Cross-laminated timber, glulam, and other mass-timber products have attracted significant attention as lower-carbon structural alternatives to concrete and steel. The climate argument has two components: substitution (replacing a more emissions-intensive material) and sequestration (storing biogenic carbon in the building for its service life). Both are real, but the magnitude of each depends on assumptions that vary considerably across projects and markets. The USDA Forest Service and the IPCC AR6 Synthesis Report provide frameworks for thinking about forest carbon accounting. The key variables include: how the forest is managed, whether harvested wood would otherwise decompose or burn, manufacturing process emissions, transport distance, how the building compares to a reference structure, how long the building stands, and what happens to the timber at end of life. "Carbon negative" and "carbon neutral" characterisations of mass-timber buildings depend on which of these variables are included and how they are valued. For residential use, mass timber is relevant primarily for structural floors, roofs, and walls in multi-storey or larger single-family projects. Acoustic performance, fire resistance, moisture detailing, and procurement from responsibly managed forests require attention. Structural efficiency matters: using timber where it substitutes for concrete or steel, rather than as decorative cladding, is where the climate benefit is most defensible. ISO 14040 provides the methodology for lifecycle assessment; any comparative claims should declare their system boundary, reference building, biogenic-carbon method, service period, and end-of-life scenario. Editorial caution: Never describe mass timber as inherently "carbon neutral" or "carbon negative." State the LCA boundary, reference building, forest assumptions, service period, and end-of-life scenario behind any comparative number.