Understandingthe end of the decay phase in a compartment fire is important forsafety design, as it can indicatewhen structural hazards in a compartment have ceased. However, smouldering, a slow, persistent, and flameless form of combustion, can continue for days and hours following flames. Smouldering of mass timber has rarely been reported intimber experimentsdue tomany experiments being stoppedshortly after flames.This work presents for the first-timeobservations following the end of flames in three large compartment experiments withcross-laminatedmass timber ceiling and glulam columns, known as CodeRed. The analysis focuses on initiationand growthof smouldering over48 hours. 0.012–0.58hotspots per m of timber edge developed in 19 locations, andninespreadthrough the CLT, formingholesin the ceiling, compartmentintegrityfailure, andcollapseof an unloaded column. Visual and infrared imaging wasused to track smouldering, extinction, suppression,transition to flaming, and formation of holes.This paper shows that smouldering poses a hazard to mass timber buildings because it is hard to detect and can weaken thestructure over days followingflames, andtransition to flaming, startingnewflaming fires.Thesefindings are importantforbuilding post-fire recovery.