A series of upward flame spread tests on different wood products was carried out on 2.4 m and 7.2 m high wall assemblies. The ignition source was a 1.2 m wide and 0.1 m deep propane diffusion burner applied usually at a rate of 100 kW. The measurements included gas and surface temperatures, heat fluxes to the specimen, rate of heat release, etc. After an initial growing fire, a period of decay was observed until the lower parts of the wall started to burn through. If the rear side of the board was insulated, a second phase of increased flame spread was observed, whereas in the case of a conducting substrate the intensity of the fire remained low. A thermal flame spread model was successfully applied to simulate the rate of heat release as a function of time.