Roof Drainage Design

Having sufficient drainage on a flat roof is vital.

Roof drainage design.

Hence this is by far the most common roof drainage situation. For larger roofs siphonic drainage systems should be installed. For most dwellings you ll find that gutters range in size from approximately 100mm diameter to 150mm diameter assuming gutters with a half round section profile and downpipes range from approximately 60mm diameter to 90mm diameter assuming circular downpipes. In addition to the primary roof drains there will also be overflow drains which are used for redundancy and sized in the same manner.

Secondary drainage shall be provided through the use of sidewall scuppers or internal roof drains. Secondary or emergency roof drainage is required wherever the buildup of water is detrimental to the roof structure. One gully can drain at least 150 m2. In either case the discharge shall be located so that it is readily visible by building occupants.

Inner drains are resistant to freezing but can require careful maintenance to prevent problems. These drains are distinguished by having higher rims. On many occasions drains are too high and water cannot flow off. Inner roof drains are used on flat roofs and include a low section or channel that collects water and feeds it into a concealed drain that leads to an underground drainage system.

When designing a roof it is essential to pay close attention to where the drains are. As we will see in steps 3 4 the size of the roof drain lines is dependent on the square feet of the roof area being served by each roof drain. The main benefit of inner drains is that the system is hidden from view.