A skillion roof or sometimes shed roof is a single-pitch or mono-pitch roof, in contrast to the traditional dual pitch gabled roof, where the two ‘pitches’ slope down symetrically from a central ridge to the longer walls of the building, producing the triangular gables on the shorter walls.

The skillion roof is generally defined as having a pitch (or gradient or fall) of at least 3 degrees or so; roofs shallower than that are usually referred to as flat roofs.

The use of skillion roofs in residential buildings seems to have originated in Australia, with architects such as Robin Boyd employing them as early as the 1950s, but the skillion roof remained largely confined to ‘magazine architecture’ for many years.

Much of its present popularity, and many of the ‘architectural’ examples of the form from the 1990s on, can be traced to the influence of a single figure: Australia’s defacto architect laureate, Glenn Murcutt, though his influence seems rarely acknowledged (Nemo propheta in patria?)

Murcutt’s skillion roofs are typically clad in corrugated iron, with unlined eaves supported on tapered steel or timber rafters and purlins and sometimes struts, a clerestory of sashless glazing running around the perimeter, and a clear datum separating the clerestory from the walls or glazing below. The roof runs up to the north (southern hemisphere), the ceiling follows, and the depth of the eaves overhang on that side is carefully designed to exclude summer sun but allow deep penetration of winter light. Shading of the glazing below the datum is accomplished with external adjustable louvres.

Simpson-Lee House by Glenn Murcutt, 1993

But where Murcutt’s skillion roofs - influenced by his love of high modernism, fastidiously detailed, and genuinely functional - bought the form to a higher degree of refinement than earlier examples, the skillion’s later diffusion, first across the architectural world and then ‘down’ into the ‘builder’s vernacular’ to the point that it is now an established element and a common sight in volume-built subdivisions (though it has never come close to supplanting the gabled or hipped roof in popularity), has seen it often reduced to the status of empty stylistic gesture, a lazy, shorthand way of bringing together those shadowy twin concepts of contemporary and sustainable.

Examples at the ‘architectural’ end of the spectrum are often shamelessly plagiarised from Murcutt, but rarely executed with either his aesthetic subtlety or his fine-boned structural clarity.

This is a winery, not a house, but a good example of what can happen when all you have is “ecologically sustainable outcomes”.

The ‘builder’s vernacular’ skillion has boxed eaves and fascia boards right around the roof: a stumpy, graceless profile. It may be oriented any which way, and eaves depth is often arbitrary or insufficient. There might be two or more skillions pitched in different directions on the same building. There may be no clerestory and the ceiling underneath may be flat. The skillion here is purely in the service of fashion or style, not function.

A good example of the dog’s breakfast that is the skillion roof in the ‘builder’s vernacular’.

Though a well-designed and detailed skillion roof can be an effective solution to various environmental or other design considerations, one might still object to the form on a deeper level - call it psychological, or aesthetic, depending on your preference. That is, where the ceiling follows the pitch of the roof, the enclosed space, though dynamic in its asymmetrical upwards ‘loft’, lacks the stillness and serenity desired in a residential space. The space of the room ‘drains out’ through the clerestory, as opposed to the way it ‘pools’ in the cathedral ceiling, with its obvious metaphors of the inverted hull or cupped hands, or in the flat ceiling, which forms a kind of shoebox lid on the room. There is something settling about the traditional dual-pitch, symmetrical roof, with each side coming down from a central ridge to ‘cap’ the walls beneath, and in many cases eaves that project out over the walls, protecting them from weather, and if visible from within, serving as a comforting ‘cap-brim’ to the view.

One of the most characteristic elements of the 19th century Australian worker’s cottage is its roof. Steeply pitched with short spans and therefore low and compact in form, it is perfectly in keeping with the modest volumes it shelters. There are two basic types: either a parallel series of hipped or gabled units, themselves parallel to the street and separated by box gutters; or a U-shaped hipped roof, whose form is not immediately apparent when viewed from the front and sides, but becomes clear when viewed from the back: a box gutter, perpendicular to the street, runs down the middle of the house, separating the two hipped (or occasionally gabled) roofs that form the uprights of the ‘U’.

One explanation given for the emergence of these forms is that the unsophisticated colonial builders had a poor understanding of structural principles: the ceiling joists weren’t tied to the rafters to form a primitive triangulated truss and prevent the rafters from spreading the walls, and so the thrust exerted on the walls by the roof could only be controlled by keeping the span of the roof, and thus its mass, to a minimum. Low roofs with simple rise:run ratios such as 1:1 (45 degrees) or 1:1.3 (a 3-4-5 triangle, 37 degrees) were also easier to construct and required only short rafters.

Aside from these practical and material factors, early builders also no doubt had their aesthetic motivations, and understood very well that low, steep roofs suit these humble cottages perfectly and give them their unique appeal.

On the left: parallel gable roofs separated by a box gutter. On the right: a hipped ‘U’ form roof with an extremely long central box gutter (hidden).

On the left: a ‘U’ form roof with a lean-to off the back. On the right: a parallel series of three hipped roofs separated by box gutters.

A ‘U’ form roof shown from the back, with twin hipped roofs separated by a box gutter