Savill Gardens, Windsor Great Park, Windsor

Architect: Glenn Howells Architects
Builder/Main contractor: William Verry Ltd
Structural Engineer: Buro Happold & Engineers HRW
Joinery: The Green Oak Carpentry Company Ltd

Timber used for the roof structure was European larch (Larix europea), sourced from FSC plantations in Windsor Forest on The Crown Estate. Larch was specified, after sampling and testing, as being the best available timber from a local source to meet the strength requirements. Mature slow-grown larch with sapwood excluded is naturally durable and requires no additional preservative treatment. The structural timber was rigorously graded to meet the specification, the roof design then allowed the re-use of lower grade timber in less critical areas, thereby obtaining the maximum useful yield from the resource, and demonstrating the potential of home-grown timber for structural purposes. The Crown Estate’s woodlands at Windsor also supplied the English Oak (Quercus robur) for the rainscreen and timber floor building. Some 35 prime quality trees were carefully selected and felled in 2004 for the flooring and a further 70 trees in 2005 for the timber rainscreen. All timber from Windsor Forest was felled in accordance with the ongoing FSC forestry management plan. Oak was preferred for maximum durability, again by excluding all sapwood, and also for its appearance, weathering naturally to a silver-grey finish. Other structural elements of the roof were spruce LVL from Scandinavia, treated with Boron as a preservative and birch plywood treated for surface spread of flame.

The Crown Estate’s woodlands at Windsor supplied the English oak (Quercus robur) for both the internal hardwood flooring and external rainscreen cladding. The internal floor was designed alongside a specialist subcontractor to be used in conjunction with an under-floor heating system located within the screed. The oak was taken from source and processed, air and kiln dried, machined and delivered to site for installation on a plywood substrate. A natural matt oiled finish was chosen to accentuate the timber grain and durability. Oak was used for the rainscreen cladding to the gridshell roof, supported on an adapted metal support clip and timber bearer system fixed to the metal standing seam roof (which also acted as the waterproof membrane and thermal insulation). Oak rainscreen cladding panels were fixed directly to the bearers to form a chevron pattern across the roof and eaves cantilever.

The gridshell roof, a specialised form of timber construction, consists of four layers of timber, which are laid out flat and manipulated into a doubly curved shell form. The technique uses identical components (timber laths and shear blocks) brought together on site. The curved shell is braced in plane, for shear strength and stiffness, by the birch plywood sheathing that supports the roof insulation and outer cladding, thus the skin of the shell is multifunctional. For practical site joining, the larch laths are connected directly to the steel edge with fingers of Kerto LVL, which are bolted between the laths and support ledges on the tube. The interface between the glazed wall (up to 8 metres high) was designed to accommodate the movements of the shell allowing movement vertically but restraining the top of the wall horizontally.

The Savill Building roof is approximately four times the size of the Downland Gridshell (first gridshell roof in the UK) and so its construction moves the technique forward and has enabled further development of understanding of the structural design and methods of construction for such structures utilising a programme of testing at Bath University. This is the first free-standing gridshell, with all sides of the building overlooking the garden being fully glazed and supported via steel edge beam and quadrapod supports at 27 metre spacing. Unlike timber elements of the roof structure, the steel edge cannot be bent on site and therefore the shell edge was set out to lie in a single plane between each support to accommodate the pre-fabricated tube which is curved in only 2 dimensions whilst creating a complex 3D form.