Why are shower drains 2“ but tub drains 1.5”?

Although shower or tub drains serve the same function to dispose of wastewater into your pipe drainage system every one performs the task differently, and there are drain differences. Standalone shower tub drains, a tub and bathtub, and shower combination drain can’t be used interchangeably. If you’re remodeling a bathroom and the project includes replacing your standalone shower with a tub, consider hiring a plumber for the tub installation. A plumber can make sure the plumbing outlets meet the plumbing code.

Shower vs. Tub Drains

A shower uses a floor drain, which directly removes wastewater from the shower instantaneously as you bathe. A tub drain, on the other hand, needs you to manually close or open it with a lift-and-turn mechanism or a lever.

Bright new bathroom interior with glass walk in shower
You will need to push the mechanism down and then turn it clockwise for closing the drain when you will want the tub to fill with water.
Once you have finished bathing, you lift the mechanism up and turn it counterclockwise to open the drain and let the water drain. A lever works the similar way, except that you pull the lever up to close the drain and push it down to open. You can find shower drains at different stores, including Home Depot, Amazon, Wayfair.

Pipe Connection Size Differences

Shower or tub drains have different drain connection sizes to accommodate the pipe sizes of your plumbing drain system. A shower tub drain has a 2” drain connection, while a tub drain often has a 1 ½” drain connection. The drain connection for your tub is similar to the one for your sink, whereas the shower drain connection is slightly larger. When making changes to your shower and tub drains, consider that the drainage pipe can be larger than the drains. For example, your bathroom might have a 2” drain and 3” drainage pipe. Be sure you have all of the essential supplies on hand if you intend on replacing drains.

Connection Differences Between Drains

Since there is a floor drain in a shower, wastewater empties into the plumbing drain system directly. It doesn’t have to be piped from the shower outlet to the plumbing drain system, like a tub drain requirements. With this, you can make your tub a floor-level sink. A tub mandates, you install drain connections, the same as those found on your sink, for wastewater to empty into the plumbing drain system of your home.

Why are shower drains 2 inches but tub drains 1.5 inches?

That’s a vestige of older plumbing codes and good practices. Current plumbing codes (IPC as well as MA Plumbing Code 248 CMR 10 for instance) need 1.5 inches to drain for a tub with a shower. The tub can drain at leisure and the shower function when in utilize, with flow-limiting showerheads, fall within the flow rate that a 1.5” drain can handle. The drainage fixture units for a tub and shower combo doesn’t increase because there’s both a tub spigot and a showerhead, because one tends to run at a given time. Tub manufacturers make tubs with 1.5 inches drains, so this is accepted practice.

So far so good.

Shower stalls, on the other hand, are or tend to be commercial and institutional in character or are subject to customization. Tile, pan-beds, area showers, and the like. Due to the lack of limits, a 2” drain and trap are prescribed. In few homes, the so-called car wash showers have flow rates that are 4 times that of a standard showerhead. So, there are reasons, but codes are minimums. The drain has to be sized for the incoming water when the installation departs from what the code sets forth as common fixtures. Under both the International Residential Code (IRC) and the Uniform Plumbing Code (UPC) the standard size for a tub drain and P-trap is 1 to 1/2 inches and under the IRC the tub waste arm (drain line) maybe 1 to 1/2 inches but under the UPC (Uniform Plumbing Code) a 1 to 1/2 inches drain line is limited to 1dfu (drainage fixture unit) whereas under the Uniform Plumbing Code (UPC) a tub is rated at 2dfu’s, therefore under the UPC the tub fixture arm must be 2 inches.

In the case of a tub that contains an internal lever waste stopper, the drain opening should be equipped with a trim cover, which is the small flat metal cover with all the little holes in it. Code requires that the combined cross-sectional area of all those small holes must be equal to or greater than the cross-sectional area of the tailpiece. Understanding that the tailpiece is 1-inches to ½ inches it then stands that the cross-sectional area of the tailpiece is 2.25-inches If we make a cover with ¼-inches diameter holes then we need to have at least 36 holes. By carefully arranging those holes in a circular pattern they can make the overall diameter of the trim cover 1-3/8 inches or 1-7/16 inches. The problem is that a foot of an adult is enough that if the person taking a shower was to stand directly on the drain opening their foot can cover all the holes and water can’t drain out. Luckily, the walls of a tub are high enough that the tub will serve as a reservoir to hold the excess water unless the person moves away from the drain, and in a worst-case scenario, the tub has an overflow that’ll prevent water from spilling over the top of the tub.

On the other hand, a shower pan should have a minimum of a ¼ inches per foot pitch towards the drain opening and the threshold should be a mere 2-inches above the finished height of the drain opening. If we had a 1-1/2’drain opening somebody can inadvertently stand on the drain opening without realizing it, and maybe drop a washrag on the drain opening and the water will rapidly build up and overflow over the threshold. By having a 2-inches drain then, it becomes essential to have 64 ¼-inches holes in the trim cover, and letting for enough metal to offer the essential strength in the trim cover, the diameter of the trim cover is now 3 ½-inches to 4-inches.