The results presented in this paper are part of a multi-partner collaborative project named ‘The Épernon Fire Tests Programme’, the purpose of which is to analyse the different behaviours of combustible and non-combustible loaded structures when they are tested under standard and ‘natural’ fires. The project has several scientific objectives, such as quantification of the energy participation of combustible materials in fire tests, the influence of combustible surfaces and ventilation factors on the dynamics of compartment fires, and the thermomechanical behaviour of concrete and timber elements under natural fires. This paper focuses on the study of the thermomechanical behaviour of cross-laminated timber (CLT) slabs under standard and natural fires. The objective is to bring new experimental data to the complex study of the behaviour of unprotected CLT slabs exposed to fire. In addition, we propose a detailed analysis of the thermomechanical behaviour of the slabs, especially in order to better understand the failure of some of the slabs during the fire.