Location: Orangery roof at Chiddingstone Castle, Kent
Timber: Oak and Chestnut
Architect: Peter Hulbert Chartered Architects
Awards: Wood Award Best Use of British Timber 2007
A timber frame gridshell for a historic orangery
Probably our most challenging project to date, this timber frame gridshell is the world’s first gridshell to support a frameless glass roof, and is a Wood Awards winner.
With an existing grade I listed structure of questionable structural integrity it was necessary to find a solution that limited the loads transferred to it at the same time as supporting the UK’s first framless glazed gridshell roof. Gridshells, using green timber, are prone to movement fom the effects of differing slopes of grain in the lathe, and varying lengths between finger jointing. For this reason, most gridshells are allowed to ‘find their own form’ prior to covering. In this case, the demands of the glazing system made only exacting tolerances, and predictable movements acceptable.
The timber frame gridshell is constructed from four layers of alternating lathe, finger jointed to enable use of locally sourced small section green chestnut. The requirement for the gridshell to support the frameless glazing meant that the usual method of construction which allows the gridshell to ‘find its own form’ and to be able to flex was inappropriate. This timber frame grid needed to be predictable and rigid, while specially designed node clamps allowed for small amounts of settling in movement. A further complication was that the design required the lathes to bend over tighter radii than they would naturally go.
• The roof had to be designed to sit within the confines of the existing listed structure
• Loading had to be transferred to the new foundation under the stone floor below not to the existing structure • Whilst similar to a number of other structures we’ve previously completed Chiddingstone was unique in that it had to support a frameless glass roof
• The initial concept required the lathes to bend to radii far in excess of the accepted engineering tolerances, meaning that the risk of breakage was high
• To inform the design process we carried out numerous tests on the cable and clamping system and bending tolerances of the lathe, this provided the data that dictated the curvature of the roof. Excessive curvature was designed out wherever possible, but was unavoidable where the lathes interfaced with the ring beam
• Our designers worked in close collaboration with the glass manufacturers to develop a node that would clamp the chestnut lathe, the cabling and support the 12mm toughened glass panels, allowing the glazing to float over the frame while accommodating small amounts of movement of the lathes