Why Are Shower Drains 2 Inches but Tub Drains 1.5-Inch?

Shower and tub drains serve the purpose of draining the water. They also get rid of the hair and gunk that can collect in the tub after a bath. Since they do similar things, it would make sense that the drainage systems are similar.

Then, why are shower drains 2 inches but tub drains 1.5-inch? 

This slight difference might not seem like a big deal, until you try to renovate. Pay careful attention to your new drain because you can’t swap the drains in showers and tubs.

Plumbing codes are not universal. They can get downright confusing. But, the reason for the drain size difference comes down to unique features. Tubs have a p-trap like a sink but showers don’t.

Shower Drains Versus Tub Drains

Most showers use a standard 2-inch drain to remove wastewater, soap, hair, and all the other waste. Tub drains are 1.5-inch. They involve a lever mechanism to stop or clear the pathway for water to exit.

You have to turn the lever in a tub in a half rotation to close the drain so that water can fill the tub. After you have finished filling the tub and bathing, you can reverse the process to drain the tub.

It may seem strange that the drains are different sizes. It used to be that older homes and municipal codes allowed for 1.5-inch drains. Over time, developers noticed that larger pipes are better at removing wastewater and debris.

Also, since hair and soap build-up in a pipe, a 2-inch pipe is more desirable than the traditional 1.5-inch drain. The larger the pipe, the easier it is for you to flush it clean.

Why Are There Different Sizes?

Shower drains are often 2 inches, while tub drains are 1.5 inches. A 2-inch drain is best for passing large volumes of wastewater and debris, so why not use it in both? 

Some municipal codes state that a shower drain can directly empty wastewater into the sewer system. They usually don’t restrict drain size.

Tubs have different requirements than showers since you have to plumb them in a way that’s like a sink. Also, just like a sink, tubs require P-traps

P-traps prevent smells and waste from returning through the plumbing and causing odors or overflows. That’s a pretty good reason for restricting the drain size. It certainly would be gross if waste was able to backfill into the tub.

Wondering how p-traps actually work? Check out this video.

Most sweeps, S-traps, and P-traps are a standard size of 1.5 inches. It is only appropriate that the tubs requiring such parts use the same sizing standard.

Why Are Shower Drains 2 Inches but Tub Drains 1.5-Inch?

It seems that 1.5-inch drains are desirable for tubs. You can use different parts that ensure adequate drainage. Still, 1.5-inch pipes are more restricted than 2-inch pipes.

Having a 2-inch drain would be more desirable because it could move larger volumes. We might wonder why plumbing designers haven’t standardized 2-inch drains.

Old plumbing codes, such as the IPC, and various US state plumbing codes, such as 248 CMR 10, state that tubs with showers need to use 1.5-inch drains. 

Due to such codes, 1.5-inch drains remain the standard even though they are less functional. Finding loopholes is also incredibly difficult. For example, if you install a “tub shower,” the codes say you must use the tub standard. That means you’re stuck with a 1.5-inch drain.

Commercial Versus Residential

We certainly are not looking for loopholes, but again, I think a 2-inch drain is better. Shower stalls like you find at gyms and spas are customizable. This would also be true for custom shower pans, open area showers, and other similar projects.

In any situation, I recommend a 2-inch drain and trap. Municipal codes might make you nervous about citations. Just remember that most codes are more like recommendations. They often represent the minimum requirements.

The International Residential Code and the Uniform Plumbing Code state that any tub drain should have a p-trap. The p-trap is 1.5 inches, so the drain line should be the same measurement.

It is a bit of a contradiction, though. The drainage fixture unit limits a 1.5-inch drain to one under the International Residential Code.

This figure contrasts the Uniform Plumbing Code, which indicates a two drainage fixture unit. It means that the plumbing should be of 2-inch diameter.

What if you have a tub with a stop plug? These tubs should have a channel opening with a trim cover. A trim cover is the metal lever on the cover with several little openings in it. Code expects that the trim cover should act like a filter or funnel to support the larger opening. 

Understanding that the rear end is 1.5 inches, it then stands that the cross-sectional area of the rear end is 2.25 inches. If we make a cover with .25-inch measurement openings, we want to have about 36 openings as compared to volume. They can make the general measurement of the trim cover 1-3/8 inches or 1-7/16 inches.

Phew! That is confusing with a lot of measurements. The important thing to remember is that the foot of the tub is enough to contain the volume of any overflow if the drainage isn’t sufficient. 

Fortunately, the walls of a tub should be high enough to contain any overflow. There are a few exceptions. For example, say the individual created distance or a break from the channel and the tub floods. The water will pool and fail to drain.

In layman’s terms, the municipal codes set requirements for tub drainage. The volume of water the tub can hold needs to be able to escape through the drain without issue. If you somehow obstruct your tub’s drain, you’ll have a mess of your own making. 

That said, a shower drain should have at least .25 inches per foot pitch towards the channel opening. The limit ought to be a simple 2 inches over the completed level of the channel opening.

In real life, let’s imagine a 1.5-inch drain opening in the tub. Somebody, say a curious child taking a bath, “accidentally” blocks the drain with a bath toy of some sort. The right plumbing setup prevents the water from overflowing too quickly. It also may save the poor parent from a terrible mess.

The water could quickly build up and spill over the edge, but as a result of installing a 2-inch drain, the time by which the shower can overflow decreases.

Final Notes

Let me answer the question of “why are shower drains 2 inches but tub drains 1.5-inch.” It comes down to the drain connection sizes to pipe size of your current plumbing drain system. Larger pipes need larger drains, and vice versa.

2-inch drains are better than 1.5-inch drains. Municipal plumbing codes can be a bit difficult to adhere to. In principle, the concepts seem reasonable. If you want to move larger quantities of water quickly, you need a bigger drain.

That said, experienced plumbers highlight the value of a 2-inch drain over a 1.5-inch drain. If you are a DIY type of person, keep that in mind. The larger the drain, the more waste drains at a faster rate.

Also, be aware that a tub’s function is to hold a specific volume of water. You don’t want the water to escape, so you want a balance between 1.5 and 2-inch drains.

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