Fire risk assessment is mainly concerned with the evaluation of the probabilities of occurrence of undesirable events which can cause significant damage to life and property. Such events include flames reaching the ceiling of the room where a fire starts and the fire spreading beyond the room. For this purpose complex stochastic models (Ramachandran, 1995b) have been developed to predict the growth of a fire within and beyond a room as a function of time. However, a simple technique known as fault tree analysis (pp. 114-16) is generally adopted in practical problems to evaluate the probability of occurrence of an undesirable (top) event in the presence or absence of a fire protection device. A reduction in this probability is a measure of the effectiveness of the device. Fault trees and their inverse, success trees, are known as logic trees.