood followed by a secondary glulam beam structure that forms the insulation and ventilation zone. This was then laid with 50 x 25mm timber boards, laid in opposing directions to avoid matching joints and to provide the substrate for the copper roof-sheet.

The roof structure

Steve Atkinson, structural engineer and director of Built Engineers Ltd, explains the structure of the timber roof.

‘The timber roof frame is an orthogonal double curvature beam grillage. Support to the roof surface is provided by the end concrete walls, and also by a single intermediate thermally broken stainless steel post that is incorporated within each long glazed edge of the pavilion. The lower edge of the timber roof is framed out with 300 x 300 x 90mm L-profile glulam sections to provide a constant fixing perimeter to the radial timber ribs.

‘The double curvature surface directs the spanning forces to perimeter framing elements. Forces are predominantly in compression and tension, though the pronounced offset and asymmetric arrangement of the roof-light means that the beam grillage surface has also to additionally deal with forces in a flexural mode – notably on the wide end slope where predicted flexural deflections were highest, and also the corner ribs which exhibited critical bending and shear. In order to achieve economy in the roof build, fixity of elements became a key driver; following discussions with Züblin Timber, the original high fixity model was re-appraised and many elements were moment-released. The upshot of this was the minimum depth ribs increased from 180 to 200mm deep (11 per cent increase), maintaining the 60mm width. This resulted in an efficient 37 per cent increase of major axis bending stiffness in the radial ribs. The reduction of joint forces when the joints were modelled as notional pins rather than with full moment fixity allowed Züblin Timber to simplify their jointing methodology and hence deliver the roof within acceptable budget constraints.

‘The ribs and lateral rails were first glued and clamped in a single axis curvature with layers as thin as 4mm. These first pieces were then re-sawn and glued to the final 3D curvature; though full use was made of the digital model to create the clamping jigs, this remained an intensively manual process. The radial ribs of the whole roof frame were trial assembled before the butt jointed lateral ribs were cut and manually fitted between the radials in an impressive effort of patience and skill. Connection details were agreed to Züblin Timber’s preferred design, employing skew drilled timber framing screws where possible and also incorporating specially fabricated steel bracketry to carry the higher loads most frequent at the lower ends of the radial ribs.’

The resulting form is an impeccably crafted timber structure which brings light and drama to the living space. It transforms what was once a constricted site cramped between tall houses into a serene piece of architecture, spacious and filled with light.

Awards

RIBA Regional Award 2019
RIBA House of the Year 2019, Longlisted
AIA (UK) Excellence in Design Award 2019, Professional Commendation
AR House Award 2019, Shortlisted
ArchDaily Building of The Year Award 2019, Winner
AZ Awards Best Single Family Houses 2019, Winner
Wood Awards 2019 Structural Award, Winner

Completion Date:

September 2018

Building Type:

Private house

Location:

Notting Hill, London

Architect:

Gianni Botsford Architects

Structural Engineer:

Built Engineers

Main Contractor:

New Wave

Roof Structure:

Züblin Timber GmbH

Joinery:

New Wave

Staircase Manufacturer:

SteelOne

Timber Supplier:

Züblin Timber GmbH (roof), Dinesen (floors, walls, ceilings, staircase)

Timber Elements:

roof structure, floors, walls, ceilings, staircase

Timber Species:

PEFC-certified European spruce, birch and Douglas fir

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