Basil Spence designed the main spiral staircase to the house as a separate element linked to the end wall by a short landing and enclosed in an octagon of black-stained vertical cedar slats. These had deteriorated and when the new extension was added the architect designed a new 16sided enclosure - of glass and vertical cedar fins - around the original staircase. The form is a 'homage' to Spence's slate-finned 'Chapel of Industry' at Coventry Cathedral.
The original stair - a steel spiral with afazalia treads - and landings have been retained. The concrete base has been partly cut back and a new circular edge beam cast.
Sixteen 269 x 69mm cedar fins and 9mm toughened glass panels act as a composite structure to support the roof without the need for bracing.
At the base of the fins runs a stainless steel ring cut from 10mm plate, which is chemically bolted to the concrete edge beam.
A series of 150 x 10mm steel flitch plates are welded to the ring; the fins are slotted over the plates and are pinned to them with H10 stainless steel dowels, cut flush to the face.
The glass panels are faceted between the fins and set in rebates secured with silicone to form a composite structure. They are covered with cedar beads that are cut at 11.25mm degrees to match the angle of the glass facet.
At the head the fins are bolted to a faceted 150 x 75mm steel angle.
The roof is a stressed skin plywood structure filled with insulation and covered with terne-coated stainless steel.
As the stair is a transitory space the walls are single glazed. A grille-covered trench heating unit in the floor supplies background heating.