GI duct is fabricated from Galvanized Iron. Fabricator cuts GI sheet and bends it in required shape mostly in square and connect with GI rod using tools like hammer, GI sheet cutter and GI sheet bending machine. They create several pieces and then connects them to achieve the required length of GI duct. After that, they paint it with the appropriate paint to make a good contrast with the surroundings or outside of the building, usually with the black paint. Here in the below picture, the owner of the building has painted them black.
This is a house in Kenya, and it is more than 20 years old. You can see the black GI duct, the contractor or plumber have installed it to drain rainwater from roof to ground level. Now, as this pipe is vertical, they have mounted on the wall using GI Clamp. They have provided a big opening at the top, we can say catch point, and its details have been shown in the below picture. Its opening is wider and open to the sky. In this case, blockage chances are increases if kids put something or something get in from somewhere, but to avoid draining big things from the roof, they have provided 50mm GI pipe to stop big elements from getting in.
The roof is bunded with concrete, and these two GI pipes have been placed during casting so in the future they don’t leak. The duct size is 100x100mm, which is good enough to drain all rainwater without an overflowing roof over the bund. In another way, they could have put the bend in the concrete bund directly, and cover the opening with the mesh to keep big particles or leaves out.