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The Home Depot

Why is My Fiberglass Bathtub Cracking?

Pro Referral > Home Guides > Plumbing > Why is My Fiberglass Bathtub Cracking?
Why is My Fiberglass Bathtub Cracking?

Fiberglass bathtubs are inexpensive and come in a variety of colors to complement moth bathroom designs. However, when they crack it becomes impossible to use them as the water can leak on to and damage the sub floor. Additionally, the water that seeps out can stagnate and cause the growth of mold. The bathroom too might smell musty because of the constant dampness. Replacing a cracked fiberglass bathtub can be expensive as this will involve dismantling of the tub as well as the surround. Instead, you need to understand why fiberglass bathtubs crack to ensure that this does not happen again.

  1. 1.Improper Installation

    Improper installation of the fiberglass bathtub is often the cause of the cracking. The fiberglass is flexible and bends when weight is placed on it, for instance, when a person gets into the tub, unless it has adequate support. This support should ideally be provided before the tub is installed. A fiberglass bathtub should be placed on a concrete and cement mixture that plumbers call mud. By embedding the tub in this mixture, the bottom gets adequate support to not flex when weight is placed inside it. This prevents cracking of the tub.

  2. 2.Inadequate Support

    Even if the fiberglass tub has been installed without a concrete base to support it, you can introduce additional support under the tub when it first develops a crack. This will ensure that the crack does not reoccur or the tub does not crack in another place after the original crack is mended. This additional support can be introduced in the form of expanding polyurethane foam under the tub. This foam is mixed in the nozzle of the canister and hardens within five minutes of being applied. This foam will fill the space between the subfloor and the bottom of the bathtub and ensure that it does not flex and crack when weight is placed on it.

  3. 3.Manufacturing Mistakes

    Sometimes fiberglass bathtubs crack because of manufacturing mistakes. To manufacture a fiberglass tub, a gel coat is applied over a mold and chopped polyester resin and fiberglass applied over it. A laminator uses a roller to compact the resin and fiberglass into the gel. During this process, the laminator has to work in different concentrations of resin and fiberglass in critical areas. If the laminator fails to do this, the fiberglass tub will be weak in areas that take more stress and crack.

  4. 4.Cleaners

    Fiberglass tubs lose their gloss and become weak when harsh cleaners are used. Bleach, scrubbers, and other cleaners should never be used on a fiberglass tub. Instead, you need to wash the tub by spraying it with a cleaner that is safe to use on your hands. Just let the cleaner sit for five minutes to loosen the soap particles and wipe clean with a wash cloth to keep the fiberglass bathtub clean and sparkling.

  5. 5.Clumsiness (Dropping things!)

    A fiberglass bathtub will crack if heavy objects are dropped on it. For instance if plumbing tools being used in the bathroom or heavy and hard toys are dropped into the fiberglass tub, it can become cracked. To avoid this scenario, you should never place heavy tools on the ledge of the bathtub or near it. Additionally, children should be restrained from taking heavy toys into the bathroom. Instead, have a selection of soft and floatable bathing toys in a basket in the bathroom and encourage your children to play with these during bath time.


    This is also unwise as well to have foreign objects in or on the bathtub because you can stub your toe when trying to step into the bathtub if something is lying on top of the bathtub. Also, this tool or object could slide into the bathtub without your acknowledgement or knowledge. Then when you step into the bathtub you can step on top of this object and lose your footing and slip.

  6. 6.Get Your Bathtub Repaired

    When a fiberglass bathtub becomes cracked, it can cause a lot of inconvenience to the household. The bathtub cannot be used until the crack is fixed as otherwise the water will leak onto the subfloor and damage it. Additionally, the dampness can lead to the formation of mold or mildew that can cause allergic reactions in the residents. Replacing a bathtub is an expensive and time consuming process as the tub and the surrounds have to be replaced simultaneously. Instead you can get the bathtub repaired by an experienced plumber who has expertise in repairing fiberglass bathtubs.


    While a cracked fiberglass bathtub can be a major inconvenience in a home, it can be repaired with ease by retaining the services of a reliable plumber with expertise in this area. The plumber will identify the cause of the crack and fix any underlying issues as well such as providing additional support to the fiberglass tub before repairing the crack. This will ensure that the repair lasts long and you do not need to worry about replacing the tub soon.

Though fiberglass bathtubs are inexpensive and durable, they can crack when installed improperly or cleaned with harsh chemicals. However, this can be easily fixed by a competent plumber so that the crack is sealed and sanded, making the patch merge with the rest of the tub. This repair is more cost effective than replacing the tub and the surrounds.


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