D-3.2 Functions of Pipes in Storm Drainage Systems

A storm drainage system is composed of storm water collection devices and the associated piping connected to the collection devices that transport the stormwater to an approved disposal location. This chapter describes the array of stormwater collection devices utilized and their function in the system.

Parts and Function

Roof drain: a drain installed through the roof deck or slab. It is generally a circular body made of cast iron with a dome-type strainer that receives stormwater on all sides of the drain. Roof drains are also available in spun copper (Figure 1) or aluminum styles that may or may not be furnished with strainers. Whenever you install a flat roof drain, ensure that strainers or domes are in place to prevent debris and other potential obstruction elements from getting into the drainage system.

 

This diagram illustrates a **spun copper roof drain with a strainer**. The components include the **roof membrane**, which is a waterproof layer that protects the building, and the **spun copper flange**, which helps to secure the drain to the roof. The **strainer or dome** is placed on top of the drain to prevent debris from clogging the drainage system while allowing water to flow through.
Figure 1 Spun copper roof drain with stainer. (Skilled Trades BC, 2021) Used with permission.

Flow-control roof drain: used to drain the roof at a controlled rate. Excess water is allowed to accumulate on the roof under controlled conditions. The water then drains off at a lower flow rate after a rain event. The roof must be structurally designed to temporarily store the maximum amount of water without overloading during periods of heavy rainfall.

 

This diagram shows a **flow-control roof drain**, which includes a **flow control weir**. The weir is designed to regulate the rate of water flow through the drain, helping to manage roof drainage more effectively, particularly during heavy rainfall. The flow control mechanism ensures that the drainage system does not become overwhelmed, thereby preventing water buildup on the roof.
Figure 2 Flow-control roof drain. (Skilled Trades BC, 2021) Used with permission.

Siphonic roof drain: a drain designed to operate at full pipe capacity, which creates a siphonic condition. The siphonic condition allows water to drain faster than using conventional gravity roof drains. Because of the flow rates generated, a minimal number of roof drains are required, and they can be connected into a single rain leader.

Rainwater leaders (RWL): may be attached to the outside wall of a building or located within a building. They may be a sheet metal leader, a non-circular tubing material, or a pipe (Figure 3).

 

This diagram illustrates two types of rainwater leaders: **interior** and **exterior**. - The **interior rainwater leader** directs water from a roof drain through a pipe running inside the building's walls or columns, ultimately channeling the water to the building's drainage system. - The **exterior rainwater leader** shows a sheet metal leader that directs water from a roof gutter to the outside of the building, discharging water through a **kickout**, which helps prevent water from pooling near the building's foundation.
Figure 3 Interior and exterior rainwater leaders. (Skilled Trades BC, 2021) Used with permission.

Deck drain: a drain similar in all respects to a roof drain except that it generally has a flat strainer and is located in an area such as a walkway (Figure 4).

 

This diagram shows a **walkway deck drain**. It includes a **flush mount strainer**, which is designed to be level with the deck surface, and a **walkway membrane**, which helps waterproof the area and directs water into the drain. The setup ensures efficient drainage while maintaining the deck's flat, walkable surface.
Figure 4 Walkway deck drain. (Skilled Trades BC, 2021) Used with permission.

Roof gutter: a collection device attached along the entire lower side of a pitched roof. Typically constructed of aluminum or steel with a corrosion-resistant coating and terminating at an external leader.

Roof scupper: a box-like collection device located on the exterior of a building and that receives stormwater on one side (Figure 5). Emergency scuppers are designed as overflow protection on flat roofs and are typically installed in conjunction with flow-control roof drains and parapet walls.

 

This diagram illustrates a **roof scupper** system. It features a **roof drainage channel** that directs water to the **scupper**, an opening in the wall or parapet of the roof. The scupper then allows rainwater to drain off the roof, preventing water accumulation and potential damage. This design is often used in flat or low-slope roofing systems to aid in water drainage.
Figure 5 Roof scupper. (Skilled Trades BC, 2021) Used with permission.

Catch basin: a part of a storm drainage system used to drain paved areas, such as parking lots. Catch basins lead to storm sewers, which release the untreated water through outfalls directly into nearby streams or rivers. It is designed to trap debris to prevent it from entering the drainage pipes (Figure 6).

 

The figure shows a **catch basin** with a **drainage grate**, containment grout, and a removable elbow for cleanout, serving a paved area. **Ladder steps** provide access for maintenance.
Figure 6 Catch basin serving paved area. (Skilled Trades BC, 2021) Used with permission.

Storm drainage piping systems: may be designed for gravity or pumped flow to a point of discharge. The AHJ regulates the discharge rate for stormwater systems according to a municipal stormwater management strategy.

Combined drainage piping systems: designed to collect both storm drainage (rainwater and snowmelt) and wastewater (sewage from homes and businesses) in the same pipe. These systems have serious drawbacks because during periods of heavy rainfall or snowmelt, the additional volume in a combined sewer system can exceed the capacity of the sewer system.

Although most jurisdictions have replaced or are replacing these systems with dedicated sewer systems for storm and sanitary drainage, there are existing systems that have not been converted. If this situation exists, some jurisdictions employ combined sewer overflows (CSOs). CSOs are designed to overflow and discharge the excess flow directly to a predetermined disposal area, such as a river, without reaching the sewage treatment plant.

Self-Test D-3.2: Functions of Pipes in Storm Drainage Systems

Complete Self-Test D-3.2 and check your answers.

If you are using a printed copy, please find Self-Test D-3.2 and Answer Key at the end of this section. If you prefer, you can scan the QR code with your digital device to go directly to the interactive Self-Test.

D-3.2 Self Test QR Code

 

References

Skilled Trades BC. (2021). Book 2: Install fixtures and appliances, install sanitary and storm drainage systems. Plumber apprenticeship program level 2 book 2 (Harmonized). Crown Publications: King’s Printer for British Columbia.

Trades Training BC. (2021). D-3: Install storm drainage systems. In: Plumber Apprenticeship Program: Level 2. Industry Training Authority, BC.

Media Attributions

All figures are used with permission from Skilled Trades BC (2021) unless otherwise noted.

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