The efficiency of fuel breaks installed in wildland–urban interfaces to reduce fire hazard depends strongly on the conditions of spread (rate of spread, flame height) of a surface fire through the shrub on the ground and also on the possibility of a transition for this fire from the understorey vegetation to the canopy. The aim of the present paper was to study (using numerical simulation with a physics-based model) the behaviour of surface fires propagating through Mediterranean shrub and to evaluate, from the characteristic dimensions of the flame, the onset of transition from a surface fire to a crown fire. The geometry of the flame was defined from the energy loss in the gas resulting from the radiation emission of soot particles, the flame contour was reconstructed from a threshold level fixed at 60 kW m^–3. The numerical results were compared with experimental correlations of the geometry of the flame obtained for static and spreading fires. Extensive calculations were performed through a shrubland (Quercus coccifera and Brachypodium ramosum) for various fuel depths H_fuel ranging from 0.25 to 1.5 m and for wind speeds U_H ranging from 1 to 10 m s^–1. Then this study was extended to situations including a supplementary fuel layer representing the canopy of small trees (Pinus halepensis). The numerical results were analysed, introducing a dimensionless physical parameter, the Froude number, defined as the ratio between the inertial force due to the wind flow and the buoyancy. The results obtained with an upper fuel layer highlighted the role played by radiation heat transfer for the transition of surface fire to the crown. Some calculations were also carried out to study how a reduction of surface fuel on the ground can affect the vertical transition of the fire.