We use recent advances in material flame spread physics to emphasize that empirical flammability indices might be replaced by fundamentally derived parameters. Flame spread rates in upward wall fire situations can be described both for charring and non-charring materials by using a characteristics length scale,lm, and a characteristic time,tp. The length scale,lm, is related to the distribution and magnitude of the heat flux from the flames to the wall surface as well as to burning material properties and mixing processes in the turbulent wall flow. The same characteristic time,tpdescribes both the spread rate, by the determination of ignition time of yet unpyrolyzed material, and the transient pyrolysis of the solid for situations where such pyrolysis can be approximated by a thermal pyrolysis model.