SKILLION ROOFS

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.

A skillion roof (a) and gable roof (b)

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.

Date House by Robin Boyd, 1955

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. 

STEEP AND LOW ROOFS

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

MOULDINGS VIII - THE CYMA RECTA AND THE CYMA REVERSA

In contrast to the simple mouldings covered in previous posts, in which each moulding consists of a single curve, the cyma recta and cyma reversa are examples of compound mouldings: mouldings that consist of two or more curves. In the case of the cyma (from the Greek for ‘wave’) mouldings, there are two curves in each, an ovolo and a cavetto, arranged in series.

The cyma recta consists of an ovolo at the bottom and a cavetto at the top:

In the cyma reversa, the order is reversed, with the cavetto at the bottom and the ovolo at the top:

The cyma recta and cyma reversa are examples of the general group of compound curves known as ogees (pronounced with a soft g), defined as double curves or arcs, one concave and the other convex, joined at a point of inflection, and where the unjoined ends of the curves or arcs point in opposite directions and have parallel tangents; that is, if the ogee curve were in a road you were travelling on, you would be travelling in the same direction upon exiting the curve as you were when entering it. In the recta, the ends extend horizontally; in the reversa, they extend vertically.

In terms of their structural and psychological functions, the recta is a supporting moulding with an ‘upwards’ emphasis, and the reversa is a terminating moulding with an ‘outwards’ emphasis.

The cyma mouldings come in infinite varieties and expressions, depending on whether the curves used are arcs, ellipses, parabolas, or hyperbolas, and on the flatness or depth of the profile. The relative size of each curve in the profile can also be varied; a cyma reversa with a small cavetto topped by a large ovolo, for example, has a much more robust appearance than one with a large cavetto under a small ovolo.

Cymas are typically better employed as the uppermost or lowermost mouldings in a group than they are in an intermediate position. They are almost always combined with small fillets above and below, to isolate and define them against the background planes of the wall or soffit, or against other moulding profiles in the group.

As for remembering the difference between the two, all I could come up with is that the recta resembles a breaking wave, which, if you were surfing it, might mean you were about to get ‘rect’. Not great, but if you have a better mnemonic please let me know!

WINDOWS ARE PICTURES

WHEN YOU WANT to hang a picture in your house, you choose one with a size and shape that suits the wall and the room.  A rough rule of thumb is that you need to be able to stand at least as far away from a picture as the length of its diagonal: i.e., for a 3.0m x 4.0m picture, you need a room at least 5m deep.  Another way of looking at it is that the picture should lie completely within a solid angle (subtended from your eye) of no more than 40°.  That's why you don't put big paintings in hallways.

    Windows in modern buildings are basically designed to be ignored, regarded as just holes in the wall to be looked through, not at, but that wasn't always the case.  There's a lot to be said for dimensioning and placing a window in the same way as you would choose and hang a picture for a particular wall: by paying close attention to the subject (the view), the size, the proportions, and the frame.  It makes sense, for example, to leave some wall around the whole perimeter of the window, which preserves the legibility of each as separate elements, and allows the eye to either focus on the window and its view, or see the wall as a coherent whole.  If all the windows in the room are designed this way, the eye can flow right around the room without being visually ‘blocked,' and is able to perceive the continuity of the walls bounding the room, which gives a sense of containment and security.  This continuous band of wall between openings and the ceiling is called in Japanese ari-kabe, or ‘ant wall,’ supposedly because it would allow an ant to do laps of the room on this unbroken ‘track’.  

Somewhat ironically, advances in glassmaking technology have been a major factor in the degradation of windows as design elements. Traditionally, glass panes were created by ‘puddling’ (resulting in a characteristic ‘bullseye’ ripple pattern) or later by hand-blowing glass cylinders, cutting them open and flattening them out, resulting in relatively ‘flawed’ glass and small panes that could only be assembled into large windows by the use of muntins - the slim vertical and horizontal timber members that divide and hold the individual panes. These muntins and the bubbles, ripples and optical distortions of the glass give these windows great charm and make the windows impossible to ignore.

In light of all the above, it is a pity that these days the primary consideration when choosing windows for views seems to be raw size, to say nothing (for now) about the obvious shortcomings in thermal performance guaranteed by the heat pouring in or out of these vast expanses of glass (double glazed, low-e or not). Better that we conceive of windows as subjects worthy of contemplation in themselves, as well as portals to a view- as things to be looked at as much as through.

A framed view.

Might be time to go outside?