28 CONTEXT 185 : SEPTEMBER 2025 availability, their carbon cost, especially when transported intercontinentally, is significantly higher. Local sourcing offers not just heritage compatibility but also meaningful reductions in embodied carbon. Production emissions The production emissions for roofing slate vary significantly across sources, but interpreting these values requires caution, making direct comparisons between imported and indigenous slate problematic. EPDs for Spanish slate, which dominate the dataset, report production emissions (normalised to a standard thickness and coverage of 39 kg of slate/m² of roof) ranging from 3.0 to 11.1 kgCO₂e/m², although most were in the region of 4 kg kgCO₂e/m². For UK natural slate, estimated emissions are towards the upper end of this range, at 9.0 kgCO₂e/m² for slate of comparable thickness. The UK estimate is based primarily on data from Crishna et al (2010 and 2011), now over a decade old, and may not reflect improvement in quarrying practices, energy efficiency or grid decarbonisation. A more recent assessment could be expected to yield a lower figure. An advantage of UK slate that still needs to be evaluated relates to co-product utilisation. The EPDs for Spanish slate imply that the extracted slate that does not meet the specification for roofing slate is returned to the quarry as waste. In the UK, anecdotal evidence suggests that a large proportion of quarried material is used for other valuable purposes, such as walling stone, paving or aggregates.² This means that in the UK, part of the environmental burdens associated with the initial steps of extraction and manufacturing can be allocated to the co-products, thereby reducing the share of embodied carbon attributed specifically to the roofing slate. These issues highlight the need for UK-specific EPDs, allowing betterinformed material choices. Without them, the comparison remains uncertain, and UK slate may be unfairly represented in carbon assessments despite its low transport emissions. Alternatives The study also reviewed EPDs for concrete, fibre cement and clay tiles. These were from European sources, as no UK-specific EPDs for these materials were identified at the time the research was carried out. Reported production emissions (per m² of roof coverage) are: • Fibre cement tiles from Belgium and Ireland: 6.8 and 16.2 kgCO₂e/m2 respectively • Concrete tiles: 9.1 kgCO₂e/m², with significantly higher transport emissions (1.5–7.5kgCO₂e/m²) due to weight and distance from Denmark • Clay tiles from Germany: 7.2 kgCO₂e/m² These alternatives have somewhat higher production emissions than those shown in most of the slate EPDs. Transport comparisons are less useful here, as in practice at least some of these materials may be sourced more locally. Durability and replacement The need to replace a roofing material during a building’s lifetime can substantially increase its total embodied carbon, which is assessed over both 60-year and 120-year periods according to RICS guidance (2023). A single complete replacement of the roof in the study period would result, approximately, in a doubling of embodied carbon, with further replacements increasing it still further. A full assessment also needs to cover any maintenance and repair requirements. This is a key issue for less durable roofing materials, but UK slates – such as those from Wales and Cumbria – may avoid the need for replacement altogether even over a 120-year period, providing a clear carbon benefit. While data on durability is not formally recorded, UK slates are considered more durable than imported alternatives, with anecdotal evidence from historic buildings suggesting lifespans exceeding a century.³ By contrast, many imported slates and alternative products have been in widespread use only for a few decades, and their long-term durability in the UK climate remains less proven. Calculated transport emissions from quarry region to construction site ² Private conversations with Ian Ramsey of Burlington Stone. Scenario No material decarbonisation With material decarbonisation Two complete replacements 2 1.17 One complete replacement of roof covering 1 0.5 One replacement with reuse of existing roof covering 0.3 0.15 No replacement 0 0 Note: ‘No material decarbonisation’ refers to an assumption used in lifecycle assessments (such as whole-life carbon calculations) where the carbon intensity of materials remains constant over time. It represents a conservative, worst-case assumption where today’s carbon intensities persist into the future. This is often contrasted with material decarbonisation scenarios, where future replacements are modelled with lower emissions, reflecting expected industry improvements and cleaner energy grids. ³ Opinions gathered from practitioners, experts and industry, such as Terry Hughes, Ian Ramsey and Alison Henry Total cradle-tosite embodied carbon (A1–A4) for roofing slates and alternatives A4 GWP: kgCO₂e/m2 25 20 15 10 5 0 Wales (Penrhyn) Cumbria (Kirkby) San Pedro: sea route San Pedro: overland Minas Gerais (Brazil) Shaanxi (China) London Leeds Glasgow
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