Shower bases, or pans, made of acrylic, ABS, or fiberglass make life easier in so many ways for homeowners and for installers.
Instead of building up ceramic tile bases from wet mortar, installers only need to set them in place on a mortar bed before the walls are finished, then connect them to the drain system. Some pans do not even require mortar. Pans can be paired up with tiled walls for more design possibilities. For homeowners, single-piece shower pans are easier to clean than tiled shower floors.
Yet for all of their advantages, shower pans do have a few downsides. Plastic is not an inherently strong material; it always must be supported in some way, whether with support ribs or with a mortar base. Sometimes, severe flexing can stress the drain fittings and may cause a leak around the shower drain. In serious cases, a shower pan may even develop cracks in its floor surface.
Why Your Shower Pan Is Moving or Flexing
Identifying the cause behind the moving shower pan is nearly as important as the fix itself. Once you know the cause, you may be able to make minor repairs that stabilize the pan or protect the delicate drain fittings.
Normal Amount of Flex
Few prefabricated shower pans will feel as rock-solid as a tiled shower base. If you are accustomed to a tiled shower base, you will immediately sense the difference. Shower pans’ hollowness can be perceived as weakness, and this is not necessarily the case. Some small amount of flexing is expected. This is not a problem as long as no water leakage has developed.
Thin or Poorly Braced Shower Pan
All plumbing fixtures come in a range of quality standards and shower pans are no different. Some prefabricated shower pans will have a thinner floor or less substantial support ribs beneath it. This results in a base that still is operable but is more susceptible to flexing. Flexing can be minimized or eliminated if a thin or poorly braced shower pan is re-installed in a mortar bed. Any insubstantial shower pan that rests directly on the subfloor is likely to flex underfoot.
Shower pans have weight limits. Consult the manufacturer's instructions for information on weight limits for the shower pan. Do not exceed safe weight recommendations. Very heavy usage might contribute to cracking and pulverization of the mortar bed, leading to structural failure.
Improper Cleaning or Usage
Improperly cleaning the shower pan over time may deteriorate its structural quality. When using drain cleaner, always rinse the pan immediately and thoroughly with water. Be especially careful to avoid using chemicals known to deteriorate acrylic, ABS, polystyrene, fiberglass, or plastic. Solvents such as turpentine, mineral spirits, paint thinner, and acetone can crack the surface of the pan.
Prefabricated shower pans come in one of two types: those that are installed in a bed of mortar spread over the floor and mortar-less pans that are designed to rest flat on the floor.
If a mortar bed is not installed with a shower pan that requires one--even with higher-quality pans--the shower pan will not be adequately supported underfoot. The pan will move and flex when someone showers. The condition will worsen over time.
Shower pans that do not require a mortar bed have support ribs beneath them that radiate out from the drain opening. They are designed to support the shower pan as a whole. If the subfloor is uneven, the support ribs will not rest firmly on the floor across their entire length.
Also, if a shower pan has been shimmed on one side to level it, the ribs may be unevenly supported, with gaps in the center. Wood shims will rot and collapse.
Poorer quality shower pans may not have enough support fins to adequately reinforce the pan. For these reasons, some plumbers use a mortar bed even when the shower pan manufacturer does not specify that mortar is required.
How to Fix a Flexing or Moving Shower Pan
Leave It As-Is
If your shower base and drain fitting have been correctly installed, minor flexing will likely cause no problem at all. The flexing may be completely within the expected performance of the shower pan.
Add New Plastic Shims
A flexing shower pan resting directly on the floor without a mortar bed may need to be more adequately supported along the length of the support ribs below it. This can be a difficult thing to do after the pan is already installed. It may be possible to access the bottom of the shower pan from a room below the shower. Make sure that you use plastic shims to avoid the possibility of wood rot.
Install a Flexible Fitting
Plumbers who anticipate the flexing or moving of the shower pan often will install a special flexible drain fitting in the drainpipe below the shower.
The fitting is designed to accommodate the bending and flexing that occurs in prefabricated pans. A flexible fitting serves as a kind of rubber shock absorber that compresses and springs back when the shower base flexes around the drain opening.
This fitting can also be installed retroactively from below the shower if your shower is flexing enough to cause leaking in the drain connections. Installation from below requires cutting an access opening in the ceiling below the shower, which can be patched after the fitting is installed.
Remove and Replace the Shower Pan
In severe cases—especially if the flexing has caused the shower pan itself to crack—the only solution is to remove the base to make the necessary repairs. This can involve better shimming for the support ribs beneath the pan or laying a mortar bed to support the pan over its entire surface.
Replacing a shower pan is a major project. If you are going to this length, you might want to take the opportunity to install a new shower pan that is more resistant to flexing. If the present shower pan was installed with mortar, you most likely will need to purchase a new shower pan since the current one may become damaged when you remove it.