Introduction
Why should we talk about stormwater drainage? Well, in short, water is heavy. A building’s stormwater drainage system must be designed to drain all rainwater from a building’s roof in a quick and efficient manner. If water is not properly removed from a roof during a heavy rainstorm, the buildup of water and the subsequent load on the roof can potentially cave it in, as seen in the above image. All it takes is one point of failure and an entire roof area can flood into a building. This blog will delve into the components needed for stormwater drainage systems to function efficiently and safely for a building’s occupants and infrastructure.
‘100-Year Storm Code’ Requirements
To be prepared for the most torrential downpour, we follow ‘100-year storm’ code requirements when designing storm drainage systems for buildings. This means that storm drainage systems are designed to handle a rainfall intensity that occurs just once every 100 years. However, rainfall intensities differ with geographic location, so different parts of the country will see varying piping sizes for a given roof area. Higher rates of rainfall will subsequently need larger pipe sizes to accommodate the higher flow rates. Rainfall rates in the United States range anywhere from 0.6 inches per hour in Juneau, Alaska to 4.7 inches per hour in Miami, FL. A 5,000-square-foot roof in Miami could see a total flow of 244 gallons every minute!
Secondary Drainage Systems
Not only do we design primary storm drainage systems with the capability to handle rare, heavy storms, but we also include a completely redundant secondary system, drain for drain, in case there is a blockage in the primary system. The inlets of secondary roof drains are raised higher than primary drains by generally 2 inches. This is demonstrated in Figure 1 where a 2” high ‘water dam’ prevents water from entering the secondary drain unless the water level exceeds the height of the dam. This will prevent water from flowing through the secondary system during normal times. Therefore, any significant discharge at a secondary system outlet would be a clear indication that there is a problem with the primary system. As seen in Figures 2 and 3, these discharge points can be located up high above the top floor of a building or be routed down to a lower elevation to discharge at grade. An important rule is that the piping of the primary and secondary systems MAY NOT be combined at any point! The emergency system must independently discharge to the building’s exterior.
Drain Coverage Considerations
So, how much area can one drain handle in a stormwater drainage system? There are several factors that determine the answer to this question, so here is an example. We have a building in Atlanta, GA. Suppose a storm stack, as seen in Figure 4, can be no more than 4 inches in diameter to minimize the chase space provided for the pipe’s passage. A 4-inch roof drain could be utilized to cover up to 1,800 square feet where the piping is offset horizontally at 1/8 inch per foot prior to dropping into the stack. If the drain is located directly above the stack, or the horizontal offset is routed at an increased slope of 1/4 inch per foot, a 4-inch roof drain could then cover up to 2,400 square feet. It is generally best practice to provide space for the storm stack immediately below the drain location.
External Stormwater Drainage Systems
The internal roof drains previously described are not always needed to accomplish proper roof drainage if the roof can be sloped to the perimeter of the building. By installing scuppers, collection boxes and downspouts as shown in Figure 5, the system can collect and discharge rainwater at the perimeter parapet wall of a building. Again, a secondary method of roof drainage needs to be provided in case the primary system is blocked. In Figure 6, a secondary opening is provided adjacent to and higher than the primary opening on the building’s parapet wall. The increased height of the secondary inlet is so this path is only used if there is a problem with the primary system. One thing to keep in mind is that frequent wall penetrations are needed for this type of installation. The required openings can quickly grow huge if we try to cover too much area with one drain. This is due to the how the openings’ capacity depends on the allowable depth of water (to develop a static head and increase flow). Due to structural considerations, we are limited in how much ponding we can allow to create the openings’ depth, so the only other option as required flow increases is to increase the width. Consequently, we can reduce required flow by providing more frequent collection points.
Closing
In conclusion, storm drainage is a critical design element of any building due to the implications of inadequate flow and potential catastrophic damage. In every case, building designers, owners, and contractors should be aware of the required systems in place for primary and secondary means of drainage, and of the area limitations a single drain should be designed to serve!
