Are systems where the pipes from the gutter go down the wall and underground and then up into the tank. Many systems are “wet” because the size of buildings and the placement of tanks away from the buildings mean that there are long runs of pipe underground leading to a riser at the tank. Because the pipes are underground and below the entry point to the tank, even during periods without rainfall water remains in the pipes. "Wet" systems can be converted to "dry" using In-Ground Water Diverters, improving water quality and saving water in the process.

Because the water that sits in underground pipes has a tendency to become stagnant and provides a potential breeding habitat for mosquitoes, it is critical to insect proof the pipes in a 'wet' system. Legislation broadly says that where pipes hold water they must be screened with a non-corrosive screen of not more than 1mm aperture to prevent the entry of mosquitoes and vermin. Mosquitoes are known to be responsible for many diseases including Dengue Fever and Ross River Fever.

Rain Harvesting has developed a range of products to insect proof rainwater systems and these have been designed to meet all legislative guidelines, including rain heads, insect proof screens and flap valves, tank screens and vent cowls.