Storm drain location must be coordinated with the architectural design of the building. The roof structure must be able to support the weight of residual water by design.

Location of Structure Penetrations

When planning the location of the roof drains, deck drains, scuppers, gutters, and rainwater leaders, consideration should be given to placing an overflow drain adjacent to each roof drain.

Scuppers are often used on flat roofs or on roofs with minimal slope for overflow protection. The amount of drainage provided by overflow drains or scuppers may be reduced on roofs that are flat or nearly flat. This is because under such circumstances, stormwater can flow to another drain before enough water can accumulate to cause damage to the structure. These overflow drains may be required by some local jurisdictions, so it is always good practice to check with the local plumbing officials. Even if the area served by a roof drain is relatively small, it is good practice to provide at least two drains in all individual roof areas.

Routing

The first step in planning a route for the storm drainage system is to find the locations of the drain outlets and their relation to interior walls and structural columns. This will help you determine the space available for installing the leaders. Locating the vertical leaders within the building as opposed to the building exterior has several advantages, such as convenience, safety, appearance, and freeze protection. However, leaders located on the exterior can be installed at a much lower cost and do not take up valuable floor space.

The piping layout must be coordinated with other trade disciplines that may be affected by the route, so to keep the number of leaders to a minimum, combine flows from more than one roof drain, clearwater wastes, or any combination thereof. If leaders are to be located at building columns, the column footing design must be coordinated with the structural engineer to take into consideration the leader location. Avoid running horizontal piping above the ceilings of computer rooms, kitchens, and food-preparation areas. A pipe rupture above one of these areas could cause major damage and contamination.

Pipe Supports

Expansion and contraction of the piping system installed without proper anchoring could cause roof drains to be pushed above the roof deck, destroying the integrity of the roof waterproofing by tearing the flashing and the waterproofing membrane. This problem can be more apparent in high-rise buildings and buildings where the exposed leader is subject to cold rainwater or melting snow and ice that enter piping at the ambient temperature of the building. Consider an expansion joint at the roof drain or a horizontal section of the branch line to accommodate the movement of the leader without affecting the roof drain.

Self Test D-3.4: Planning the Storm System

Complete Self Test D-3.4 and check your answers.