Shower pans made of fiberglass, acrylic, or similar materials often are designed to rest on a flat subfloor, without a mortar bed for supporting the base. If your shower pan is flexing, it could be due to a few different problems, including the floor, the installation, or the shower base itself.
One Reader's Dilemma
A reader wrote in with the following problem, which happens to be a fairly common problem with shower pans. She had a contractor install a new shower pan and walls. As it turns out, the pan seems to flex—but with no rocking—when her husband walks around in the shower. When she asked the contractor about this, he confirmed that the pain should not do this.
For the installation, the front threshold of the shower pan rests on existing tile, while the back of the pan rests on some loose tiles. The loose tiles act as shims to make the pan level. The installer insists that the manufacturer recommended against setting the pan in mortar or sand.
The reader reports that if you shine a flashlight in the drain you can see movement when her husband walks around in the shower. The flexing is minimal and cannot be felt underfoot, but clearly there is movement in the drain. The reader asks whether she should be concerned about the pan leaking in the near future because of the flexing and/or the drain movement?
The contractor may be correct in his claim that the shower pan does not require a mud base. There are many shower pans that install "dry," meaning they are designed to be installed directly over the subfloor and screwed into the walls studs. However, if the pan is designed to install over a flat, continuous subfloor, it shouldn't be set over shims. If the shims are placed only along the edges, the pan is spanning the distance in between and is essentially unsupported over this span. That could cause some flexing.
If a pan requires full support but the subfloor has dips or humps, the standard solution is to level the subfloor with a floor leveling compound before installing the pan.
Some shower pans have built-in spacers underneath that raise the pan off the subfloor. These spacers effectively work like shims but they're strategically placed to provide support to the pan. If you're installing one of these pans and the subfloor needs leveling, you may be able to shim directly under the spacers, keeping the support system intact.
Flex In the Pan
Acrylic and other polymer shower pans—as opposed to a shower base constructed completely of tile and mortar—will always feel hollow, no matter how good they are. High-quality, heavy shower pans offer more solidity, but even these don't feel as solid as a traditional tile shower base. At the other end of the quality spectrum, a cheap fiberglass or plastic shower pan may simply flex within the pan area. This could possibly be helped with a mortar bed, if the manufacturer recommends it.
Will the Drain Leak?
A small amount of flexing around the drain of a shower base may or may not cause a leak over time. Due to the materials used in the drain, there is a small amount of flexibility built into the assembly. The pipes are usually PVC, which can tolerate some movement. The vertical pipe under the shower drain fitting often is surrounded with a rubber gasket to keep it from moving; this, too, may help compensate for some of the flexing.
If the drain does leak due to flexing of the shower pan, the water will almost certainly fall very close to the drain and will probably show up on the ceiling below the shower, although it may run along the top (backside) of the drywall (if the ceiling below is finished) before making a stain or dripping into the room below. In any case, it's best to keep an eye on the ceiling below the shower for any signs of leaking.