A method of analysis has been developed to simulate the behaviour of composite floors exposed to fire. The governing differential equation, based on the orthotropic plate theory, is solved by the finite difference method.The solution requires the calculation of plate rigidities as influenced by the thermo-mechanical properties of the materials. This is achieved by first calculating the temperature distribution in the floor. Numerical integration of stresses calculated from non-thermal strains, compatible with the appropriate temperature dependent stress-strain relations, enables determination of the stress-resultants (moments and forces). Equilibrium of in-plane forces is satisfied first. The resulting moments and curvatures enable the plate rigidities to be determined. For each stage of fire, starting with ambient conditions a small number of iterations are required to achieve equilibrium. The response of the structure, in terms of deflections, is repeatedly calculated as the fire develops. This defines the fire duration. The method has been validated through comparison with a number of published fire test results.