How To Fix “Flex” In The Bottom Of A Bathtub By Yourself By Consumer Reports

This is familiar trouble that users would experience with any bathtub product produced from non-metal materials. Some of these materials may be acrylic, mixtures of polyester resins and glass fibers, fiberglass.

If you’re wondering how to fix “flex” in the bottom of a bathtub, you’re in the right way! Keep reading this article, and I will show you the most economical and efficient way to tackle this problem. 

What causes a flex in the bottom of a bathtub?

The crack might be going to appear in case someone is standing on the bathtub or excessive water weight creates downward pressure consistently through time, and the bathtub bottom appears a flex.

Also, manufacturing defects such as loose or broken corner stiffeners may lead to a flex that makes the bathtub leak when you fill it with water.

A flex may happen because when you installed the bathtub, it didn’t sit squarely or attach tightly on the floor. Therefore, there are gaps between the bathtub bottom and the floor.

Manufacturers may attach blocks of high-density foam in the bathtub bottom to aid in leveling the bathtub. If the manufacturers added blocks like the ones below to the base, you need to install the bathtub fitly to avoid forming flexes at the bottom of a bathtub.

As you can see, gaps between the bathtub bottom and the floor are the main reason for flexes at the bottom of your bathtub. Keep reading, and you will know how to fix it at home economically and efficiently.

How to fix “flex” in the bottom of a bathtub

You can fix the flexing by adding the amount of polyurethane foam into space or blank between the bathtub bottom and the sub-floor. Here is a step by step guidance! Let’s check it out.

Step 1: 

 If you don’t know, a bathtub weight is about 60 to 70 pounds. The first step will be the most troublesome problem when you want to tackle a flex at the bottom of a bathtub. 

You need to reach any side under the tub regardless of its weight. If you can reach under the tub from both sides then much better.

Step 2

 You need to have a compound of 2-pound or 5# thickness foam to achieve the vital structural quality to halt the flex. You will fill the hole in the bottom of the bathtub with that mixture.

My “recipe” will include 2-pound foam expanding to about one cubic foot and #5 foam increasing about one and a half cuft. You’d rather remember these estimates to determine the exact amount of foam to use to settle the flex.

Using the excess amount will make the bathtub bottom distort, and it’s the worst experience you have ever had. Therefore, please determine the exact amount.

In most situations that we’ve been through, the foams which are thicker than #5 won’t expand adequately to fill gaps or spaces in this situation.

Also, retailers usually sell regular foam, which is only one and a half-pound density. You can use it as a thin cover for protection, not structural adjustment support.

The photo displays several kinds of foam that include polyurethane foam and containers for injection. These products are highly recommended for the best foam to stop flexing in the bathtub.

Step 3: 

You need to drill through the layer of the bathtub or the support board to inject the foam into the blanks.  A 1/4″ diameter hole is the most suitable size to work.

Step 4: 

You fill the gap by filling the foam into it. If the blank is larger than you expect, you need to inject more than one point.

Selecting the correct foam for the right problem is significant. If the blank is bigger than three inches, you should choose the 2-pound foam.

Step 5: After the foam is dry, add a repair paste or putty and finished to match the original color of the bathtub.


After reading this article, I hope the issue “how to fix “flex” at the bottom of a bathtub” won’t bother you anymore. There is the easiest way to resolve your trouble, but you need to be patient.

I’m looking for your feedback about my tips to fix flexes in the bathtub bottom. Leave a comment if you are having trouble while applying it.

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