If you are anything like me, you bought at least one ticket to win the Powerball when it reached $1.5 billion. And, if you are like me, you didn’t win. I’m sorry. But, as we all waited for the drawing, many of us daydreamed about what we would do with the money if we won — vacations, big houses, exotic cars, instant retirement. Life would be good, wouldn’t it? No more work — never having to worry about money, being able to truly live life to the fullest. That would have been fantastic, but you are like me, so you didn’t win. Given that the odds of winning were so remote, you likely had a backup plan. My backup plan primarily revolved around continuing to go to work every day.

They say that in order to truly be successful you have to take risks. To win the Powerball you took a tiny risk. Perhaps, to win a lucrative bid or start your own company or change careers you took a big risk. I am a realist, so I try to take calculated risks while also preventing disaster and preparing for a rainy day; hence the reason I did not quit my job after purchasing my ticket. When it comes to stormwater, there is a balance that has to be struck between building design and dealing with the rain that lands on the roof. While the building design needs to please a great many people, the roof drainage system also has to be designed to prepare for a rainy day; in this case, a very rainy day.


Roof drainage methods

There are three basic methods covered in Chapter 11 of the Uniform Plumbing Code that are used to deal with stormwater that falls upon the roof or in and around a building. Which method is used will depend on how the building is designed and/or the type of public sewage system used in the area. The methods are:  

  1. Gravity drainage — Stormwater simply drains off the roof and other areas to the ground. Stormwater flows away from the building naturally to storm drains or, in some cases, to the street. For most single-family residences, the roof is pitched, allowing rainwater to flow by gravity to a gutter system and discharge to grade.
  2. Separate storm and sanitary drainage systems — Water may accumulate on the roof because of building design and a building storm drainage system is installed, which will consist of a primary and a secondary system. Stormwater is then carried away to either a collection point or to the graded area around the building and flows naturally away from the building. The building and public sewer system is only a sanitary system and may not collect stormwater. There may be a separate storm sewer system, but it is independent of the sanitary system.
  3. Combined storm and sanitary systems — Water may accumulate on the roof because of the building design. The public sewer is a combined storm and sanitary sewer system; therefore, the building will have a primary and secondary storm water drainage system that will collect the stormwater and convey it to the combined sanitary/storm drainage sewer (see Figure A). 


Primary roof drainage

Paramount to eliminating the threat of roof collapse due to water accumulation is the installation of roof drains or gutters. Roof drains are generally installed on flat roofs or roofs surrounded by parapet walls. Pitched roofs not surrounded by parapet walls are generally drained by gutters. Close coordination with structural engineering relative to location and roof pitch, as well as proper sizing in accordance with local climactic conditions, are the basis for the design of the roof drainage system.

Rainfall rates are established by the U.S. Weather Bureau based on 100-year storm occurrence with a 60-minute duration and can be located in Table D 101.1 of Appendix D of the UPC. The Table of Recurrence Intervals and Probabilities of Occurrences from the U.S. Geological Service provide a measure of the probability and frequency of these storms. This table explains that the size of a storm represented by a 100-year storm has a probability of 1 in 100 of occurring in any given year and that there is a 1 percent chance of a 100-year storm occurring in any given year. This type of storm is used as the basis for sizing roof drainage systems to ensure that the system can handle the largest estimated storm that could occur in any given year. When using Appendix D, make sure that local area requirements are also observed. Many times the amount of rainfall shown in Appendix D is increased locally for a greater measure of safety.


Secondary drainage

To ensure rainwater accumulation on a roof is drained away, two independent systems of roof drainage are required. They are the primary roof drainage system and the secondary roof drainage system, commonly referred to as an “overflow system.” The primary system consists of roof drains, piping serving those drains and the discharge method used – either surface or gravity drainage or pumped drainage. The secondary system ensures that if the primary system or primary drains are plugged or overloaded, the secondary system will handle the rainwater and drain it away, protecting the building and its occupants from harm (see Figure B).

The secondary roof drainage system can be of two methods— roof scuppers, open sides of parapet walls, or secondary roof drains. The secondary roof drains can be of two types — roof drains with an independent piping system or roof drains that combine with the primary roof drainage piping, which will require an increase in piping size. 

Once the required rainfall data has been determined, the primary and secondary roof drainage system must be sized in accordance with Section 1103.0 using Table 1101.12 (vertical drains and pipe), Table 1101.8 (horizontal drain and pipe) and Table 1103.3 (gutters).

An open-sided roof area used for secondary roof drainage can be just a simple opening in the parapet wall surrounding the roof. A scupper can be a piped penetration in the parapet wall, as in Figure C. When either method is used, they must be sized by Table 1101.12. The width of the opening must be equal to the circumference of the drain required. For example, if the roof drain is 4 in., the opening of the scupper or the open-sided roof opening must be 12.56 in. in length. If it is a piped scupper, it must be a 4-in. or larger pipe size. 

If roof drains are provided for the secondary system, they must be located so that they will be a minimum of 2-in. higher than the inlet of the primary roof drain. The drain must also be placed so that it will serve the area of the primary drain. Care must also be taken that the ponding of water, which will occur if the primary drain does not drain, does not exceed the structural capacity of the roof. The roof must be designed with the weight of that standing water in mind.

The secondary roof drain piping may be installed using the method in either Section 1101. or Section 1101. The secondary piping may be an entirely independent system consisting of roof drains, conduits and leaders, which independently discharge to the outside of the building. This secondary system is a mirror of the primary system, sized and piped identical to the primary system. If this method is used, the primary drainage piping must be piped to the curb, gutter or other approved area. The secondary system, basically an emergency overflow system, may be piped to an open area even if it is a walkway or other traveled outside area. The reason for this is the secondary system should seldom be used, and if it is used, it would give a warning that something is wrong with the primary system. This secondary piped system gives the building owner the largest margin of safety for the building and its occupants.

The other method of secondary roof drainage piping is to connect the secondary roof drains to the piping serving the primary roof drains (see Figure D). The connection of the secondary drain must be in a vertical section of piping downstream of any horizontal piping below the roof. If this method is used the size of the conduits, leaders and downspouts downstream of the connection must be increased by using twice the rainfall rate required for the primary piping. For example, if the rainfall rate for the area is 3 in. per hour the size of the combined piping system must be calculated using 6 in. of rainfall per hour.

When it comes to design and installing the roof drainage system of a building, it simply is not enough to plan for the average day. In fact, you cannot even gamble that a 100-year rainfall will not happen while you are alive or able to be held responsible for not providing adequate protection. That 1-in-100 chance does not mean that rainfall may happen in 100 years. I remember as a child, dealing with a “100-year flood” twice while living in the same house, a home we only occupied for six years. Remember, the odds of winning the Powerball are far more remote than that of a historic rainfall. 


"This article was originally posted on ww.reevesjournal.com."