Hence, the owner of the building has to pay this cost every 20 years. Since green roofs have a higher lifespan than conventional roofs, reduced cost of replacing a conventional roof is considered as a benefit. If the probabilistic analysis (Monte Carlo) randomly selected a time frame of 40 years, the cost of replacing was considered as $160/m2. Furthermore, if the Monte Carlo simulation randomly selected a time frame of more than 40 years, an owner would have to replace the conventional roof at least two times. In such a case, the benefit is estimated as double of the benefit of one conventional roof replacement. Therefore, a benefit of $320/m2 (for green roofs) was used to estimate the NPV.