sunk as much as below sea level. The highest roof point is 67 metres above sea-level which is the same height as that of a 22-storey building. The roof is made of 2,194 pre-cast concrete sections, which weigh up to 15 tonnes each. Although the roof structures are commonly referred to as "shells" (as in this article), they are precast concrete panels supported by precast concrete ribs, not shells in a strictly structural sense. Though the shells appear uniformly white from a distance, they actually feature a subtle chevron pattern composed of 1,056,006 tiles in two colours: glossy white and matte cream. The tiles were manufactured by the Swedish company Höganäs AB which generally produced stoneware tiles for the paper-mill industry. Apart from the tile of the shells and the glass curtain walls of the foyer spaces, the building's exterior is largely clad with aggregate panels composed of pink granite quarried at Tarana. Significant interior surface treatments also include off-form concrete, Australian white birch plywood supplied from Wauchope in northern New South Wales, and brush box glulam. Of the two larger spaces, the Concert Hall is in the western group of shells, the Joan Sutherland Theatre in the eastern group. The scale of the shells was chosen to reflect the internal height requirements, with low entrance spaces, rising over the seating areas up to the high stage towers. The smaller venues (the Drama Theatre, the Playhouse and the Studio) are within the podium, beneath the Concert Hall. A smaller group of shells set to the western side of the Monumental Steps houses the Bennelong Restaurant. The podium is surrounded by substantial open public spaces, and the large stone-paved forecourt area with the adjacent monumental steps is regularly used as a performance space. Performance venues and facilities The Sydney Opera House includes a number of performance venues: Concert Hall: With 2,679 seats, the home of the Sydney Symphony Orchestra and used by a large number of other concert presenters. It contains the Sydney Opera House Grand Organ, the largest mechanical tracker action organ in the world, with over 10,000 pipes. Joan Sutherland Theatre: A proscenium theatre with 1,507 seats, the Sydney home of Opera Australia and The Australian Ballet. Until 17 October 2012 it was known as the Opera Theatre. Drama Theatre: A proscenium theatre with 544 seats, used by the Sydney Theatre Company and other dance and theatrical presenters. Playhouse: