The predictability of heavy timber (or mass timber) structures in fire is often given as a benefit, and while this is generally true in relation to standard fire resistance tests, it is not necessarily the case in real fire events. To enable engineers to better design structures to not collapse during or following a fire event, it is essential to consider how the additional fuel contributed by the timber structure changes the fire severity and what the maximum resultant depth of char is likely to be. Char depths depend on a range of factors, including situation and fire conditions and nature of the wood and of adhesives. Fire protection methods for structural timber include encapsulation by non-combustible boards or the application of coatings that affect the combustion process, as well as fire sprinkler systems. Relying solely on results obtained from standard fire resistance tests may lead to some structures that are vulnerable to failure during or after fire particularly in jurisdictions where the prescriptive fire resistance requirements are relatively low. There are still many research questions needing to be answered including: the conditions needed for burning extinction of mass timber, the performance of structural connections, the effect of wind on burning behaviour in fire, as well as understanding the basic fire dynamics. While there are many advantages and opportunities for constructing more buildings using heavy timber there is also a need for caution and a considered approach to their fire design.