Purlins? an unusual word. The origin is obscure, so we’ll leave it at that. As for their purpose, purlins span the gap between rafters, a term that is more well known among non-roofers.

The last post described the main roof area as a butterfly roof. This roof form comes in all shapes, angles and sizes and is also called a ‘V roof’, among other names. Butterfly roofs have been around longer but came into their own in the middle part of the last century, as one of many  visible and iconic symbols of modernist architecture.

The word modernist is often defined as ‘form follows function’. It’s not a bad start at a definition, though it reduces a very nuanced and complex approach to the built environment to a slogan.

Hobart hill house’s roof is unashamedly a case of form following function; the angles created by the varying heights of its three planes serve real purposes. And modernism, as the linked images above show, doesn’t have to look or feel cold, dull, dry or unattractive. Far from it.

For this house, the two planes closest to the road form the butterfly and the third plane, a ‘flat’ roof, covers the studio at the far end of the house. The studio roof is on an angle compared to the ground but it is in one plane, without hips or valleys, so it gets the ‘flat’ label. All the roofs use rafters and purlins.

Nathan, the site foreman, decides to start building the roof structure first over the studio, once the scaffolders have finally finished building the platforms to make working at roof level safe and possible.

Day 77 (18 June): Nathan (left) and Damien fix rafters on the studio. These rafters are made of the same lightweight but structurally strong Hyspan LVL timber that is used throughout the house. These rafters will be entirely hidden from view after the metal roof, soffits and flashings go on.

The studio roof is on a 10-degree pitch. The high end, at the rear of the studio, faces south, away from the sun, and soars 4 metres (13.1 feet) above the finished floor. The lower, north-facing end, where Nathan is, will have eaves to stop the late morning and early afternoon sun from entering the studio through the 2.1 metre (6 ft, 2 in.) high sliding glass door and two fixed glass panels within its 2.85 metre (9 ft, 4 in.) ceiling height.

Form follows function, and, as another saying goes, passive solar houses need active people. The idea of architects Uta and David for this studio roof form is to capture the magnificent view to the north – whether for inspiration or subject matter – use the warming northern sun for heat from sun-up until the studio is being used or the sun’s rays are lost behind the rest of the house, but above all, enable plenty of steady, ambient light from the south to enter the studio. On bright, sunny mornings, it will be possible to close off the glare of the harsh northern sun using curtains or blinds and still be able to create art under natural light. How? The entire upper half of the back wall of the studio will be made of a special glazing material called Danpalon, instead of glass, and there will be nearly 8 sq. metres (9.6 sq.  yards) of it.

By the middle of Day 79 (20 June), the rafters have been fixed on the studio roof and the team is two-thirds of the way to completing the rafters for the main roof, which forms one wing of the butterfly.

The rafters here are noticeably different from those used on the studio roof. These butterfly roof rafters are Tasmanian oak, a eucalyptus hardwood that is not an oak at all; this timber will feature elsewhere inside the house. The carefully tapered ends of these rafters will be permanently visible. They will be left unsealed, protected by the roof’s eaves from the sun’s damaging UV rays and by small metal end caps to prevent moisture from entering the end grain.

Unlike the studio roof, which is oriented to allow maximum ambient light from the south, the long and large roof on the north side of the butterfly is pitched higher on the sunny side to maximise the sun coming into the house and warming it in the winter time, when the sun is lower on the northern horizon. This passive solar effect only requires the occupants to be aware of the season, the sun’s position and to act accordingly in the morning by raising the blinds to let the sun’s rays in.

Keeping the house cool in the summer months can also be done passively, with rafters long enough to create an eave area that will block the sunlight from entering the house when the angle of the summer sun is much higher. Another summertime benefit of the butterfly roof will be in cross-ventilation: cooler air from the shady south side of the house will enter through highlight windows and be drawn up the slope of the roof and ceiling in the kitchen-dining-and-living area; as it warms up it will leave the house on the northern side via openable windows below the eaves. Again, form follows function.

With the main roof plane coming along well, steel fabricators Mark and Justin, of Light Construction and Welding, return on Day 79 to install the steel channel that will help support the roof rafters on the smallest of the three roof planes. A small crane places the steel.

First the vertical section …

And then …

… the horizontal section.

Day 80 (23 June) sees the rafters on the main roof plane fully installed and the steel channel welded up so that the rafters on the smaller roof can be installed.

The next step is to block out between the rafters, a task already under way by Damien in the photo above, while others continue on to prepare rafters for the smaller plane of the butterfly roof.

While blocking out and other tasks continue, with the aid of a new team member who has come on board, 4th year carpenter’s apprentice Zac, it’s also purlin time.

Zac (left) and Jib on the purlins. The tapered, exposed rafters for the smaller of the butterfly roof planes (right) are on too.

The small plane of the butterfly roof has its function too. At a 22-degree pitch and north-facing, it will be the perfect spot for a 3Kw solar photovoltaic system to be installed once the roof sheets are on.

Day 82: Purlins span the gaps between the rafters on the studio roof. The metal roof sheets will be screwed into the purlins by the roofers.

Day 83 (26 June): the main roof structure, with its purlins almost done, is nearly ready.

The sun casts a great shadow pattern thanks to the purlins.

Friday, 27 June, marks a significant milestone. No, the roof sheets have to wait still, but the project is far enough along for Jib to work his last day, ready to head over to be the site foreman on another of Ian’s projects starting Monday.

Building a house is more than functionality, materiality and delight, more than form follows function in the thinking, the making and in the completion. It is about people, the right ones, who make a contribution to our built environment through their labour. We will miss Jib on this building site – and we wish him all the best.

Day 84 (27 June): Jib’s last day on our project; he is welcome back anytime.

The next post will look at some of the details of the preparation of the roof structure. Once those details are completed, the roof sheets go on!