Lack of Support
Fiberglass tubs are often installed with a lack of support between the underside of the tub and the floor. A lack of support gives the tub a flimsy feeling. Plaster, cement or wood shims should be installed under the tub to give it support so it will not flex. Otherwise, each time the tub is used, it flexes a bit. Everyone entering the tub in the same place makes the problem worse. Flexing anything often enough will cause fatigue and the result is a crack.
The proper cleanser should be used to clean a fiberglass tub. Some soap products are too harsh and contain alkalies that will cause cracks in the finish, according to the Ask the Builder website. The mildest soaps should be used and the tub should be cleaned regularly. Consult the manufacturer for recommendations for cleansers. Additionally, scouring pads will scratch the surface. Instead only use a wash rag.
Fiberglass tubs are constructed using the same materials and methods used in fiberglass boat building. A gel coat is allowed to cure when applied to a mold, and chopped polyester resin and fiberglass are applied over the gel coat and then worked into the surface. The worker, also known as a laminator, uses a roller to compact the fibers in order to make a strong tub. However, the strength of the tub depends on the ability of the laminator to apply the correct amount of resin and fiberglass in the critical areas. If this is not done correctly, the tub will develop cracks over time, often in the bottom of the tub.
Jumping in a fiberglass tub will cause cracks whether the tub is full or empty. Dropping hard objects in the tub, such as tools used while working around the tub, will cause cracks in the surface. Hard children's toys can also damage the tub.