Welcome to our comprehensive guide on sizing a building’s storm drain system using the 2018 International Plumbing Code (IPC). An efficient storm drain system is a crucial component of any building infrastructure, protecting your building from potential water damage and flooding caused by heavy rainfall. Correct sizing of these systems not only ensures the safety and integrity of your building but also helps in the proper management of stormwater runoff, a critical aspect of urban water management.

The 2018 International Plumbing Code (IPC) is a set of comprehensive regulations developed to guide the design and installation of plumbing systems, including storm drain systems. These rules, developed by the International Code Council, are accepted worldwide and used by architects, engineers, and builders to ensure safe, efficient, and effective building practices.

In this blog post, we will guide you through the process of sizing a storm drain system as per the 2018 IPC. We’ll take you through preliminary considerations such as building type, location, and the area of the roof. Next, we’ll delve into understanding rainfall rates and calculating roof drainage areas. We’ll discuss the intricacies of IPC Table 1106.1, which provides valuable information for sizing the storm drain system components. Additionally, we’ll touch upon the role of secondary roof drains in your building’s drainage system.

Whether you’re a seasoned professional looking to brush up on your knowledge or a newbie trying to understand the basics, this guide will serve as a valuable resource. Let’s dive in and navigate the waters of storm drain system sizing together.

Understanding the 2018 International Plumbing Code (IPC)

The International Plumbing Code (IPC) is a set of standards developed and published by the International Code Council (ICC). This code offers comprehensive guidelines for the design and installation of plumbing systems in residential and commercial buildings. The 2018 version of the IPC is a universally accepted guide that’s commonly used by engineers, architects, builders, and local government bodies to ensure that the plumbing systems within buildings are both safe and efficient.

The IPC is a crucial tool when it comes to building construction. It provides details about plumbing system components, installation methods, testing, inspection, and maintenance. It also covers aspects related to water supply, sanitation, and storm drainage systems, among other things. These guidelines ensure that plumbing installations comply with recognized best practices, thus guaranteeing the safety, health, and welfare of building occupants.

The IPC is particularly valuable when it comes to sizing a storm drain system. The code provides clear guidelines on how to design these systems to efficiently manage stormwater runoff. It includes formulas for calculating the correct sizes for gutters, downspouts, and other components of the storm drainage system based on the roof area and local rainfall rates. In addition, the IPC offers guidelines for secondary drainage systems and overflow scuppers, ensuring a complete solution for managing stormwater.

In the following sections, we will dive deeper into the specifics of using the 2018 IPC to properly size your storm drain system. By the end of this guide, you will have a clear understanding of how to apply these universally accepted standards to your specific building project.

Preliminary Considerations for Storm Drain Sizing

Before diving into the calculations needed to size a storm drain system as per the IPC, let’s take a moment to discuss some essential preliminary considerations. These factors can significantly influence the size of the storm drain system you’ll need.

The **type of building** you’re working on plays a significant role in your drain system sizing. Different buildings have different roof structures and different usage requirements, all of which can affect the amount of stormwater they need to manage. For example, a large commercial building with a flat roof will require a different storm drainage approach than a small residential house with a sloped roof.

The geographical location and climate of the building is another critical factor. Rainfall intensity and frequency vary significantly from region to region. A building located in a region with heavy and frequent rainfall will need a larger and more robust storm drain system compared to one in a drier area with less frequent rainfall. Therefore, it’s important to consider local climate and rainfall data when planning the storm drain system.

Finally, you’ll need to consider the area of the roof. The size of the roof directly impacts the volume of rainwater it can collect during a storm. A larger roof will collect more water, meaning the storm drain system will need to be sized to handle a larger volume of runoff. For flat roofs, the entire roof area contributes to stormwater runoff. For sloped roofs, the effective area, which is usually less than the total roof area, contributes to stormwater runoff.

Taking these preliminary considerations into account will provide a solid foundation for you to start sizing your storm drain system in accordance with the 2018 IPC. As we progress, we’ll show you how to use these considerations to make informed decisions about your storm drain system design.

Understanding Rainfall Rates

A key component in sizing a storm drain system is understanding rainfall rates. The design of your storm drain system must be based on the maximum expected rainfall in your building’s location to ensure it can handle the water volume during a storm. But how do we quantify rainfall, and what does it mean for storm drain sizing? Let’s dive in.

Rainfall rate, also known as rainfall intensity, is measured in inches per hour. It describes how much rain falls in a specific location over a particular period. The higher the rainfall rate, the more water your storm drain system must be capable of handling.

One commonly used concept in calculating rainfall rates is the “100-year storm” event. This term doesn’t mean a storm that only happens once every hundred years. Rather, it represents a rainfall event that statistically has a 1% chance of occurring in any given year. Using the 100-year storm event as a basis for design helps ensure that your storm drain system can handle even highly unusual and intense rainfall events.

Local weather data is used to determine the expected rainfall rate for a 100-year storm in your building’s location. This information is often available from national meteorological agencies or local authorities and is sometimes presented in the form of maps or charts that provide rainfall rates for different geographical areas.

For example, if the local rainfall rate for a 100-year storm is 3 inches per hour, then you’ll need to design your storm drain system to manage this amount of rainfall. The 2018 IPC includes specific guidelines on how to do this, which we’ll discuss further in the following sections.

Understanding rainfall rates and how they impact storm drain system sizing is a fundamental part of designing a system that will effectively protect your building from water damage during storms. In the next section, we’ll look at how to calculate the roof drainage area, another crucial component in storm drain system design.

Calculating Roof Drainage Area

One of the primary inputs for sizing a storm drain system is the area of the roof that contributes to stormwater runoff. This area is referred to as the roof drainage area. In this section, we’ll provide a step-by-step guide on how to calculate this crucial figure.

1. Identify the drainage area of the roof: The first step is to identify the area of the roof that contributes to stormwater runoff. For flat roofs, this is typically the entire roof area. For pitched or sloped roofs, however, the effective area contributing to runoff is typically less than the total roof area and is based on the area projected onto a horizontal plane.

2. Measure the roof dimensions: Next, measure the length and width of the drainage area. This might require taking multiple measurements if your roof has a complex shape or multiple sections.

3. Calculate the area: Multiply the length by the width to calculate the area. If the roof is divided into multiple sections with different lengths and widths, calculate the area for each section, then add them together.

The result of these calculations will give you the roof drainage area in square feet. This is the figure that you’ll use in conjunction with the local rainfall rate to determine the appropriate size of your storm drain system components.

Calculating the roof drainage area can be relatively straightforward for simple roof structures. However, for more complex roof shapes, you might need to divide the roof into multiple sections and calculate the area for each section separately. Always remember to take accurate measurements to ensure the calculations are as precise as possible. Once you have the roof drainage area calculated, you can proceed to size the storm drain system using the 2018 IPC guidelines.

Sizing the Storm Drain System

You are correct, my previous explanation didn’t accurately reflect the approach to storm drain sizing outlined in the 2018 International Plumbing Code (IPC). The IPC actually uses Gallons Per Minute (GPM) as the basis for determining the size of storm drain pipes, rather than square footage of the roof area. My apologies for the confusion, and I appreciate your patience. Here’s a corrected overview of the process:

Calculating Gallons Per Minute (GPM)

The first step in sizing your storm drain system is calculating the maximum expected flow rate in gallons per minute. This calculation is based on your roof drainage area and local rainfall rate.

The formula to calculate GPM is:

GPM = (Rainfall Rate in inches/hour x Roof Drainage Area in square feet) / 96.23

This formula works because 1 gallon per minute is equivalent to 96.23 square feet of water 1 inch deep flowing in 1 hour.

Using the IPC tables for pipe sizing

Once you’ve calculated your expected flow rate in GPM, you can use the IPC tables to determine the appropriate pipe sizes for your storm drain system. These tables provide a list of pipe sizes in inches, along with the maximum flow rates they can handle in GPM, for both horizontal (Table 1106.2) and vertical (Table 1106.3) storm drain piping (also known as leaders).

To use these tables:

1. Locate your calculated GPM in the “Flow Rate” column of the appropriate table (horizontal or vertical).
2. The corresponding entry in the “Pipe Size” column will give you the minimum pipe size needed to handle your calculated flow rate.

Example of sizing a storm drain system

Let’s illustrate this process with an example. Suppose your building has a roof drainage area of 2,000 square feet and is in a location with a local rainfall rate of 3 inches per hour.

First, we calculate GPM using the formula:

GPM = (3 inches/hour x 2,000 square feet) / 96.23 = 62.3 GPM

This means that under a rainfall rate of 3 inches per hour, your roof drainage area will generate a flow of 62.3 gallons of water per minute.

Next, we use the IPC table for pipe sizing. For instance, using Table 1106.2 for horizontal storm drain piping.  We first select what slope will work.  For this example we will assume 1/8″ per foot.  The corresponding pipe size is 4″.  The vertical leader is not impacted by slope and a 3″ pipe is sufficient.

In the following sections, we will delve into the specifics of incorporating secondary roof drains and other considerations you should take into account when designing your storm drain system.

Secondary (Overflow) Roof Drains

Secondary roof drains, also known as overflow drains, are a critical part of the design of a storm drain system. They serve as a backup to the primary storm drain system and help prevent potential roof damage or collapse due to excessive rainwater buildup.

These drains are particularly crucial in regions with heavy rainfall, or for flat roofs where water might accumulate if the primary drains are blocked or fail to handle the total water volume. The 2018 IPC has specific requirements regarding secondary roof drains which we’ll cover in this section.

According to the 2018 IPC (Section 1108.1), secondary (overflow) roof drain systems should be installed and arranged in such a way that they kick in if the water depth on the roof reaches a certain level – typically not more than 2 inches (51 mm) above the roof surface.

The overflow system needs to be separate from and independent of the primary system. This is to ensure that if the primary system becomes blocked or overwhelmed, the secondary system can still operate unhindered.

Additionally, the code stipulates that the secondary roof drain system must be sized to accommodate the same rainfall rate as the primary system. This means that the secondary system must be able to handle the full capacity of expected rainwater runoff.

Therefore, when calculating the size of your secondary roof drains, you’ll use the same process as you did for the primary system. You’ll calculate the expected GPM based on your roof area and rainfall rate, and then use the IPC pipe sizing tables to determine the appropriate pipe size for your overflow drains.

By incorporating appropriately sized secondary roof drains into your storm drain system, you can ensure that your building is well protected from water damage, even in the event of a failure or blockage of the primary system. The next section will touch upon some final considerations before concluding our guide on sizing a building’s storm drain system using the 2018 IPC.

Final Considerations and Conclusion

We’ve now covered the main aspects of sizing a storm drain system based on the 2018 International Plumbing Code, from preliminary considerations, understanding rainfall rates, calculating roof drainage area, to sizing the primary and secondary systems. However, before we conclude, let’s consider some additional points.

Local Regulations: It’s important to check with your local authority having jurisdiction (AHJ) for any additional requirements they may have. While the IPC provides a strong baseline, local ordinances can further specify or adjust these standards. Always ensure your design complies with both the IPC and any local regulations to avoid future complications.

Professional Advice: While this guide provides a comprehensive overview of the process, each building and situation can have unique elements that may affect the storm drain system design. As such, it’s often beneficial to seek professional advice or a review of your design from a qualified engineer or architect, particularly for larger or more complex buildings.

Maintenance: A well-designed storm drain system is crucial, but regular maintenance is equally important to ensure the system functions correctly. This includes regular cleaning to prevent blockages, inspections for any signs of wear or damage, and immediate repair or replacement of damaged components.

In conclusion, proper sizing of a building’s storm drain system is a vital aspect of building design. Using the guidelines provided by the 2018 IPC and the steps outlined in this guide, you can ensure your system is effectively designed to handle stormwater runoff, protecting your building from potential water damage. However, always remember that storm drain system design is a technical task that often requires professional assistance to ensure optimal function and compliance with local regulations.

Summary and Key Takeaways

In this guide, we explored the process of sizing a building’s storm drain system as per the 2018 International Plumbing Code (IPC). Let’s summarize and review the main points we’ve covered:

1. Preliminary Considerations: Before embarking on storm drain system sizing, we evaluated several crucial aspects – the type of building, the geographical location and climate, and the area of the roof. These factors are key determinants of the requirements for your storm drain system.

2. Understanding Rainfall Rates: We discussed the concept of rainfall rates and how they are crucial to the design of your storm drain system. These rates, typically based on the “100-year storm” concept, help ensure your system can handle even highly intense rainfall events.

3. Calculating Roof Drainage Area: To estimate the amount of stormwater your roof will generate, we measured the roof drainage area. This is the total roof area contributing to the runoff.

4. Calculating Gallons Per Minute (GPM): The flow rate of rainwater from your roof, given in GPM, is calculated using the formula: GPM = (Rainfall Rate in inches/hour x Roof Drainage Area in square feet) / 96.23.

5. Sizing the Storm Drain System: Using IPC Tables 1106.2 and 1106.3, we determined the size of horizontal and vertical leaders (pipes) needed based on the calculated GPM.

6. Secondary (Overflow) Roof Drains: As a safety measure, we also incorporated secondary roof drains into our system design. These drains serve as a backup to handle overflow in case the primary system fails or gets blocked.

7. Final Considerations: Lastly, we discussed the importance of compliance with local regulations, the value of professional advice, and the necessity of regular maintenance for the long-term efficiency of the system.

This guide provides a foundation for understanding and applying the 2018 IPC rules in designing a storm drain system. It’s crucial to note, however, that professional guidance is often necessary to address unique circumstances and ensure proper system design and installation.

Resources for Further Reading

For those looking to dive deeper into the specifics of storm drain system design or wanting to broaden their understanding of the plumbing code, here are several resources that can assist in further learning:

1. 2018 International Plumbing Code: This is the fundamental resource for all the information we’ve discussed. It’s comprehensive and provides clear guidelines on storm drain systems and various other aspects of plumbing.  Free access to this code and all other international codes can be found here: Free Building Codes!

2. ASPE Plumbing Engineering Design Handbook: The American Society of Plumbing Engineers (ASPE) has a detailed handbook that covers a wide range of topics, including stormwater management and drainage design. This can be a great resource for expanding your understanding beyond the IPC. 

https://www.aspe.org/publications-news/aspe-plumbing-engineering-design-handbooks/

3. Local Building Department or Authority Having Jurisdiction (AHJ): They can provide resources or references tailored to your specific location, which can be particularly useful given that local regulations and climate considerations can have significant impacts on storm drain design.

4. Online Courses and Webinars: Websites like Coursera, Udemy, or the Engineering Training Institute offer online courses on stormwater management and drainage design. Webinars from professional organizations like the American Society of Civil Engineers (ASCE) can also be beneficial.  We offer an in-depth Udemy course here: Storm Water Design Course

5. Consult with Professionals: Reach out to engineers, architects, or experienced builders in your area. They often have practical insights that can be invaluable when navigating storm drain system design.

Remember, continuous learning and staying updated with the latest rules and regulations is essential in effectively designing and maintaining storm drain systems. These resources can provide further support as you continue your exploration of this important aspect of building design.

Here are some other guides that may be useful:

• 5 Steps to Size a Natural Gas System: A Comprehensive Guide for Commercial and Industrial Buildings – Equation Method and Calculator for Gas Pipe Sizing
• Size Water Pipe: A Guide Based on the 2021 International Plumbing Code
• A Comprehensive Guide to Sizing Condensate Piping and Pumps according to the International Mechanical Code
• A Comprehensive Guide to Sizing Sanitary Drainage Piping According to the 2021 International Plumbing Code