To understand how roof drains work, one should not only consider pipe size and plumbing piping slopes, but also understand drainage anatomy. Flow control devices slow the rainwater runoff into the storm system, leaving a higher water load on the roof for a longer period. Ballast guards are used to keep stone/debris from blocking drains and entering the plumbing piping.
In a retrofit project, depending on local bylaws and codes, rubber compression seals or watertight mechanical compression seal (Figure 1) might be installed in retrofit drains, but these can decrease the size of the drain and reduce the effective drain capacity.
Redundant drains are another option to protect a roof from ponding water. If the main drains get blocked, or heavy rainfall causes water to back up on the roof, overflow scuppers can be installed to relieve the roof from water after it reaches a certain level/depth. Overflow drains can also be placed beside the primary drains, or evenly spaced along the roof perimeter for added protection.
Common pitfalls for roof drainage
Poor location of drains
When drains are located at the high point of the roof deck, the result will be standing water. Depending on the depth of the water and if the weather is favourable, evaporation can occur quickly or water may remain there for a long time. Water is typically considered ponding if it remains standing on the roof for more than 48 hours after weather is conducive to drying. In a conventional roof assembly, where the membrane is exposed, the membrane manufacturer may void a new roof warranty.
Rooftop equipment obstacles
Rooftop equipment can block drainage paths by acting as a barrier for water flow. Concrete pavers used for walkways or as setting surfaces for equipment can also block drainage paths. The result of these obstacles is water that cannot drain efficiently and locally pool at critical locations.
Dirty snowballs
When dirt and debris build around drains or gain access to the drainpipes, blockage can allow water to back-up on the roof. The presence of water can encourage unwanted wildlife and vegetation contributing to the debris, creating a snowball effect.
