Asked on Jul 16, 2013

Hairline crachks in bottom of fiberglass tub

Trixie63Phil MillerWoodbridge Environmental


I have recently observed hairline fractures in the bottom of my fiberglass tub.I belive they are a result of the tub not being supported properly when it was installed. The bottom of the tub has a very small grid pattern that is raised. Dur to this I think It wiil be difficult to repair with a fiberglass kit. Does anyone have sugestion? I am on a very tight budget.
7 answers
  • You need to support the bottom of the tub to prevent any further movement. Spray foam will help, but is really not a cure all for this. Ideally you need to work some cement under the tub to stop all the movement once and for all. Fiberglass tubs and showers have what is called a Gel Coat on the surface. This is what makes the tub shiny and easy to keep clean. With movement over time this gel coat surface begins to crack and with tiny scratches makes it harder to keep clean. The fix for the very tiny cracks is to paste wax the surface to fill in the voids. But if they are getting larger then the only method is to refinish the surface of the tub. This is best left to pros that do fiberglass repairs. Not that this is difficult to do, but using special tools to apply the new clear coat combined with the pre cleaning work required makes it more likely they will result in a more new look then what the average home owner can provide not doing this on a daily bases.

  • Karen M
    on Jul 18, 2013

    This happened to one of our tubs years ago. It may not be the installation that caused it. Ours was a manufacturers defect in the application of the materal backing. We were refunded and the tub was repaired nicely. [resealed and coated] My husband builds homes. Our little 3 yr old girl noticed the first crack - and we were concerned she'd get pinched.[she turns 40 this year- the tub is still in use though we moved!

  • Phil Miller
    on Jul 18, 2013

    Tubs, no matter what kind, need to be supported on a bed of sand. Cement should not be used at all. As for the fiberglass fix, there are kits to repair them from the bottom side if they are leaking. And then there are tub liners if you want to try one of them. But first, you need to pull the tub and put a bed of sand under it. Be sure it is level.

  • Wri200925
    on Jul 18, 2013

    our tub in a trailer- over 10-started a crack- water ran in and under- Fixed it with Plumbers Putty- which when mixed together-gets hot and then real hard! Fixed for years.

  • Phil the issue I would have with sand it is fluid. It will settle out and move every time the tub is stepped on. After a while the support is no longer there. Using a closed cell spray foam will work, but it needs to be at least a 2lb mix. Anything from a can is to soft and it will flex with the tub. I do not think at this time their tub is leaking but the gel coat surface is starting to crack. Over time this will result in a failure of the fiberglass structure. I do think however if the tub itself has cracked your idea of a new tub liner being installed is a good one. But replacement fiberglass tubs are not all that expensive other then the cost to repair the wall tile when it is pulled out.

  • Phil Miller
    on Jul 20, 2013

    Sand has been used for years before foam was invented. I have used it with no problems.

  • Trixie63
    on Sep 22, 2015

    My husband sells fiberglass tubs. He says either someone put a ladder in the tub to work on the ceiling or something. Or like you suspect the tub was improperly installed. If this is the case, a repair kit will do no good as it will crack again. He suggests contacting the person who installed it or the only other option is to get s new tub and a new contractor. I hope you figure out the best solution for you.

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