The heat conduction through lightweight roofs is modeled with melting and decomposition processes involved. The derived s e t of equations is solved using a numerical difference procedure. Which has been enlarged widely for these processes. The computer simulations for several roof assemblies are compared with experimental results from large scale fire tests with flat lightweight roofs exposed to point fire sources underneath the roof. The simulation results show good agreement with data points as to the tendency of the curves. The maximum temperatures reached show minor deviations from the test data and give a good estimate for real situations. Differences among the curves are due to scarcely known physical parameters with temperature and perhaps to some simplifying assumptions whose validity will be studied in further work.