## 2 Methods
### GWP Values Analysis of Investigated Steel Products  
LCA was used to analyse the impact on the environment in terms of the metal products' GWP value. By utilising the LCA approach, the GWP value of the products could be quantified throughout the selected building life cycle stages (Figure 3). The LCA in this study referred to ISO 14040:2006 [70], in which four steps were conducted: goal and scope definition, inventory analysis, impact assessment, and interpretation analysis. Figure 3 illustrates the selected building life cycles (inside the red box) in analysing the metal cladding product's GWP value in this study, which was adopted from the building life cycle stages and modules in the BRANZ database and BS EN 15978:2011 [57,75].  
According to Figure 3, the scope of the system's analysis in this study involved the upstream processes of the supply and manufacturing of raw materials (A1-A3), water processing (C3), disposal (C4), and material recycling, reuse, and recovery stages (D). The selection of the investigated life cycle stages of the products was made in reference to the primary goal of this study, which was to analyse the GWP values of the specific steel products. The A1 through A3 modules in the production phases were required to be computed when analysing specific architectural components, while the other stages are optional computations. In addition, the construction and in-use stages for the specific metal roofing and cladding products were not assessed due to a lack of knowledge or information on how these processes occurred in the New Zealand construction industry. Thus, the production stages, EOL stages, and potential carbon offset from the recycling process were assumed to be the critical life stages and became the focus of the study. In addition, the geographical boundary in this study was the six mentioned cities in New Zealand, with 90 years of service life based on the BRANZ database [57]. The functional unit of this study was one square meter of roofing and wall cladding metal products used for 90 years in New Zealand buildings.  
Furthermore, LCAQuick [71] was performed to quantify the GWP values of the investigated steel products. Two main data sources were required to calculate the GWP values using the LCA tool: internal and external data. The internal data were sources of information about the buildings or products, such as the products' quantities, building functions, and service time, whereas the external data involved the impact factors of energy  
Figure 3: Selected building life stages of the investigated metal cladding products.  
and water and the environmental product declaration (EPD) of the specific metal products. Therefore, the spatialised material inventory map was included in the LCAQuick tool for the carbon emission calculation in collaboration with the LCA approach and GIS technology.  
However, due to the limitation of the LCAQuick tool in calculating GWP values from specific metal products, an additional formula was used to develop the analysed data from the tool. The equation to calculate the GWP of these specific metal roofing and wall cladding products is given below in Equation (1). Two essential elements in this equation are the total quantity of the product and the emissions from the product's life cycle stage.  
\[\text{GWP}_{T\&S,cladding}=\sum_{t}(MS_{T\&S(t)}\times LCE_{T\&S}\times CF_{T \&S(t)}), \tag{1}\]  
The formula was used to measure the GWP values of specific metal roofing and wall cladding products (\(T\&S(t)\)). The term \"\(LCE\)\" refers to the life-cycle emissions contributing to the GWP value of the metal products per square meter (kgCO\({}_{2}\)eq/m\({}^{2}\)). This value was calculated based on the calculation from the LCA tool. \(CF\) was a characteristic factor derived from the life cycle impact assessment model. From the construction stock analysis, the \(MS\) value was found. It was based on the mapping of metal products that were in use in the project (m\({}^{2}\)).