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Selecting the best roof surface for an effective rainwater harvesting system

Each rainwater water treatment system needs to be designed according to the surface from which the rain will be harvested. In many instance the roof is the first surface that rainwater comes into contact with, before it is transferred into the rainwater harvesting system. Thus it is the first area where the water can be contaminated and it is the biggest surface of the rainwater harvesting system.

Contaminants on the roof surface depend on the material, location and presence of organic material or animals in the area. The best roof surface for rainwater harvesting should be smooth, made of inert material, angled slightly, clean, and have no moss or bacteria growing on it.

Aluminum or pre-painted Zinc or Aluminum steel is the best roofing material for a rainwater harvesting system but it does not mean that you cannot harvest rainwater from a tile or cement roof. This has been done with great success and water results prove that you can harvest potable quality water from a tile and concrete roof.

Each roofing material has its own benefits and disadvantages with regard different types of contaminants. It just means that the roof surface has to managed and controlled more than ideal roof surfaces. Here are some of the most common materials used throughout South Africa

Clay and slate tiles
These roof tiles can be found in many old houses. The tiles are rough and form many crevasses and gather moss easily. The roof does not leave any trace metals in the water and absorbs water before the water will start flowing. The roof needs to be cleaned once per year with a high pressure washing system. Some people paint the roof with a lead free paint to reduce the porosity and prevent moss and algae from growing on the roof.

Modern concrete roof tiles
They have very similar properties to clay tiles and need to be treated in the same way

Cement based roofs
Concrete roofs are difficult to install at a slope/angle and in big commercial installations they form big puddles on the roof. The roof gets very dirty easily and in many instances they are covered with a bitumen based or membrane roof sealer. These surface need to be manage more than any other roof surface. The roof surface needs to be to be dry swept regularly and puddles need to be engineered out. The first flush system for a concrete roof would also need to be increased in size to accommodate the slow cleaning process at the start of the rain

Bare galvanized sheets.
Works as well as aluminum or pre-painted steel. The only difference is that the water is exposed to bare metal for a short period and will contain very small amounts of cadmium and zinc but water results has proved that it is negligible.

Roofs covered with soil and plants and or grass are not recommended for rainwater harvesting if the water will be used for potable use.

Throughout the world you will find many types of roof surfaces and roof materials being used. Irrespective of the material being used, consideration should always be applied to the concept of clean surface harvesting with minimal surface and material contaminants. The water quality will always depend on the quality of the surface installation and design.