Fiberglass showers and tubs are susceptible to cracking due to excessive flexing when adequate support is not present below the surface. During the installation of your fiberglass tub or shower, proper support may have been overlooked.
Damage resulting from improper support, caused by the additional flexing, can include cracking of the gelcoat (the finished surface of the tub or shower), and/or cracking of the fiberglass structure.
When the gelcoat cracks you may notice many small hairline cracks in the finished surface of the tub or shower. The cracks can be short or long and they typically run along the corners. These cracks are often called spider cracks. The presence of spider cracks do not necessarily indicate there is damage to the underlying fiberglass structure. It is likely that the spider cracks have been created due to an excessive amount of flex in the bottom of the tub. We'll need to address the cause of the flexing first.