<p>Biochar is increasingly applied to soils to enhance carbon sequestration and mitigate greenhouse gas emissions, yet its role in regulating methane (CH₄), a potent greenhouse gas, in engineered substrates remains poorly understood. In particular, there is a lack of field data on how biochar amendments influence CH₄ fluxes from engineered substrates in urban environments, where substrate composition, shallow profiles, and intensive management fundamentally differ from natural soils. Here, we present a five-year period (2020–2024) field study examining the effects of biochar amendment on CH₄ fluxes from engineered substrates used in extensive green roofs dominated by stonecrop species (<i>Phedimus kamtschaticus</i> and <i>Sedum</i> spp.). Biochar-amended modules (~5% v/v; 20 t ha^-1) consistently exhibited greater CH₄ uptake than unamended controls across seasons, with peak uptake rates in spring 2023 nearly fivefold higher (−1.91 ± 0.25 vs. − 0.40 ± 0.10&#xa0;nmol⋅m^-2⋅s^-1). In addition, biochar addition was not accompanied by increased CO₂ emissions. Variability in CH₄ uptake was strongly associated with substrate moisture and water vapour flux, suggesting that biochar modifies gas diffusivity and oxygen availability through moisture-mediated physical controls that favour microbial CH₄ oxidation. These results demonstrate that biochar can substantially enhance CH₄ uptake in engineered green roof substrates and extend the application of biochar-based mitigation strategies to engineered systems beyond conventional soils. The mechanistic insights provided here are broadly relevant to understanding CH₄ cycling in biochar-amended engineered substrates.</p> Graphical Abstract <p></p>