# Improving Sustainability of Steel Roofs: Life Cycle Assessment of a Case Study Roof
## 1 Introduction  
Human activities are the primary drivers of change to the Earth's natural processes, potentially triggering abrupt or irreversible environmental changes that could be disastrous for human well-being, such as climate change [1]. The construction business emits a substantial amount of carbon dioxide, the leading cause of climate change [2]. It accounts for one-third of global carbon emissions and over 40% of primary energy consumption [3]. In New Zealand, the construction sector is responsible for 20% of gross national emissions [4], and it has increased by 66% in the last decade [5]. Approximately half of these building-related emissions are from embodied carbon in the structures [6], although this number may be higher in New Zealand [7]. The manufacture of building materials, such as steel, cement, and glass, is identified as a significant source of these emissions [8]. Also, building activity contributes to a number of environmental problems, such as eutrophication, acidification, and the depletion of resources [9]. In New Zealand, the steel material for housing roofs accounts for more than 75% of the market share [10]. Steel roofs with timber battens are the most common type of roofing for residential buildings in the country due to their low cost and high performance. With current practice, the great majority (approximately 95%) of the nation's steel roofs are fabricated from base steel produced by local producers, with the rest generally being imported from South Korea [11]. Steel roofs in the country typically comprise thin sections of cold-rolled steel coil folded to form profiles such as corrugated, trapezoidal, or standing seam. These profiles can sustain the harsh wind, rain, and snow, as well as the maintenanceand access loads exerted over the roof's lifespan. Research related to steel materials has been widely conducted [12; 13; 14; 15], not only for roofing but also for other building components such as columns and beams [16; 17; 18; 19; 20; 21; 22]. Several studies analysed the strength of this material [23; 24; 25; 26; 27; 28] and its ability under certain conditions (e.g., high temperature) [29; 30; 31; 32; 33], as well as proposed the design rules and assessment method of the steel product [34; 35; 36; 37; 38; 39]. However, despite its good performance and the large market for steel materials in the country, the environmental impact of the steel industry cannot be negligible and needs to be considered [40; 41]. The steel industry accounts for about 8% of global carbon emissions [42]. Swalec & Shearer [43] estimated that 61% of global crude steel is produced using blast furnace-basic oxygen furnaces (BF-BOF). The use of coil in this process results in significant carbon emissions, which are approximately 2.2 tonnes for every tonne of steel produced in a typical BF-BOF. On the other hand, the electric arc furnace (EAF) route, where a natural gas-base can be used for the reduction reaction, can lower emissions to 1.4 tonnes of carbon per tonne of steel. [PERSON] [44] noted several pathways for emissions reduction, such as the use of scrap steel in EAF, biofuels, or carbon capture with BF-BOF and EAF, and blue or green hydrogen for the reduction reaction. The International Energy Agency, through the \"Net-zero by 2050\" report [45], estimated a need for over half of all steel production to be from EAF and approximately 30% of steelmaking to be equipped with hydrogen use and significant carbon capture adoption. It also noted that 40% of the emissions reductions would need to be achieved from material efficiency in design and recycling, requiring a 20% reduction in global steel demand by 2050. Efficiency in steel roof design can reduce the weight of raw material needed in the manufacturing process, which can greatly reduce the embodied carbon emissions of the product. As previously proposed [46; 47; 48; 49], structural optimisation techniques can be used to improve the efficiency of steel product design by optimizing form, shape, and topology while not compromising design quality.