Can You Use A 1-1/2 Inch Pipe for Shower Drain? (Find Out Now!)

Dennis Howard
by Dennis Howard

Replacing an existing standard bathtub with a walk-in shower has become a popular remodeling project for many homes. While these replacements are usually straightforward installations, there can be some unforeseen issues. Bathtubs usually have a 1-1/2 inch drainpipe. Can you use the standard 1-1/2 inch drainpipe with a shower?

The quick answer is it depends. The longer answer is you must check your local building code to find the exact requirements for drainpipe sizing. If your local construction code requires a two-inch drainpipe for a shower, you may be required to replumb the old bathtub drain to be legal. Your building codes department is the best source of information.

Requiring a larger drain on a shower than a 40-gallon bathtub may not make much sense at first thought. However, understanding the physics behind water flow in a drain often gives more insight into the reasons for certain building codes. However, there is almost certainly a reason for the requirements

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A Forty Gallon Bathtub versus a 2.5 Gallon Per Minute Shower

A standard bathtub holds about 40-gallons of water. It would seem that draining a full bathtub would require a much larger drainpipe than a shower head that flows only 2-1/2 gallons of water per minute. The logic seems to defy the demands of the building code.

One would think that 40 gallons of water trying to get down a 1-1/2 inch drainpipe would cause problems. On the other side, 2-1/2 gallon of water per minute flowing into a shower makes a 2-inch drainpipe seem like overkill. In fact, we are talking about two different sets of water flow problems.

Drainpipe Construction – Differences Do Matter

One factor contributing to the seemingly illogical requirements of the building code is the way drains for bathtubs and showers are constructed. Understanding these differences dispels some of the confusion.

Many people don’t understand the differences in how shower drains and bathtub drains operate. It isn’t always about the amount of water in the tub or the flow rate from the showerhead. How water moves and flows is a major factor in drain design and installation.

Air and Water – It takes Both for an Effective Drain

Drains don’t just move wastewater. Drains also capture and entrain air as the wastewater flows away. For a drain to operate effectively and efficiently, there must be a source of air for the drain. Bathtub drains usually have a dedicated air vent that allows the drain to capture air as the wastewater flows through the pipes.

On the other hand, Showers usually don’t have a dedicated vent pipe to allow extra air for the wastewater to flow through the pipe. This need for extra air means that the drainpipe must get the necessary air through the drain opening. Hence, a larger pipe allows proper airflow into the drainpipe.

Of Drains, Overflows, and Catastrophes

A bathtub drain includes an overflow drain that helps prevent overfilling the bathtub. The overflow typically bypasses the bathtub plug and discharges any water directly into the drain. Should something plug the drain, such as a washrag or other object, the overflow prevents water from topping the bathtub and damaging the bathroom’s flooring.

Showers don’t have overflows. An inadvertent plug, or a drain that doesn’t flow properly, can cause the shallow shower pan to fill and overflow into the bathroom. An overflow can be the start of an expensive catastrophe for your home.

A larger drainpipe on a shallow shower pan can mitigate drain flow and drain capacity problems to help prevent an accidental flood in your bathroom. These facts explain some of the reasoning behind a larger drainpipe for a shower than a bathtub.

Rules and Exceptions – Get the Facts

For every rule, there is an exception, and building codes are no different. Take, for example, the city where I live. My municipality of residence has adopted the International Plumbing Code for both commercial and residential occupancies. Let’s look at what the IPC has to say about drainpipe sizes.

A quick search of the IPC returns a lot of data about drains, pipes, sizes, fittings, and connections. Sifting through all this information, we find an interesting table. This table defines the specific requirements for drainpipes sizes according to the type of fixture. Looking closer, we find the following information about bathtubs and showers.

  • A standard bathtub with or without an overhead shower or a whirlpool attachment requires a 1-1/2 inch drainpipe.
  • A shower with a showerhead that delivers less than 5.7 gallons per minute of water requires a 1-1/2 inch drainpipe as well.
  • If the showerhead flows between 5.7 and 12.3 gallons per minute, the drainpipe must be at least two inches in diameter.
  • The required drainpipe size increases as water flow in gallons per minute through the showerhead increases . (International Building Code, Chapter 7, Section 709, Table 709.1)

This information points out the need to be aware of the requirements of the building code adopted by your city. Other model building codes may have different requirements. Your city may have also enacted more stringent requirements than the model building code. A call to your local building inspection department is the best source of correct information.

What’s a Person to Do?

No doubt the problem of installing a shower over an existing bathtub drain can be a problem. Often, the cost of replumbing the drain to meet building codes exceeds all the other project’s costs combined. There may be some alternatives to the situation.

Get A Waiver

An understanding building codes inspector might see the problem and issue a waiver to the building code requirements. A waiver would allow your plumber to install the shower pan with a 1-1/2 inch drainpipe and still get a passing inspection.

Change Your Plans

Changing your remodel plans is certainly not the answer anyone wants. If the whole reason for the project was to install a walk-in shower for convenience, going back with a standard bathtub may not be an acceptable option.

Consider a Walk-in Bathtub

Another option would be a walk-in-style bathtub. New technologies have made these bathtubs with doors a much more viable option. Since they technically are a bathtub, the 1-1/2 inch drain requirements work. Most of these walk-in bathtub installations include showers. You can still walk in but have a hot soak in a bathtub as well.

Do It Anyway

Let’s understand each other. We never advocate blatantly breaking or ignoring building codes and regulations. For the most part, these codes are in place for good reason. Construction done to standards leads to fewer problems.

However, under some circumstances, bending a rule is necessary. If you are the homeowner and understand the codes and the chances that come with bending the codes, you are probably within your rights. Using good techniques to plumb a shower drain with a 1-1/2 inch pipe can mitigate many of the problems that can ensue.

The Dangers of Bending the Rules

If you elect to bend the rules and go against your cities building codes, you must be ready to accept the possible consequences.

  • If inspectors discover the installation in question, you may have to tear out and redo all the work you have done.
  • If the non-code compliant installations are discovered, you may also face fines and penalties.
  • When you try to sell your home, a home inspection may reveal the discrepancies. Potential buyers may walk away or report the non-code compliant to the codes department.
  • The smaller drainpipe installation may prove problematic and need recurring drain cleaning, service, or repairs which can eventually cost more than putting in a correct drain installation.

For these reasons, we don’t advocate anyone skirt the building code requirements. If you can’t get a waiver from the code’s inspection department, accept the inevitable and perform the required upgrade work.

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The Shower You Want

The bathroom and shower you want is the key goal. Understanding the requirements, you need to meet to reach that goal are almost as important. The temptation may lead you to try circumventing some of the code requirements. We urge you to accept the situation and allow your contractor and plumber to meet the codes to get the inspection. In the end, you will be happier with the results. You will also probably endure less frustration and trouble in the future.

Dennis Howard
Dennis Howard

Dennis is a retired firefighter with an extensive background in construction, home improvement, and remodeling. He worked in the trades part-time while serving as an active firefighter. On his retirement, he started a remodeling and home repair business, which he ran for several years.

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