An experimental study was conducted to elucidate the effects of thermal penetration on delamination and the potential changes in failure mode of CLT. The first test series studied thermal penetration depths at various heat fluxes. The second test series consisted of single lap shear tests at homogeneous elevated temperatures followed by a third test series that used the results of the previous tests to study the overall behaviour of CLT as a thermal wave progressed through the material. If charring progresses at similar rates as the thermal wave, then failure due to loss of cross section prevails. Results from the tests showed that the thermal penetration depth before the onset of charring increases at lower heat fluxes. Bond lines with elevated temperatures affected the failure mode of CLT beams. If the thermal wave propagates much faster than charring, then bond failure dominates the behaviour of the beams. The evolution of the thermal loads exhibited in compartment fires therefore have the potential to change the failure mode of CLT members from charring timber failure to cohesion failure with delamination of timber plies.