Roof drainage systems are located at the top of the many branches in a large drainage system.  They are generally discharged directly into the on-site detention/infiltration facilities or property drainage systems. Because the areas are usually small and there are fewer complications, roof drainage can be designed using simpler methods than those employed at larger scales. It is recognized that many buildings in Malaysia are, and will continue to be, constructed without roof drainage systems. This practice is appropriate in some situations, such as minor buildings in rural areas.  It therefore applies to those buildings where roof drainage is specified for reasons of runoff conveyance and collection to storage/retention facilities as well as for comfort and safety of occupants and the protection of the building structure.

More information's:

Buildings with pitched roofs can have a variety of drainage systems. With a sufficient overhang, water can drain directly to the ground without being intercepted at the roof edge. Usually, pitched roofs end in gutters that are drained by downspouts. Low-slope roof drainage is accomplished in one of three ways, namely, without gutters or downspouts, with gutters and downspouts, or by downspouts that go down through a building's interior. Drainage without gutters and downspouts can damage the exterior wall with overflow. If the roof has no gutters and downspouts or interior downspouts, carefully examine the exterior walls for signs of water damage. Gutter and downspout materials are usually galvanized steel, aluminum, copper, or plastic.

Conclusion:

Make certain all gutters are clean and slope uniformly, without low areas, to downspouts. If there is a screen or similar device to prevent anything but water from flowing into the gutter, check its condition, fit, and position, to be sure water really can enter the gutter. Check gutters without screens or similar devices to be sure that basket strainers are installed at each downspout. On roofs with lower slopes make sure gutters are placed close to the roof's surface. Hangers should be placed no more than three feet apart. Where ice and snow are long lasting, hangers should be placed no more than 18 inches (460 mm) apart. Wherever a gutter is exposed, check the strength of its fastening to the roof fascia or building exterior. Rusted fasteners and missing hangers should be replaced.

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