No-hub fitting restraints

What you need to know to avoid disaster

When designing the above ground cast iron hubless piping system for a building, many engineers and contractors make a common but potentially disastrous mistake: they underestimate the importance of using no-hub fitting restraints.

No-hub fitting restraints are systems of braces, blocks, rodding or other devices designed to keep pipes and fittings from separating under thrust force. And as some spectacular piping failures have shown, you really overlook the use of these restraints at your own peril.

The danger of skipping the restraints

A hubless waste and vent system that has not been properly fitted with restraints is likely to separate at the couplings if the system is subjected to significant thrust forces. This is what happened in the Lucas Oil Stadium disaster.

When the Lucas Oil Stadium was being built for thee Indianapolis Colts in 2008, a long series of delays created pressure on the plumbing contractor to hurry up and finish so that the rest of the building could be outfitted.

During construction, a major rainstorm hit the area just after millions of dollars’ worth of IT equipment had been moved into the stadium’s basement. Three of the rainwater leader system’s 15-inch cast iron downspouts failed, flooding the basement and ruining the equipment. Huge lawsuits and project delays ensued.

A similar disaster happened in 2012 at the Translational Medical Research Facility in Philadelphia. The project had been completed and turned over to the owner, who was in the process of occupying the building. A massive rainstorm occurred, hitting the facility with about five inches of rain per hour. Needless to say, this deluge caused significant head pressure in the piping systems. Without no-hub fitting restraints in place, the system buckled. An unrestrained, 12-inch rainwater leader separated, and a double offset fell through the ceiling in the fifth floor stairwell and flooded the building.

The No. 1 reason why pipes and fittings fail: Hydraulic jump

Many piping failures can be explained by the concept of hydraulic jump. As water travels down a vertical pipe it reaches its terminal velocity. Then, it hits the bottom and changes direction to flow along the horizontal pipe at the system’s base. Here’s where the problems hit.

Although the water that’s flowing horizontally will slow down, the water behind it will continue at terminal velocity until it changes direction, too. When the water that’s coming down rapidly runs into the water that’s now traveling more slowly, water can start backing up in that vertical pipe. This is what’s referred to as “hydraulic jump.”

With a hard enough or sufficiently prolonged rain flow, this backup can create enough thrust to cause the pipe and fitting to separate. This is what happened at the Translational Medical Research Facility in Philadelphia. After that system failed, the engineer calculated that 82 pounds of thrust caused the 12-inch rainwater leader to separate.

Another reason why pipes and fittings fail: No overflow point

Problems can also occur in high-rise structures that lack overflow points. Take the example of a 17-story building that has residential units on the top 14 floors and commercial space on the bottom three floors. If a blockage occurs at the street level in a system where the stack serving the residential floors does not have an overflow, two or three stories of head pressure can quickly build up. This, of course, can cause the pipe joints to fail.

What the International Plumbing Code says about the need for restraints

Since 2009, the International Plumbing Code, section 308.7.1, has stated that a cast iron soil pipe that is greater than four inches shall be restrained with braces, blocks, rodding or other suitable methods as required by the coupling manufacturer, at changes of pipe diameter greater than two pipe sizes, and at all changes of direction and branch openings of the system.

Similarly, the Cast Iron Soil Pipe Institute has long recommended that cast iron soil pipes that are five inches in diameter or larger be restrained at all changes of direction and branch openings.

When should you specify the use of no-hub fitting restraints?

To determine whether or not a particular joint should be fitted with a no-hub fitting restraint, I recommend that you look at the following.

If the pipe is greater than four inches in diameter, use the restraints. Because all pipes and fittings on the drain, waste and vent (DWV) systems must be tested to a minimum of 10 feet t of head pressure, DWV systems are all designed to accommodate this amount of thrust forces. If you believe that during the system’s life it might encounter thrust forces greater than this, you should certainly put the restraints in.

However, it is my opinion that 10 feet of head pressure is far too low of a bar. Given the fact that pipe failure can be catastrophic, this is woefully insufficient. I believe that your design focus should be on creating a system that restrains pipes and fittings against a 50-foot column of water – not just a 10-foot column of water.

No-hub couplings by themselves are not restraints, even with only 10 feet of head pressure. Couplings are always the weak point in the system. Without something to restrain the pipe and fitting against thrust, a no-hub coupling will not prevent pipe and fitting separation.

While pipe restraints may not be necessary on single-story projects, you need to consider them for any project that is two stories or more, where the potential to exceed that 10-foot head thrust exists.

In seismic applications, the piping restraints should be used in conjunction with other types of restraint methods, such as additional sway bracing, as prescribed by a seismic engineer.

Why needed restraints are often not installed at all

If you were to travel across the country to visit projects that were completed in the past five to 10 years and take a close look at anywhere that you see exposed cast iron waste piping, such as in a parking structure or the garage of a high-rise building, chances are you would not see any restraint systems in place. Why is this?

Doing it right takes time and money. On large projects, outfitting the entire storm, waste or vent system, as prescribed by both the International Plumbing Code and the Cast Iron Soil Pipe Institute, can be a labor-intensive and costly endeavor. It’s therefore no surprise that unless it is specified as a requirement of the job, chances are it will be overlooked or omitted.

Many installers are unaware of the code. Prior to 2009, this issue was not addressed at all by the Uniform Plumbing Code or the International Plumbing Code. Instead, these codes simply directed the installer to install the system per the manufacturer’s installation requirements. Now, the need to install no-hub fitting restraints is addressed in Chapter 3 of the International Plumbing Code – even though the rest of the codes that address drain, waste and vent systems are in Chapter 7. It is easy to overlook issues relating to these systems that are addressed in other chapters.

Many inspectors are unaware of the code. Shockingly, many municipal inspectors are unaware of the requirements for restraints. Knowing this, there are a lot of plumbers or installers who are aware that they should be installing restraints, but wait to see if the inspector calls them on it.

They’re attitude is, “if the inspector notices, then I’ll go back and put the restraint in; otherwise we’re good to go.”

Prior to 2009, this wasn’t always taught in trade school. Most trade schools have done a good job addressing installation methods and requirements for installing hubless pipe and fittings, while others have not.

Regardless of the “excuse,” those who fail to install proper restraints leave themselves open to damages and liability in the event that the system fails due to thrust forces that it cannot restrain.

The problems with field-devised restraint methods

In addition to the major problem of plumbers not installing restraints at all, to avoid disaster you also need to be aware of the problems associated with field-devised restraint methods. Because the code does not specify exactly how the pipe and fitting need to be restrained, many installers will get quite creative in their use of readily-available materials to make their own pipe restraints out in the field.

This do-it-yourself (DIY) approach uses untested methods. By definition, there is no standardization or uniformity in field-devised restraint solutions. What you get is limited only by the installer’s imagination and ability to jerry-rig a restraint device.

The DIY approach also uses untested materials. Field-devised restraints are made out of whatever material is on hand, which rarely turns out to be materials that have been tested for this use. For example, riser clamps are commonly used in field-devised restraint systems for hubless pipes and fittings. But, riser clamps are not designed to be used as a thrust restraint – they’re designed to support pipe every 10 feet or at every floor. Riser clamps have not been measured or tested for use as pipe restraints, and do not come with specifications for turning them into pipe restraints.

DIY is very labor-intensive. It can take anywhere from 30 minutes to an hour to measure, cut and fabricate a field-devised system. If a plumber is being paid, for example, $60 to $70 an hour, and he’s taking the time to fabricate a DIY restraint system for each coupling that requires restraint on a 20-floor building, the labor cost for this approach can be astronomical.

DIY exposes you to liability. After all of that effort, you’re left with a system that you cannot really rely on to stay together during a significant rain event. When you take into consideration the amount of liability you assume by putting in an untested, non-standardized system, the potential costs are even higher.

Field-devised restraints are often seen as temporary. Before the issue was addressed in the International Plumbing Code, the only time most plumbers thought about shackling the pipe and fitting together was when they were going to be putting a test on it. Since the test required blocking off the pipe system and forcing it to carry water up to a 10-foot head level to ensure there were not any leaks, plumbers recognized that some type of restraint was needed.

However, once the system passed the test and the inspector was satisfied with the installation, it was common practice to pull the plug, get rid of the shackling, and assume that the system was complete.

Unfortunately, even today many plumbers and installers still take this “jerry-rig a restraint system until the test is over” approach. The only thing worse than a field-devised restraint is no restraint at all. Which, of course, is what you get when the field-devised restraint is treated as just a temporary measure.

How engineering firms can mitigate their liability

There are two key things that engineering firms can do to mitigate their liability in this area:

1. Require the use of engineered solutions.

Put engineered no-hub pipe fittings into your specs, and specifically disallow the use of field-devised restraint solutions. This eliminates the chance that the contractor will either consider the restraints to be discretionary, or assume that a field-devised solution is acceptable.

What might this look like in your specs? If you’re allowing no-hub cast iron pipes and fittings to be installed on a project, chances are your specs already contain a series of bullet points describing acceptable methods of installation, hangar spacing, the type of couplings to be used and so forth. This is where you can add a bullet point stating: “Install pipe and fitting per the Cast Iron Soil Pipe Institute’s Designation 3011.” This can be followed with a sentence stating, “Restrain pipe and fittings using [product], installed as per manufacturer’s recommendations.”

For municipal or government projects where you cannot mention a particular brand of products in your specs, you can simply specify a performance standard in place of a manufacturer’s name or product number. For example, “Pipe restraints must be rated to hold 500 feet of head pressure.”

2. Do your due diligence on the rest of the system.

Look at the system’s design requirements and what the system is likely to be subjected to, and plan accordingly. For example, say you have a 30-foot horizontal run of 12-inch pipe, meaning you have three sections of cast iron hubless pipe in play, and the code only requires you to restrain them at branch openings or changes in direction. If there are no branch openings or changes in direction on that 30-foot t run, you cannot assume that no restraint is necessary.

Why? Because if something happens to cause the system to experience significant head pressure, this could force the pipe in the middle of the run to separate. So, even though it’s not required by code, the best practice is to restrain those couplings as well.


Pipe joints can and do come apart under pressure, often to catastrophic results. Although the International Plumbing Code specifies the use of restraints for cast iron soil pipes greater than four inches, the code leaves room for untested field-devised solutions.

Engineered restraints, such as no-hub pipe restraint systems, are faster and less expensive than field-devised solutions, and reliably restrain the pipe and fittings together for up to 50 feet of head pressure (far more than the 10 feet of head pressure required by the code). To avoid disastrous pipe failures, engineering firms should require that pipe couplings be properly restrained using proven engineered solutions. 

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