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

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

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.

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.

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.

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.

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.