Roof forms

There is a wide range of different roof forms including level (planar, saw-tooth, space frame), sloping forms, linear vaults (folded and arched forms), rotational forms (pyramids, lanterns and dormers), and complex forms (Vierendeel frames).

Level (flat) roofs are ideal where height is limited or where there are difficult junctions with other parts of the building.

These forms are often used when interior volume or exterior height is limited, and when intersection with other elements of the building makes other forms difficult to resolve. Space frames can easily provide the structure for level roofs. At the East Building of the National Gallery in Washington DC I. M. Pei has used a super-scaled space structure of triangular pyramids in built-up steel members with cast steel connecting nodes. A great deal of sophistication has gone into the detailing to make it look simple. The whole roof is supported on nearly invisible projecting lugs at the edge nodes.

Very shallow roof slopes are now possible.

For most of the history of level glass roofs, the glass surface had to be pitched or vaulted in minor elements in order to drain. However, Planar glazing now makes a near flat (1-2% slope) roof glass possible as there is no projection to block run-off. The first example was BDP's Waverley Market, Edinburgh where steel joists at 1200 mm centres pick up glass sheet corners and also carry a secondary safety glass layer. A pre-engineered space frame supports the planar glass atrium roof at the Porsche showroom in Woking. Each frame node supports a cluster of bolts picking up the corners of four double-glazed units.

The BDP design for the J P. Morgan Headquarters uses a box-section lattice to support adjustable pedestals carrying the corner bolt clusters and give a coffered ceiling appearance.

Conventional construction methods can be used if the roof is not fully glazed.

When the roof is not to be primarily glass, as it rarely is in tropical latitudes, the temptation is to use conventional joist structures supporting decking. The Plaza of the Americas, Dallas, shows this low key form, with strips of rooflight cut across the decking. Visual banality is the risk

Saw-tooth roofs are a derivative of early greenhouses.

Saw-tooth roofs go back to the first glasshouses, and a good example is the garden atrium at the IBM Building in New York by Edward Larabee Barnes. The triangular volume provides the spanning depth required. External cleaning can usually be done from the gutters which are used as walkways. Internal cleaning can use rolling catwalks between the lower chords of each angled frame.

Sloping roof forms can be constructed using rolled steel beams, lattice girders or space frame construction.

Glass will drain itself in conventional framing or in patent glazing bar systems if held on a slope. Support at approximately 3 m centres maximum is needed and a variety of simple solutions is possible. Rolled steel sections give least obstruction but use most material. The roof at the Charles Englehardt Court, Metropolitan Museum, New York City, by Roche and Dinkeloo is built that way.

However, lattice members are most commonly used for the support grid. St Enoch's Centre Glasgow, by GMW/Reich Hall demonstrates large scale sloped glazing support of this form. The most convincing use of a space frame for this task would seem to be at the Vancouver Law Courts by Arthur Ericson. Steel trees spring off the underlying concrete frame to carry nodes in a suitable distributed manner. The presence of the sky is clearly felt through the gauze of members.

Penzoil Place, Houston, by Johnson Burgee, shows a large span version of the space frame.

Linear duo-pitch roofs (folded vaults) are common and can be constructed using a variety of structural forms.

Pitched glass roofs over linear glazed spaces are common, though few use truss supports as they might if roof decking were to be supported, perhaps fearing the industrial aesthetic which could result. One exception is Helmut Jahn's Board of Trade extension in Chicago.

Roche Dinkeloo's John Deere office building uses the twice-folded gambrel roof forms of barn construction to roof the garden of this agricultural machinery giant, the steelwork being weatherng steel.

Greenhouse technology is used at the University of Trondheim by Henning Larsen, in the form of lightweight latticed frames and the rolling sections which open the streets to the summer sun.

Arched forms are structurally very efficient and can be very light, but care is needed in detailing the central flat area.

Arched roofs over galleria are an exceedingly popular form, and have become almost a cliche in shopping malls. Arches can span a long way with an elegant light framework as they minimize bending stresses. There are several pre-engineered systems available using the glazing bar as the arch structure. On very wide spans special designs are needed. Eberhard Ziedler's Eaton Centre, Toronto, shows a low-pitched, multi-centred lattice arch, with the upper chord rising to a point at the centre to avoid too low a glass pitch.

The conservatory roofs of Francis Machin use reflex, ogee curves in polycarbonate to avoid the flat top and stiffen the plastic for its span between glazing bars.

Arched forms can rise to enormous scale, one such example being the Mile High Centre in Denver Colorado where Johnson Burgee added a vast atrium to the existing Pei tower of the early l960s. However, the near level glass on the upper curved areas has been a problem, with packed snow turning to ice sliding off in avalanches.

Arched structures can be constructed using pre-bent steel sections.

Rolled steel joists or hollow sections can now be bent easily, and arched forms in rolled steel are appearing. The Palm House atrium of the Ealing Broadway office centre by BDP uses pressed, curved toughened glass to follow the arched forms.

The antiforms of the arch, namely the 'catenary' can also be used for atrium structures.

Norman Foster has used a catenary structure, which is a very efficient form, at the HongKong Bank HQ as a glass ceiling, and at the Century Tower in Tokyo as a roof. At the Bank the glass plane is cut by escalators at the prescribed Fen Shui angle, creating extraordinary details at the junctions. (Fen Shui is Chinese geomancy, a mysticism linking the forces of good and evil to spatial arrangement).

Rotational forms, such as pyramids, domes and lanterns, are suitable for courtyard spaces.

Courtyard spaces with a vertical emphasis are complemented by a rising roof form, which offers other advantages such as the ability to catch sunlight and to provide a clear storey for the release of hot air and smoke. Regular forms over square or rectangular spaces facilitate the use of cleaning rigs and make for the simplest structures. Domes spanning up to 30 m can now be bought from catalogues, though individual designs, such as the arched frame at the Royals Centre in Southend on Sea, are still competitive.

Pyramids are naturally stable structures, and the frame chosen can be a simple one to provide support for glass framing, or a space structure picking up corner nodes. Pei's structure for the Louvre entrance atrium is a highly complex form, to support planar glazing. Another example is the half-pyramid which covers a food court at a shopping centre in Dallas by SOM.

Lantern light is a term used to cover all non-platonic tall shapes. The Broadgate and Finsbury Avenue complexes by Arup Associates have a series of lantern topped octagonal atria, each with a ring beam near their summit to carry a cleaning gantry.

The large lantern over the central space at Copley Place, Boston, is of a similar form.

One of the most remarkable lantern lights in terms of shape and size is the one devised by Helmut Jahn for the State of Illinois Building, Chicago in which a vast circular drum is cut on the slope, with trusses used to carry each surface.

Vierendeel roof frames are very efficient structural forms suitable for spanning complex areas.

Complex forms and plan arrangements are difficult to span economically with space frame or vaulted forms and the solution is often to use rigid 'Vierendeel'frames in both plan directions, as at the Scotia Bank Toronto.

An older example is Crystal Court Minneapolis by Johnson Burgee where the roof comprises rigid cellular frames which are supported off the surrounding structures and carry pedestrian galleries on suspension rods.