Feilden Clegg Bradley Studios has renewed the Hayward Gallery’s pyramid rooflights

Buildings.

Words
Chris Allen, FCB Studios

Photos
Richard Battye, FCB Studios, Morley von Sternberg

The 66 glass pyramids on the Hayward Gallery form one of the most striking features of London’s 1960s Southbank Centre, its serrated glass roofscape offsetting the concrete forms below. In spite of the ingenious use of lighting baffles to provide indirect daylighting to the upper galleries, however, the pyramids never fully delivered. Their thermal performance was poor and the fabric began to fail quite quickly. Over time, measures were taken to manage the failings and maintain stable internal conditions, but the end result was artificially-lit galleries, far from the original intention.

The challenge for Feilden Clegg Bradley Studios was to retain this important roofscape element, to realise the aspirations of the original brief by bringing daylight to the upper galleries, and to improve the roof’s environmental performance to achieve the conditions required for the loan of artworks and world-class touring exhibitions. Replacing the pyramids became an urgent necessity.

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A range of software tools was employed to test and visualise the design, while physical models were used to evaluate ideas and communicate architectural intent. The consultant team used Revit 3D software to coordinate the design and as an aid for understanding the complex buildings.

The ceiling design was developed using adjustable parameters to visualise the effect of ceiling depth, apertures and angle. Combined with analytical software to test the solar shading of the emerging pyramid design, fine tuning of the proportions of the internal coffers was possible.

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Engineer Max Fordham provided detailed daylighting analysis to test the internal illumination levels under varied skies, from overcast to bright sunlight, and it created custom software to analyse how sunlight would reflect off and between the pyramids.

A 1:50 working model was used to test various pyramid shading possibilities. In one design review meeting the serendipitous moment of sunlight falling on the model revealed the importance of reflections on the inner face of the pyramid glass. This fed back into design, with the omission of two rather than one side of glass from each pyramid, and in the adoption of a diffusing etched surface for the inside face of the glass.

Various combinations of translucent PVB interlayers, added between laminations of low-iron glass, were sampled to test light transmission and visual appearance. Too much light would require the blinds to be closed frequently to meet the standard 200 lux illumination level. But if the glass was too opaque, it would appear dark when seen from the gallery below and would not illuminate well at night. The chosen solution had eight PVB layers to balance transmission and opacity.

A full-size mock-up was erected on the Southbank terrace to review the complete assembly, including a pyramid with two translucency options. Another benefit was being able to review the position of welds in the stainless steel pyramids and this led to a refinement of the frame to a more slender detail. It also allowed the contractor to test the assembly and lifting of the pyramids.

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A large quantity of scaffold was required to work safely with the difficult, multi-level building. A temporary roof was erected to protect the galleries below, and a scaffold false floor built up to the underside of the existing ceiling to provide safe working.

The old pyramids, roof, ceiling and gallery infrastructure were removed but the original steel trusses retained to support a new timber roof. Upstands for the horizontal rooflights were built on top of the steel trusses, and a bitumen-reinforced roofing membrane installed with insulation.

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The new low-iron, double-glazed flat rooflights were installed as long runs, reducing the number of upstands and junction details. The pyramids, assembled and glazed offsite, were delivered four to a lorry and lifted in sequence onto the roof. Each corner of the pyramid frame was bolted to a specially fabricated bracket that was fixed back through the horizontal rooflight, using a standard brise-soleil detail.

The internal board-marked concrete is a hallmark of the Hayward, setting it apart from many contemporary ‘white cube’ galleries. The quality of the original work was due to the skills of the 120 carpenters who made the tongue-and-groove formwork, and the use of ripsawn Baltic Pine which was deemed to provide the best grain impression.

However, 50 years of use had left its mark on the Hayward’s walls, and options for cleaning the concrete were explored that would remove the ingrained dirt without damaging the surface texture.

The use of water or an abrasive that could potentially damage the retained finishes within the gallery were ruled out. The aim was not to artificially colour the concrete, which would result in an unnatural uniformity, but to present the concrete as it was originally. The chosen product, Arte Mundit, a natural latex spray, was applied directly to the concrete surfaces. Often used in conservation work – not least for the 1000-year-old Westminster Hall – the moisture within the latex is absorbed into the surface, and as it cures and dries, moisture is pulled from the concrete and with it the surface dirt. The latex layer is then carefully peeled from the wall, removing the dirt.

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