How To Fill Void Under Bathtub- 4 Easy Steps

Are you tired of bathtub water getting out the gaps? Do you want to fill up the gaps under the bathtub or know how to level a bath? If your answer is yes, then take a glance. Well, I know how to fill void under bathtub. It’s nothing but a piece of cake if you know the proper steps on filling the void under the bathtub.

Water tries to expand whenever they get void. Bathtubs having a void is every household’s nightmare. We’re sure you’re not a pro plumber. But you can learn the process step by step without spending money on it. To be honest, I’m a professional plumber. So I certainly know how to do the job with ease.

In this post, we’re going to write down the easy steps and share extra tips for your better understanding. So make sure you’ve read this article at least once properly.

How To Fill Void Under Bathtub (Common Items And Steps)

The bathtub gets void because of shifting. It doesn’t matter if your floor has better quality installation.

The bathtub needs caulking Whenever it is loose or has voids to fill. And no matter how well you fill the voids under the bathtub, it won’t stay without removing the caulking joints and redoing it.

Common Items That You Will Need To Remove And Redone Caulking

Most of these everyday items can be easily found in our house. Without these things removing and redoing caulking is impossible. Such as:

  • Container
  • Rag or Paper Towel
  • Razer Blade
  • Bleach
  • Toothbrush
  • Silicone Caulking Tube
  • Knife

4 Steps To Remove Caulking Joints And Fill Out Bathtub Voids

1. Remove Old Caulk

 It’s important and mandatory to remove the old caulk. Otherwise, the bathtub would still have voids after redoing. Many might think you can just fill up the voids without removing the old ones. But don’t do that because the old caulk will soon get out on its own.

To remove caulk, use the following steps.

  • From the joint of the bathtub and floor, pull out the old caulk.
  • Then use a utility knife or razor blade under the remaining caulk carefully to loosen the old caulk.
  • Pull out the caulk carefully without any scratch.

2. Clean Expansion Joint

 The wall that is attached to the bathroom is an expansion joint. Expansion joints need proper cleaning too.

Sometimes the expansion joints aren’t placed properly for which the bathtub attachment gets loosened. To clean expansion joints like a pro, follow the steps. Such as:

  • At first, clean the expansion joint, then remove the caulking.
  • Vacuum out any loose dirt or debris in the expansion joint.
  • Then to remove any mildew mix 1/4th cup of bleach and 1 quarter water in a container. Scrub the joints using this mixture with a toothbrush.
  • Then rise up the joint with clean water and pat dry the area with a rag or paper towel. Ensure that the expansion joint is dried before starting work.

3. Fill Bathtub With Water

 Filling up the bathtub with water before redoing caulk is definitely compulsory. This process helps the bathtub to expand to its full width and length. If the bathtub isn’t expanded before new caulking, then caulking can budge out while using the bathtub.

Simply just fill up the whole bathtub with water to expand. After finishing the caulking process, it won’t come out easily.

4. Make Strong And Smooth Joint

In this step, we start recaulking the bathtub. Make sure the caulking is solid and smooth.

  • Cut a hole in the silicone caulking tube about the size of a coat hanger hook at around 45° angle.
  • Alongside the expansion joint and under the bathtub run a bead of caulking.
  • Then create a smooth surface using a dampened index finger or a leveler.
  • Don’t use the bathtub until it completely dries off.

Tips To Remember While Filling The Bathtub Void

  •  After doing recaulking make sure to dry it off for 24 to 48 hours.
  • Try not to use wood trim directly because it can cause mold.

A Helpful Tutorial You May Need!

Frequently Asked Questions (FAQs)

Can I use expanding foam under the bathtub?

Yes, You can use expanding foam under the bathtub. An existing installation of a bathtub helps in reducing flex and coldness. However, extra problems can occur due to improper application. So, it’s best not to use it without the help of a professional if you don’t know the proper application.

How do you fill the gap between tub and floor?

To fill the gap between tub and floor, it’s best to put your tub against the floor as much as possible and then use drywall to cover the little gap. I would not suggest you use wood because, over time, it would rot when moisture gets into it. But if you want to use wood then it’s best to use Styrofoam wood trim. Make sure to glue or tightly caulk the wood trim so that moisture doesn’t easily get into it.

What do you put under a fiberglass bathtub?

I put expanding spray foam under a fiberglass bathtub. Bathtubs are not level or flat so they can’t directly touch the floor. Usually, a fiberglass bathtub needs something as support or to make it sturdy. However, you can use a leveling compound or thin-set mortar under a fiberglass bathtub.

How do you support under a bathtub?

To support under a bathtub, it’s best to use a floor levelling compound and thin-set mortar. Also, by placing piles in series it is easier to control, which gives support under the bathtub.


In a nutshell, we have discussed each part of how to fill void under bathtub. It’s quite easy to fill up the voids just follow each part thoroughly with patience.

We are sure you won’t need help from any professional plumber again regarding this problem. Make sure to read my other articles related to plumbing.

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