Ibstock Place School, Roehampton, South West London

Introduction

The architect Maccreanor Lavington has created a new school refectory with three dramatic vaulted roofs, each a diamond-grid lattice of glulam beams rising to a glazed lantern. It stands in the grounds of Ibstock Place School in Roehampton, South West London, and the three vaulted volumes define the primary functions of the building – dining hall, servery and kitchen. Every day 1200 pupils come here for lunch – the dining hall can seat up to 500 pupils at any one time – and the lofty spaces aim to help celebrate communal school life. As the architect explains: ‘To create an interior that was celebratory whilst warm, welcoming and comfortable to use, we naturally looked at timber, both as a structural element and as a finish. In our practice, we use timber for its low carbon qualities, visual warmth and character; and because of the quality and programming benefits that prefabrication of timber components can offer.’

Ibstock Place is a private co-educational school for pupils from nursery to sixth form and occupies the house and grounds of a former 16-bedroom Edwardian mansion with additional school buildings – theatre, classrooms and new refectory – all set between mature trees in the original landscaped gardens. Running alongside the eastern garden boundary, the refectory has a backdrop of mature London plane trees and its external materials, clay tiled roofs and London stock brickwork walls, make reference to the neighbouring buildings on the site. The gentle roof pitches descend to a relatively low eaves to reduce the visual impact of the large internal volumes. A colonnade with brick piers runs along the main façade of the refectory and links it to paths leading to other parts of the school.

Although the refectory is built on the footprint of former dining facilities, the architect has trebled the usable floor area by inserting a generous sixth form study area above the kitchen and a mezzanine gallery and part of the servery, and by excavating a generous basement for stores.

The lattice structure is exposed and the diamond shaped spaces between the inclined glulam beams are filled with oak-slatted panels which provide acoustic absorption. The glulam beams rise to projecting glazed lanterns which fill the deep spaces below with natural light and support the natural ventilation strategy. Below the roof, the internal walls are panelled with matching oak slats; doors and floor finishes are also of oak.

The design of the roof

In shape, the timber roof structures are truncated pyramids with a complex geometry; they enclose spaces of different sizes and rise at different inclinations to lanterns which are of different size but which all terminate at the same height.

Each roof structure has a similar hierarchy of elements: a series of primary 160 x 520mm glulam beams with secondary 160 x 440mm glulam beams incline in opposite directions to create the main diagrid. They rise from the steel ring beam at the eaves to a timber ring-beam at the lantern or to the corner hip beams. Fixed between them are inclined 160 x 200mm glulam beams, creating a subsidiary diamond lattice.

The team also considered the idea of using the glulam beams as a two-way spanning diagrid. As Greg Nordberg of engineersHRW explains: ‘This format reduced the stresses on individual members but created more onerous connections, meaning that equivalent sized sections would be required. Overall, the hierarchical approach was just as efficient and more economical.

The stiffness of the roof geometry and the truss action of the roof were considered with separate models to provide an envelope of forces for element design and examine alternative load paths for disproportionate collapse.’

The corner hip beams act in compression to transfer the bulk of vertical loads from the roof to the steel ring beam. At the eaves, the first floor structures to the Small Hall and the kitchen provide restraint to the ring beam to reduce deflection under vertical loads. The in plane action of these floors transfers lateral loads to diagonally braced bays at the corners of the building. The lanterns consist of welded steel frames which transfer horizontal loads across the openings at the top of the roof and stiffen the edges of the ring beam.

The geometry of the diamond grid lattice was fine-tuned to minimise variations in size of the oak-slatted acoustic panels set between them. As a result, only four different sizes of diamond-shaped panel were used throughout the project and they are of a manageable size for manual handling. The oak slats are lined with acoustic fabric panels and incorporate concealed cabling for lighting, data and other services. They were prefabricated and fully finished off-site.

The assembly of beams at different inclinations was complex; the glulam had to be fabricated with millimetre precision and connections to it had to be designed and fabricated. The inclined 160 x 520mm and 160 x 440mm glulam beams are connected by concealed hangers or galvanized flitch plates depending on their load requirements. Connections between the glulam beams and the steel ring beam structure that supports them are made with galvanised steel plates which were specially developed for the project by engineersHRW and specialist timber contractors Constructional Timber. The glulam roof structures are overlaid with a CLT deck which contributes to the in plane stiffness of the inclined roof planes.

The roof installation

Once the timber-faced steel ring beam at the eaves was installed, a loadbearing scaffold was erected which mimicked the internal volumes of the roofs. The upper timber ring beams at the lanterns were fixed in position onto the scaffold together with the four corner hip beams which were dropped into place by crane and rotated to their final positions. The bases of the main glulam beams were then craned on to angled reveals set in the eaves ring beam and positioned at the correct angle onto the roof lantern ring beam or corner hip beam. The scaffolding gave tolerance to position these elements and allowed the roof to support its own weight while the rest of the structure was installed.

Andrew King of Constructional Timber adds: ‘With restricted site access, all deliveries were arranged using only rigid vehicles, which was challenging as the corner hip beams were nearly 11 metres long and weighed 1.2 tonnes. For the installation sequence, it was key to fix the level of the lantern ring beams off the loadbearing scaffold supplied by the main contractor. The lantern ring beams were levelled to the correct height before the twin hip beams were craned into position. The members were all supplied fully protected and with their fire treatment applied so as to reduce site time and potential for damage.’

Timber finishes

It was important to the architect to ensure that the different timbers used – spruce glulam structure, oak timber linings and joinery, and oak engineered wood floors – would harmonise in the long term, in particular to avoid the possibility that the glulam might be affected by sunlight exposure and become a contrast to the more muted hue of the aged oak finishes. The practice worked closely with the timber coating company Renner Coatings which undertook accelerated ageing of the different finishes in their laboratory and produced a white tinted fire-proof finish which is UV-stable.

Sustainability

The refectory has been awarded a BREEAM ‘Excellent’ rating for its long life-span and low carbon impact in-use. Timber is the primary material for structure and finishes and is all either PEFC- or FSC-certified. Passive measures include lanterns for stack ventilation and daylight, colonnades which contribute to shading and cooling, together with on-site renewables.

Awards

Wood Awards 2021 Education & Public Sector – Highly commended
Structural Timber Awards 2021 – Architect of the Year – Finalist
Structural Timber Awards 2021 – Project of the Year – Winner
AJ Architecture Awards 2021, School Category – Winner
New London Awards 2021, Learning Category – Winner

Prepared by the publishing team with contributor Susan Dawson.

Completion date:

December 2019

Building type:

Refectory

Location:

Roehampton, South West London

Architect:

Maccreanor Lavington Architects

Structural engineer:

engineersHRW

Main contractor:

Feltham Construction Ltd

Structural timber supply and installation:

Constructional Timber (Manufacturers) Ltd, Barnsley

Joinery:

D Smith (Royston), AAS Projects and Design (Rickmansworth)

Timber suppliers:

Mosser Leimholz GmbH (glulam) Austria; X-Lam Dolomiti Srl (CLT) Italy; Havwoods UK (floor)

Timber elements:

Roof structure, wall linings, staircases, handrails and balustrades, internal linings and acoustic panels, doors. windows and floors

Timber species:

PEFC-certified Austrian spruce, FSC-certified American white oak, FSC-certified European oak

Register to download the full case study with images and architectural drawings

Register

Suggested Reading

Carbon and timber in construction for building designersnavigation-arrow

This Wood Information Sheet covers the following topics in relation to timber in buildings: carbon emissions, carbon stores and the complex relationship between timber in the built environment and the forests it was sourced from. It also provides methods to minimise carbon use and to maximise the carbon store, and explains...

30/03/2022

State of Trade Survey 2021 Q3navigation-arrow

The construction products manufacturing sector expanded for a fifth consecutive quarter in 2021 Q3, according to the latest Construction Products Association’s State of Trade Survey. This is despite escalating cost pressures and ongoing issues on the supply side that have been further exacerbated by a shortage of HGV drivers and...

26/11/2021

Construction Industry Forecasts Autumn 2021navigation-arrow

The Construction Products Association (CPA) has revised construction output growth up for 2021 from 13.7% to 14.3% since its previous Forecasts, but also revised down growth for 2022 from 6.3% to 4.8% in 2022. With more buoyant demand so far in 2021, supply chain constraints are expected to hinder growth...

26/10/2021