IndexProblems and factors affecting rainwater harvesting of commercial buildingsRoofing materialLand use patternRainfall intensity and durationWeather conditionsIneffective use of first flush systemsIn the field of civil engineering we must consider not only the structural elements, bridges, highways, railways etc. but also the water and environmental sector. When we consider the water resource. Above all it is an essential resource in everyday life and also in the future, due to its demand for sustainability. And even when we think about ethical issues we should pay our attention not only to the use but also to the protection of this resource and to its maximum use. There are some methods to protect the water resource and reuse this resource. Rainwater harvesting is one of the best ways to protect this water resource. In reality, there are mainly two methods for collecting rainwater. Run away from harvesting and collecting on the roofs. Precipitation intensity is the amount of water that falls over a period of time. When we talk about this Sri Lankan situation. According to the data released by the irrigation department in 2017 the data is as follows: Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get Original Essay103 River BasinsMajor WatershedsMahaweli-1027kmqWalawe -2442kmqKalu -2772kmqAverage Annual Rainfall -1861mmAnnual Rainfall Volume -12x1010 m3(IRRIGATION DEPARTMENT DATA IN 2017) Sri Lanka as a tropical country receives rainfall mainly from three basic ways. Monsoons North-East Monsoon South-West Monsoon Convective Cyclonic So rainwater harvesting is a very important criterion because according to the annual rainfall 12x1010 m3, we are using very less amount of water. Sri Lanka had already paid attention to this case since last period. King Prakkramabahu had once said: “Do not let very little water pass from the sky without using it directly in the sea. When we consider the collection method, we mean storing it in such a way that it can be used in the future. For the collection method we should have adequate catchment area, it is the best method to use rooftop collection and use it for household and agricultural work. In Sri Lanka, as in the world, there is a shortage of water today. Precipitation patterns have been confusing. People in rural areas do not have piped water. Natural water resources are not qualitatively and quantitatively capable of satisfying water demand. The problem of drinking water has become so critical as hundreds of people have died due to the CKDu problem in the North Central Province who were consuming poor quality water. Therefore, rainwater harvesting is a better solution to overcome these problems. Due to the increase in human population, their basic needs have also become important. Therefore, the demand for drinking water is increasing day by day. However, nature has been able to provide humanity with the amount of pure natural water. But after the industrial revolution and through the development of new technologies, humanity has taken great care of the environment. Water was used greedily by some nations for their industrial purposes, while other nations suffered from water scarcity. There have been many natural disasters in the world. Climate models have been changed. Environmental problems such as global warming have occurred. Natural water resources have been polluted due to industrial waste dumped insurface water resources without any treatment. Water quality declined rapidly. Therefore, the amount of natural water resources was not sufficient to equitably and efficiently meet the demand for drinking water for every human being. Although water resources are abundant on earth, only a large percentage is drinkable. Even that drop of water has been polluted by man. A water shortage has arisen on this planet not because of any living being but because of humanity. According to the United Nations Department of Economic and Social Affairs, approximately 700 million people in 43 countries suffer from water scarcity today. By 2025, 1.8 billion people will live in countries or regions with absolute water scarcity, and two-thirds of the world's population may be living under water stress. Some people have to walk hundreds of miles to find a simple amount of water to drink. People tend to drink polluted water to satisfy their need for drinking water with the water they have. It is not suitable for drinking. But they have to find water in any way to satisfy their thirst. As a result of consuming that polluted water, they were affected by so many water-related and water-related diseases. Thousands of children and adults have died as a result of drinking polluted water without treatment. The cause of the CKDu problem was also found to be the consumption of polluted water contaminated with heavy metals. So people tend to find solutions for this water scarcity and save humanity from this burning problem. Rainwater harvesting can be achieved as a method. It is defined as conservation in such a way that it can be reused in the future. For the collection method we should have adequate catchment area, mainly two methods for runoff collection and rooftop collection. Rooftop harvesting is a good method not only for domestic use but also for agriculture. There are some factors that should be considered when it comes to rooftop rainwater harvesting. When considering the situation of Sri Lanka, in June 2005, the Government of Sri Lanka adopted the "National Rainwater Harvesting Policies and Strategies" with the aim of encouraging communities to control water close to its source by harvesting l 'rainwater. Accordingly, the objectives were to further water security, minimize the use of treated water for secondary purposes, reduce flooding, improve soil conservation and aquifer recharge, provide water for domestic use with proper treatment, agricultural benefits and reduction of energy consumption. In addition to this policy, a regulation was published in the Official Gazette on 17 April 2009 making rainwater collection mandatory in certain categories of new buildings in areas under the jurisdiction of the city and urban council. As a result, rainwater harvesting systems have been installed in several places in Sri Lanka. and came from different types of rainwater collection tank materials. Furthermore, they would be above ground or below ground or alternatively. Even though it is widely used in the dry zone, there are some problems in rainwater harvesting systems in Sri Lanka. The quality of stored rainwater is a unique factor that threatens the sustainability of RWH technology. Many users do not use rainwater for drinking purposes because they believe it is not drinkable. This is due to various problems existing in rainwater harvesting systems in Sri Lanka. There are a few factors that affect roof rainwater collection. Issues and factors affecting rainwater harvesting in a commercial building.