This study presents a simplified theoretical model to predict the ignition of FRP composites of general thermal thickness (GTT) subjected to one-sided heating. A simplified GTT heat transfer model to predict the surface temperature of GTT composite panels was developed, and the exposed surface temperature was used as ignition criterion. To validate the GTT model, intermediate scale calorimeter fire tests of E-glass fiber reinforced polyester composite panels at three heat flux levels were performed to obtain intermediate-scale fire testing data in a controlled condition with well-defined thermal boundary conditions. The GTT model was also verified by using results from finite element modeling predictions. This model can be used to estimate the surface temperature increase, time-to-ignition, and mass loss of FRP composites for fire safety design and analysis. Copyright © 2013 John Wiley & Sons, Ltd.