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Moen Solutions

What Is A Stacking Leak?

Tub and Shower Leaks

This guide will explain how to correct a showerhead leak when you are running the water out of the tub spout only (this is sometimes referred to as shared flow). This is called stacking and generally found in new installations.
What is Stacking?

The term "stacking" is used to describe a significant shared water flow out of a tub/shower system during normal operation (Note: This symptom is unique to tub/showers). When the system is turned on you'll notice a steady stream of water coming out of the showerhead while the tub is filling below.

The symptom is generally found in new installations of 1, 2 or 3 handle tub/shower systems. It is sometimes mistaken for a cartridge leak. The difference between stacking and a leak is that a leak begins when the tub/shower has been completely shut off, while stacking is shared flow noticed only while the tub/shower is operating.


The tub spout diverter gate is partially blocking the path of the water.


If there is debris or damage to the tub spout, the flow of water will back up and cause a shared stream.


Remove the tub spout.

If the symptom disappears:

replace the tub spout.

If the symptom continues:

Are there additional SharkBites or snap adapters being used? This maybe slowing the flow down.



Incorrect installation.


The recommended 46" provides a reservoir for the water that backs up as there is a slight delay exiting the tub spout.


Check that the spacing between the valve body and the outlets for the tub spout and showerhead are within the guidelines on the instructions. Re-plumb the outlets to the correct spacing.  

  • The tub spout is to be no less than 6" and no greater than 11" below the valve body.
  • The showerhead must be at least 46" above the valve body.  


Incorrect piping/tubing used.


1/2" metal piping is needed to allow adequate water flow to the tub spout to avoid a shared stream. The internal diameter of a 1/2" pipe varies based on material and connection type.  


The installation instructions specify that the piping used from the valve body down to the tub spout outlet be 1/2" metal pipe (Copper, Galvanized, Brass IPS (Threaded) pipe).  Replace incorrect piping. 



Multiple adapters (elbows) used between the valve body and tub spout.


Elbows slow the water down as it exits the tub spout and could create the stacking symptom (the pipe or tubing might be large enough to deliver the water to the tub spout but the fitting might restrict or back up the water flow).


Only one elbow should be used between the valve body and tub spout. IPS (threaded) tub spouts are designed to screw onto a pipe with 1/2" male pipe threads. Some models may feature a slip fit installation with no threaded adapter in the tub spout. For these installations, ensure 1/2" copper water tube is used.



Single Handle-The valve body was installed upside down. 











Single Handle-If a valve body is installed upside down it will send the water to the showerhead first instead of down to the tub spout. 










Single Handle-Check your installation to make sure "UP" is facing the ceiling. Reinstallation of the valve body may be required to correct stacking.  



Two Handle-Upside down flow director.


Two Handle-If a valve body is installed upside down it will send the water to the showerhead first instead of down to the tub spout. 


Two Handle-If the valve body was mounted upside-down to accommodate water supplies that come down from the ceiling, the flow director must be removed and installed into the side of the valve body that the water supply enters.  Remove the flow director and reinstall correctly.

Shower Arm Volume Control Valve Option

If you are not able to correct the issue creating the stacking, consider installing a shower arm volume control valve-Moen part #91888. This device will allow you to shut the water flow off at the showerhead. It is not designed to be used a substitution for the control handle and should never be used as your main on/off control.



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This guide will explain how to correct a showerhead leak when you are running the water out of the tub spout only- Also referred to as STACKING