However, speak with the architect Adrian James or developer Rycote and it soon becomes obvious that in setting themselves the task of producing a high quality, statement project they also set themselves a massive challenge in gaining planning and NHBC approval for the scheme.
The apartment block was planned as a ‘gateway’ to residential development behind it, with the original design having a shallow pitch, curved metal-clad roof devised to reduce the scale of the building, bring dynamism to the scheme and follow the sloping contours of the site. The local authority had expressed a desire for the design to be sympathetic in scale and material to neighbouring properties but, given that homes on Cumnor Hill had no unified style, the developers opted for a more contemporary architectural style.

The scheme failed to win planning approval. The architectural committee of the local planning department condemned the design with the metal roof in the strongest terms and it was back to the drawing board for Adrian James.

Subsequent changes to the design were quite minor, with the internal layout remaining the same but the roof moving. Its shallow curve was slipped to each end of the building, making clay tiles the natural and obvious choice of roofing material.

The re-submitted design, although in essence the same building, looked very different, with the same architectural panel responded by saying that although the building seemed radical, it was of a design standard that should be encouraged.

Specifying the roof tiles proved an easier task. Plain tiles were identified as the best solution, suiting both the context of the building and its curves. After much research, Dreadnought’s Staffordshire Blue Smoothfaced tiles were placed on order (all 8,500 of them) because of their strong, even and natural colour. Dreadnought also supplied matching eaves and gable tiles.

Double trouble

The design of the curved roof is a segment of a circle producing a continuously changing pitch, with the last 2m at the apex falling below a 35° pitch. Therefore the main challenge was creating a robust design for the sub roof that protected this lower pitch section without being obvious. This area of roof was constructed so that the tile covering was decorative where it fell below 35° but still matched the main portion of the working clay tile roof with no distinguishable joint.

However, the NHBC would not accept doubling up of the Tyvek membrane for the decorative section below 35° pitch due to fears over weathertightness against wind driven rain. So for the top portion of the curve, effectively 1800 – 2000mm from the ridge – an underlay system developed by Grace Ice & Water Shield was used and the construction altered to accommodate it. This underlay is a self-adhesive membrane giving premium roof protection on sloping timber boards. It was developed for northern USA and Canada, primarily to deal with the formation of ice dams on roofs.

A mock-up of this section of the roof was built and subjected to continuous flows of water to prove the watertightness of the design and the ability of the camber of the tiles to follow the curve dictated by the rafters on battens fixed at a constant and standard 100mm gauge. Standard production tiles achieved this without the expense of heavily cambered mansard tiles. The lower courses of tiling are almost vertical and much of the roof pitch is more than 60°. Consequently all tiles were twice nailed. Despite this, the consistent shape of the machine made tiles enabled them to lie closely and provide an apparently seamless surface.

The test rig experiment proved successful and the build was back on. The completed apartment building was subsequently marketed as a ‘landmark architecturally striking building’ and went on to be highly commended in the ‘New-Build Housing’ category of the Clay Roofing Awards 2008. Designer Adrian James was also the recipient of a special Clay Roofing Award for exceptional architectural merit, in celebration both of this project and a similar curved roof on a new house elsewhere in Oxfordshire.

The final words has to go to Robert Gee, of Rycote Developments: “This project demonstrates very well the use of a traditional product in a contemporary design, producing a fantastic overall result.”

Email: emma@kimbercoms.info