If you hear creaking noises when getting in or out of your bathtub, the culprit is likely the floor area under it. The squeaks can come from either the acrylic tub bottom or the subfloor that it sits on. A common solution for both is adding support underneath the bathtub, which can help stop the sound.
Acrylic tubs are typically floor creaking under bathtub with a layer of rubber or foam to prevent the bathtub bottom from rubbing against the bathroom floor, reveals Clarion Bathware. This material is molded to the bottom of the tub and often sits on top of a plywood subfloor. It is recommended that most of the space between the tub bottom and sub-floor be filled with a structural polyurethane foam or another filler designed for this purpose at the time of installation.
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The squeaks you hear can also be caused by the flexing or bending of the tub floor area and this type of movement can potentially weaken the acrylic and lead to cracks and other damage. Ambitious DIYers can try injecting a low expansion foam filler into gaps between the floor of an acrylic tub and the bathroom floor, but professional help is generally advised.
Creaking can also occur due to poorly-installed or fastened tubs, or from water leaking onto the flooring and causing the joints to expand. All of these problems require access under the bathtub, which can be a challenge unless you cut an access panel through a closet or bedroom wall, or have access to the crawlspace or basement where the tub is located.