The building sector's contribution to global CO2 emissions requires innovative approaches to reduce its environmental impact. Mass timber offers a viable solution due to its lower global warming potential during production and its ability to temporarily store carbon. This storage not only reduces radiative forcing, but also removes carbon from the atmosphere and can help mitigate climate change. The inclusion of biogenic carbon flows in LCA is therefore essential to accurately assess the environmental performance of a mass timber building. However, European standards do not include this in their guidelines for conducting LCA of bio-based materials. A systematic review of LCA studies on mass timber buildings highlighted the low presence of biogenic carbon flows and found the need for an advisory decision tree framework for accounting for these flows. Through an applied case study of a recent mass timber building project in Rotterdam, this research itself demonstrated that prolonged storage periods of wood products in construction can lead to a negative Global Warming Potential (GWP) score, thus supporting the inclusion of biogenic carbon in LCA models. However, the variability of LCA methodologies makes it difficult to compare studies on bio- based materials. To address this, a harmonization process among standardization organizations is recommended. Therefore, the decision tree framework is intended as a first approach to guide practitioners on how to model biogenic carbon, emphasizing the importance of accounting for temporal storage and forest dynamics based on a GWP-bio value.