Swimming pool, King’s College School, Wimbledon, London

Introduction

An elegant wave-like timber roof soars over this new swimming pool, reflecting the shape of water in motion. The pool is glazed on three sides and the timber elements of the roof – curved glulam beams and cross-laminated timber (CLT) roof panels – are finished with a pale tone which co-ordinates with other materials, creating a sense of tranquillity which is enhanced by views of green lawns and gardens through the glazed walls.

The swimming pool is part of a group of new and remodelled buildings, designed by David Morley Architects, which consolidates the sports activities of King’s College School, a private day school in Wimbledon, southwest London. The aim was to design a new sports complex with strong visual connections between the indoors and outdoors to encourage students to enjoy physical activities while seamlessly tying the old sports facilities to the new.

The site lies within the West Wimbledon Conservation Area and is bordered by listed buildings. The existing sports hall and squash courts occupy the north end of the site and face a garden court, now newly landscaped. The new building connects to the south wall of the squash courts and extends across the site to form an enclosure to the garden.

An elegant colonnaded lobby runs at the side of the squash courts to link the existing buildings to the new one and form a single, holistic complex.

The new building integrates three separate functions – sports hall, changing rooms and pool – into a single unit. The central element is a two-storey pavilion housing the reception, changing rooms, viewing galleries, strength and conditioning suite, gym and exercise studio. It gives access to the brick-clad six-court sports hall on its west side and to the six-lane, 25 metre swimming pool on the east.

The first floor of the central pavilion is fitted with galleries on each side so that students can view activities in the sports hall and swimming pool. Its roof is of pre-weathered zinc which extends as a tapered eaves edge to the swimming pool roof, sweeping down in an elegant curve to follow the shape of the supporting glulam roof structure while acting as a border to the insulated green roof, planted with varieties of sedum. The roof shape is emphasised by glazed walls on three sides, giving swimmers in the pool a glimpse of lawns and gardens, while from the outside pool activities are visible to students walking past on their way to the new sports facilities or the school playing fields.

The choice of timber

The architects were aware that timber works particularly well as a material for pools due to its resilience to resist damage or corrosion caused by the moisture-rich pool environment. Timber was also the ideal material to create the sweeping roof form which spans the width of the pool. It had the additional advantage that roof structure, deck and sound absorbing lining panels could be integrated as a single overarching roof material. The use of timber reduced net carbon emissions; more than 100 tonnes of carbon emissions were saved by selecting a timber structure compared to one of steel or concrete. The natural and sustainable qualities of timber reinforced the aims of the project – to promote the health and well-being of students and other users.

The pool roof structure consists of seven curved glulam beams which support a deck of CLT panels. From their lowest point along the eastwall, the beams span the pool and sweep upward above the first floor viewing gallery, to connect to the roof of the central pavilion. The top corners of the glulam beams are profiled to slope sharply down and support a series of rooflights set between the beams. The rooflights direct natural light onto the pool and, together with a row of artificial uplighters at the sides, eliminate the need for overhead light fittings.

The timber elements were finished with a pale Adler Lignovit translucent water-thinnable glaze to maximise the effect of light reflected from the moving water surface.

The structure

The seven glulam beams span more than 17 metres between the roof of the two-storey pavilion and the seven cast in-situ fair-faced concrete columns which run along the glazed east wall of the pool. The beams support a green roof, with each beam supporting a load of up to 33 tonnes in weight.

Each glulam beam is 280mm thick and profiled to curve in a flattened S-shape, increasing in depth from 745mm at the column head to more than 2415mm, with the top corner of the beam profiled to support the sloping rooflights. The beam is connected to the reinforced concrete downstand beam by means of a galvanised bracket cast into the concrete, with a similar bracket recessed and screwed into the end of the glulam beam. Both brackets are concealed; the glulam beam was lowered into place to provide a bearing onto the cast-in bracket on the downstand beam and two long M24 rods were then threaded through both brackets to form a tie.

Along the east wall, the seven concrete columns are circular on plan at their base and gently taper to an ellipse. The column heads are linked to a 300 x 765mm deep reinforced concrete beam which, together with the columns, creates a sway-frame and helps to provide stability to the roof structure. The glulam beams are connected to the column heads by galvanised brackets. Each bracket was factory-fitted by the installer, B&K Structures, into a 700 x 30mm recess in the end of the glulam beam. A steel plate with a projecting steel square hollow section was grouted into the top of the concrete column to provide the connection and a robust shear key.

The top edges of the glulam beams were grooved to support the ends of the 120mm thick CLT panels which span between them and which act as a diaphragm to support the insulated green roof covering.

To accommodate the curved profiles of the glulam beams, the CLT panels are of varying widths, with panels of smaller widths used where the curve is tighter.

The pool structure is a concrete box set within a larger basement which provides space for plant, access for maintenance and acts as a plenum to deliver fresh air to the pool hall. The basement and pool box were built with in-situ reinforced concrete designed to BS:8007 to limit crack widths and with water-resisting additive to achieve a water-retaining structure.

Sustainability

The glulam beams and CLT panels were manufactured from Austrian and European spruce respectively, and both were supplied with full PEFC chain of custody certification. The Kerto Sonans LVL (laminated veneer lumber) sound-absorbing ceiling panels are of PEFC- and FSC-certified spruce.

For the whole development, including the pool, 86 percent was built on previously occupied land. Impact on the existing ecology and biodiversity was kept to a minimum, with trees and planting retained and protected during construction. The green roof of the swimming pool helps to modify the internal environment, assists rainwater attenuation and enhances existing natural habitats.

Awards

Wood Awards 2020 Winner – Education & Public Sector
RIBA Regional Awards 2020 shortlist

 

Prepared by the publishing team with contributor Susan Dawson.

Completion date:

May 2019

Building type:

Swimming pool

Location:

King’s College School, Wimbledon

Architect:

David Morley Architects

Structural engineer:

Price and Myers

Main contractor:

Knight Harwood

Structural timber subcontractor and installation:

B&K Structures

Acoustic panel joinery:

Suffolk and Essex Joinery Ltd

Timber supplier:

Rubner (glulam), Binderholz (CLT), Metsä Wood

Timber elements:

Roof structure, sound absorbing panels

Timber species:

PEFC- and FSC-certified spruce

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