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Structural Concepts

The design chosen to replace the existing Glen Murcutt roof relies heavily on trusses as the main structural support due to its rigidity of the triangular pattern and its ability to resist compressive and tensile loads.

Kempsey is situated on the coastal NSW, so the house would be subjected to harsh winds from the sea, as well as potentially cyclonic weather. Therefore trusses were chosen due to its structural integrity.

As seen in figure 3, the roof has a relatively high pitch at 30˚ even though the length of the rafters are short. This decision was made due to the fact that Kempsey, NSW is notorious for flooding and has a large annual rainfall. Therefore with a large pitch, the rainfall will be able to drain away faster. (Weatherzone 2013)

Saddle Truss

Girder Truss

Figure 1

Figure 2 Figure 4

Figure 3

30˚

Figure 2 shows a dutch gable roof structure covering the patio area of the house. Hip roofs generally perform better in extreme weather but require more materials. Traveller (2009) states that gable roofs are simpler in design and are easier to maintain. Therefore having a dutch gable, a hip roof with a small gable, means that the roof can get the best of both worlds. The roof area covering the patio does not have as many supports to lift that part of the roof, so the partial hip roof was chosen because it is a more stable structure than the gable roof.

The protusions to the main roof are gable roofs, as seen in figure 4. This was chosen mainly for aesthetic reasons. The structural integrity has not been compromised by choosing a gable roof as the trusses provide adequate support.

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Materials ConstructionA steel frame was chosen as opposed to the more popular timber frame as it is easier to work with labour-wise, it is termite resistant and it is less subject to thermal and moisture expansion, according to Lester (2009). Wood would not be a viable option in an area, such as Kempsey, that has a high annual rainfall and high humidity as the wood would expand and rot from the moisture content.

Tile roofing has been chosen over the alternative sheet steel/tin roof for this particular house. Terracotta tiles offer more resistance to wind suction in storms than lighter weight metal roofs. Also, tiles generally last longer than metal roofing. Tile roofs provide highly effective thermal insulation. (Lansell Homes 2012)

Insulation will be installed under the roof to provide extra thermal insulation. Steel frames are less thermal insulating than the wooden frame. Reflective foil laminates would be used as insulation. It provides not only heat insulation under roof but can serve as a vapour barrier from the condensation that forms when the sheeting becomes colder than the air in contact with it. (Lansell Homes 2012)

Due to the light weight of the material, steel framed houses can be built quickly and is much more economical to work with than timber. Also, a steel framed roof saves construction time and labour as the roof trusses are all pre-manufactured with precies measurements for each member. (JV Global 2004)

The roof will have components of the roof pre-manufactured and delivered on site, which reduces the assembly time. This is desirable in an area such as Kempsey which receives an above average amount of annual rainfall and has unpredictable weather which could dramatically increase construction time.

The steel components that are constructed off-site are generally clinched, riveted or screwed together (seen in figure 6), while the on site assembly and last minute changes to the frame are usually done by nailing the steel together.

Figure 6

As seen in figure 6 above, the roof tiles are propped and are attached on an angle by tile battens. These battens are usually nailed or screwed onto the steel frame. The tiles are held down using tile clips.

Figure 5

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Construction Aesthetic Design & Function

As seen in the model, the roof projects beyong the wall, leaving an eave of about 20cm in real life. This is the keep the upper part of the wall dry and to keep rain from seeping in where the wall meets the roof. In a high rainfall area such a Kempsey, quite a large roof over hang is required. Therefore 20cm was chosen to be appropriate.

In the model, the 30˚ angle of the roof can be seen to be quite steep. As previously stated, this is desired to evacuate the rainfall as soon as possible.

Another function of the roof is thermal insulation. This is achieved by placing insulation under the roof, which serves to keep the heat in during colder days and keep it cool during warmer temperatures.

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Reference List1. Cadden, R & Novelli, TP 2004, Guide to steel framed construction, JV Global, viewed 28 August 2013, < http://www.lib.unimelb.edu.au/recite/citations/harvard/ref242-elecSourceWebDoc.html?style=2&type=4&detail=2 >2. Lansell Homes 2012. Tiles Versus Metal Roofing, viewed 28 August 2013, < http://www.lansellhomes.com.au/_blog/News/post/Tiles_Versus_Metal_Roofing >3. Lester, B 2009, Steel versus Timber, Green Lifestyle, viewed 28 August 2013, < http://www.greenlifestylemag.com.au/features/984/steel-versus-timber?page=0%2C0 >4. Traveller, J 2009, The advantages and disadvantages of a gable roof, Helium, viewed 28 August 2013, < http://www.helium.com/items/1437737-the-advantages-and-disadvantages-of-a-gable-roof >5. Weatherzone 2013, Kempsey Long-Term Averages, viewed 28 August 2013, < http://www.weatherzone.com.au/climate/station.jsp?lt=site&lc=59017 >

constructing environments