The use of large timber elements (mass timber) in buildings instead of concrete and steel can improve sustainability, aesthetics, construction times, and costs. However, the adoption of mass timber in high-rise buildings is currently hindered by concerns regarding safety and structural performance in fire. This thesis finds a lack of experimental data for large compartment fire experiments (floor area > 100 m2). Therefore, this thesis introduces CodeRed, the largest mass timber compartment fire experiments to date, with a floor area of 352 m2, and a mass timber ceiling and columns. A fire-resistant camera array was iteratively designed to improve data retrieval and positioned around the compartments to study fire behaviour via visual diagnostics. Three hazards that have not been studied significantly in the literature were studied in-depth, including flame spread, smouldering of timber elements, and firebrands, which are burning pieces of timber ejected from compartment openings. Compared to previous large concrete experiments, inclusion of a timber ceiling increased flame spread rates, reduction of the density of the fuel load inside the compartment reduced the rate of spread, and the addition of ceiling encapsulation delayed ceiling ignition. During the experiments, approximately 4500 firebrands were ejected from a single opening, posing a potential ignition hazard. Increased firebrand quantities and velocities were observed where external flaming occurs. Over the 48 h after the end of flames, nineteen smouldering hotspots were studied over three experiments. Hotspots developed along the edges of timber elements where heat losses are minimised at a density of 0.012-0.58 hotspots per meter of timber edge. Hotspot phenomena including spread, self-extinguishment, and transition to flaming. Glulam column collapse and formation of nine holes in the timber ceiling were studied. Overall, this thesis introduces valuable findings on understudied hazards in timber buildings that can be mitigated to improve fire-safe modern timber construction.