Objective: Thermal building simulation tools take account of the thermal capacity of the walls and floors by a one-dimensional characterization. The objective was to obtain thermal equivalent parameters for ribbed or composite slab elements that can be input into one-dimensional models. Method: Transient finite element calculations (FEM) were used to establish the heat transfer to and from composite floors using four deck profiles and for daily heating cycles in compartments with defined heat gains and operating conditions. Results: The performance of composite slabs was compared to a concrete flat slab for a typical office in the UK and Germany. It was shown that a deep ribbed slab generates a maximum heat flux of 30.5 W/m(2) for a 5 degrees C temperature variation about the mean, and that the daily heat absorbed by a typical composite slab was 220 Wh/m(2) floor area. Conclusions: Using the thermal capacity of the ribbed floor slabs, the comfort conditions defined in terms of the number of hours over 25 degrees C are acceptable for many classes of offices. Practical implications: Thermally equivalent properties of ribbed slabs can be used in conventional software to predict the thermal performance. (C) 2013 Elsevier B.V. All rights reserved.