Abstract
The residential building sector regularly satisfies a diverse range of housing needs whilst addressing respective capital-cost considerations. Designers and builders must also be aware of the environmental implications of their design specifications; the work here adds to a body of knowledge concerned with carbon footprint and embodied energy demand, specifically through an examination of alternative roof-covering materials. A life cycle assessment (LCA) has been carried out, within a West Australian context, to compare impacts for the roof specification options of: clay tile; concrete tile; and sheet metal. In locations where recycling facilities are unavailable and thus disregarded, it is found that clay tiles have the lowest carbon footprint of 4.4 t of CO2 equivalent (CO2e-) and embodied energy demand of 52.7 Mega Joule (MJ) per 100 m2, while sheet-metal roofing has the highest carbon footprint (9.85 t of CO2e-), with concrete roof tiles having the highest embodied energy demand (83 MJ). Findings confirm that a sheet-metal roof can obtain significant carbon and embodied energy saving benefits (i.e. 71–73%) compared to clay tile or concrete roof covers through ongoing encouragement of recycling strategies and increased local recycling facilities able to embrace residual cradle-to-cradle material reuse.
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Appendices
Appendix A: a sample calculation of timber structure
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Details and specification were provided by drawings and the Design Engineer from City of Melville.
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General specification of a residential house:
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Joist spacing: 600 mm.
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Rafter spacing: 600 mm.
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70/75 mm frame.
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Pitch angle: 20 degrees.
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Single storey.
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Specification of material:
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Top plates (AS 1684.4 Table A22).
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Roof type: sheet-metal roof.
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Rafter Span = 9,000 mm.
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Timber size: MGP10 2/45 × 70.
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Roof type: clay and concrete tile.
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Rafter Span = 9000 mm.
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Timber size: MGP10 3/45 × 70.
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Ceiling Joists (AS 1684.4 Table A27).
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Joist span: 3600 mm.
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Timber size: MGP10 120 × 45.
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Hanging beam (AS 1684.4 Table A28).
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Ceiling joist span: 3600 mm.
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Hanging beam span: 3600 mm.
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Timber size: MGP10 240 × 35.
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Strutting beams (AS 1684.4 Table A32).
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Sheet roof strutting beam span: 4800 mm.
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Timber size: sheet metal: MGP10 2/190 × 35.
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Timber size: clay and concrete tile: MGP10 2/240 × 45 Underpurlins (AS 1684.4 TA33).
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Strut spacing: 2400 mm.
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Timber size: sheet metal: MGP10 2/90 × 45.
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Timber size: clay and concrete tile: MGP10 2/140 × 35 Rafters (HySPAN).
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Timber size: sheet metal: MGP10 120 × 35.
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Timber size: clay and concrete tile: MGP10 120 × 35 Ridge Beam (AS 1684.4 Table A36).
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Beam spacing: 2400 mm.
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Beam span: 3600 mm.
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Timber size: sheet metal: MGP10 2/190 × 45.
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Timber size: clay and concrete Tile: MGP10 2/240 × 45 Batten (AS 1684.4 Table A37).
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Rafter spacing: 600 mm.
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Batten spacing: 900 mm.
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Timber size: sheet metal: MGP 45 × 70.
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Rafter spacing: 600 mm.
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Batten spacing: 330 mm.
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Timber size: clay and concrete Tile -MGP10 35 × 42 Hip or Valley Rafters (HySPAN).
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Timber size: sheet metal: MGP10 190 × 45.
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Timber size: clay and concrete tile: MGP10 240 × 45 Roof Struts.
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Timber size: MGP10 90 × 45.
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The timber frame required was calculated by measuring the drawings to calculate the length and then calculating the volume using the dimension of the timber frame in the above section.
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Timber frame required (tile: concrete & clay):
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Total length of top plate (MGP10 2/45 × 70) = 43.2 m.
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Total volume of top plate = 43.2 × 0.045 × 0.07 = 0.136 m3.
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Total length of ceiling joists (MGP10 120 × 45) = 204 m.
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Total volume of ceiling joists = 204 × 0.12 × 0.045 = 1.1 m3.
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Total length of hanging beam (MGP10 240 × 35) = 44.6 m.
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Total volume of hanging beam = 44.6 × 0.24 × 0.035 = 0.375 m3.
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Total length of strutting beam (240 × 45) = 13.45 m.
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Total volume of strutting beam = 13.45 × 0.24 × 0.045 = 0.145 m3.
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Total length of underpurlin (MGP10 140 × 35) = 60.7 m.
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Total volume of underpurlin = 60.7 × 0.14 × 0.035 = 0.3 m3.
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Total length of rafter (MGP10 120 × 35) = 197 m.
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Total volume of rafter = 197 × 0.12 × 0.035 = 0.83 m3.
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Total length of ridge beam (MGP10 240 × 45) = 3.22 m.
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Total volume of ridge beam = 3.22 × 0.24 × 0.045 = 0.035 m3.
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Total length of batten (MGP10 35 × 42) = 317 m.
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Total volume of batten = 317 × 0.035 × 0.042 = 0.47 m3.
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Total length of valley rafter (MGP10 240×45) = 26 m.
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Total volume of valley rafter = 26 × 0.24 × 0.045 = 0.28 m3.
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Total length of roof struts (MGP10 90 × 45) = 36 m.
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Total volume of roof struts = 36 × 0.09 × 0.045 = 0.15 m3.
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Total timber volume = 3.821 m3.
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Timber frame required (sheet metal):
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Total length of top plate (MGP10 3/45×70) = 43.2 m.
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Roof cladding required:
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Clay tiles:
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No. of tiles per m2: 11.9
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No of tiles required: 11.9 × 150 = 1785 tiles.
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Mass per tile: 3.1 kg.
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Total mass of tiles = 3.1 × 1785.
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Total mass of tiles = 5533.5 kg.
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Total mass of quartz (60%) = 5533.5 × 0.6
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Total mass of quartz (60%) = 3320.1 kg.
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Total mass of clay minerals (40%) = 5533.5 × 0.4 Total mass of clay minerals (40%) = 2213.4 kg Concrete Tiles:
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No. of tiles per m2: 9.4
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No of tiles required: 9.4 × 150 = 1410 tiles.
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Mass per tile: 5.55 kg.
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Total mass of tiles = 5.55 × 1410.
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Total mass of tiles = 7825.5 kg.
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Total mass of quartz (70%) = 7825.5 × 0.7
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Total mass of quartz (70%) = 5477.85 kg.
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Total mass of Portland cement (30%) = 7825.5 × 0.3 Total mass of Portland cement (30%) = 2347.65 kg Steel Roofing:
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Required steel roofing: 150 m2.
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Mass: 4.3 kg/m2.
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Total mass of metal roofing: 150 × 4.3
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Total mass of metal roofing: 645 kg.
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Total mass of Aluminium (55%): 354.75 kg.
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Total mass of Zinc (43.5%): 280.575 kg.
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Total mass of Silicon (1.5%): 9.675 kg.
Appendix B: calculation of the effects of varying solar reflectance of roofing materials
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Physical Data: Physical data assumptions are detailed below for input into Eq. 1.
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Temperature readings are taken from the (WA) Bureau of Meteorology (BOM). The temperature (T1) is the average temperature recorded at 3 pm from 1994 to 2011 at Perth Metro WA each month. Industry representatives (thanks to TT air-conditioning), the conformable temperature will vary individual to individual. However, industry representatives note that many buildings are set at a room temperature of 24 degrees C.
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Average radiation figures for areas in Perth metro WA have been derived from BOM. A figure of 625 W/m2 for a 6 h day is typical. BOM has also provided data that the heat flow transfer coefficient is 25 W/m2.K.
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The Building code of Australia, 2005 notes that roofing requires a total R value (= 1/U) of 2.2 m2. K/W.
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According to Selby (2006) the absorption rate for clay tile, concrete tile and sheet metaling is 0.63, 0.67, and 0.38.
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Q = rate of heat flow per square metre from roof to the inside.
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U = the overall heat transfer coefficient between the ambient and inside (W/m2/K) Note that 1/U = R (the thermal resistance).
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H = Outside transfer coefficient between roof and ambient (W/m2/K).
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∂ = rate of absorption to solar radiation.
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G = Solar radiation per unit area.
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Calculation for January (Clay Tile):
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Average temperature at 3 pm: 29 degrees.
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Comfortable temperature: 24 degrees.
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Change in temperature = 29 − 24 = 5 degrees (Cooling).
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α = 0.63 (Clay tile).
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h = 25 W/m2.K
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G = 625 watts/m2.
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R = 1/U = 2.2 m2 K/W (Building code of Australia, 2005).
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Roof area = 100 m2.
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Heat Loss = (1/2)*((0.63*635/25) + 5).
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Heat Loss = 9.43 W.
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Heat Loss = 0.94 kW.
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Heat Loss over 6 h = − 0.94 × 6 = 5.66 GJr.
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Heat Loss in a month = − 5.66 * 31 days = 175.43 GJr.
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Heat Loss in 65 years = − 175.43 × 65 years = 11,403.07 GJr.
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Total Cooling in 65 years during the month that requires cooling = 59,961.61 GJr.
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Calculation for applied energy.
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1 x Mitsubishi 4.2 kW Air Conditioner.
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Energy Efficiency (Cooling): 2 stars (Sourced from TT air-conditioning).
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COP: 3.25 (Energy Aus).
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Energy efficiency (heating): 2.5 Stars (sourced from TT air-conditioning).
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COP: 3.5 (Energy Aus).
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Cool Capacity: 4.2 kW.
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Heating Capacity: 5.4 kW.
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Total Output Energy for Clay (Cooling): 59,961.61 GJr.
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COP = output/input.
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3.25 = 59,961.61 GJr/Input.
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Input = 18,449.73 GJr.
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Le, A.B.D., Whyte, A. & Biswas, W.K. Carbon footprint and embodied energy assessment of roof-covering materials. Clean Techn Environ Policy 21, 1913–1923 (2019). https://doi.org/10.1007/s10098-018-1629-9
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DOI: https://doi.org/10.1007/s10098-018-1629-9