No Concrete Under Bathtub (Here’s What You Need To Do)

Have you recently discovered that there is no concrete under your bathtub? Are you overwhelmed and contemplating what you should do?

Before you panic, you should know that even though such scenarios are rare, they are not unusual. In most homes whose bathtubs were installed during the 1950s and 1960s, the plumbers back then didn’t focus so much on creating steady support under the tub.

Therefore, if your home is relatively old, you shouldn’t be surprised when you find out that there is no concrete under the bathtub.

In the guide below, I will share professional tips on what you can or can’t do when you discover that there is no concrete under your bathtub.


What are the Risks of Not Having Concrete Under a Bathtub?

First, you should consider yourself lucky that you have discovered no concrete under your bathtub. Such a setup can be quite risky in the following ways;

• The Support is Inferior, and the Tub Can Collapse

Bathroom accidents can be severe, and no concrete under your bathtub is a recipe for disaster. A concrete floor creates adequate support that can accommodate the weight of the bathtub and the person using it.

However, the support is fragile if there is no concrete floor, and the bathtub can easily collapse, hurting the user.

• The Void Under the Tab is an Entry Point for Rodents and Insects

The absence of concrete means that there is a hole or space for rodents and insects to crawl through and get inside your home.

If you have been dealing with a rodent or ant infestation, the lack of a concrete floor under your bathtub could be to blame.

• There is a Higher Chance of Mold Growth

Without a concrete floor under your tab, mold can easily grow and spread to other areas within your home.

• Water Coming from the Bathtub Can Easily Leak into the Areas Beneath

Finally, without a concrete floor, chances are, there is no water barrier. Therefore water can easily pass from the bathtub to other areas. This can accelerate the rotting of the walls.

What to Do When You Discover There is No Concrete Under Your Bathtub?

Here is what you should do when you discover that there is no concrete under your bathtub.

1. Remove the Old Tub

The best cause of action in this scenario is to build a new concrete floor for your bathtub. And that can’t be done when the tub is still installed.

So, the first thing you should do is remove the tub. Old tubs are heavy because they are made of ceramic or cast iron. You may need an extra set of hands to remove the tub.

Start by shutting off the main water valve. Unscrew the plate of the tub from the floor. Make cuts on the drywall so that the tub can be freed.

Pry the bathtub away from the wall using a metallic bar. Take it to another part of the house so you can have ample working area.

2. Dig the Floor Beneath the Tub and Clean Up the Area

Digging the floor beneath the tub allows you to check how much space was left behind. It can also give you insight into the condition of the concrete flooring in surrounding areas. Proceed to clean up the working area so you can have enough room to lay a new concrete floor.

3. Install the New Concrete Floor

Start by pouring a base course and spreading it over the hole. Compact the base course for a much stronger base. Add some sand on top of the base course and spread it as well.

The sand does a marvelous job of creating a smooth finish and filling in the holes in the base course. Proceed to lay a sheet of polythene on top of the sand. Cut it according to the size and shape of the hole.

Considering the area under a bathtub isn’t very big, you don’t need a mesh. But if you want a strong floor, cut some mesh and add it on top. If you use a mesh, make sure to place tiny rocks between the mesh and polythene.

Mix your concrete and pour it into the opening below your bathtub. Spread it evenly and level it with a flat object. Finish up with a steel trowel.

Give the concrete time to cure. Two days should be enough time. Don’t forget to pour water on the concrete if you live in a hot area.

4. Apply a Mortar Bed Over the Concrete and Install your Bathtub

Once you create a new concrete floor, don’t be tempted to place the tub directly over it. Instead, you should create a mortar bed for additional support.

Unlike concrete, the mortar will contour to the bottom of the bathtub and hold it firmly in position once it dries. Create a mix of mortar, pour it on the area where you will install the tub, and then place it on top.

Give the mortar time to dry, and your bathtub will be adequately supported by a strong concrete floor and a mortar bed.

Can You Use Expanding Foam Instead of Installing a Concrete Floor Under the Bathtub?

Understandably, the above process of filling concrete under the bathtub seems hectic and time-consuming. But it’s what professional experts recommend.

As a homeowner, it can be tempting to use a shortcut such as expanding foam when you discover no concrete under the bathtub. But is it advisable?

Expanding foam is a very useful product that dries into a hard substance. However, expanding foam is no match for concrete regarding the level of support provided and durability.

Yes, expanding foam will fill in the hole that was to be replaced by concrete beneath your tub. But it will degrade when exposed to moisture for a long time.

Concrete is stronger. Though it’s not water resistant, it can’t be damaged even when exposed to it 24/7. You will enjoy peace of mind with concrete underneath your bathtub than expanding foam.

With expanding foam, there is the risk of using too much, which may affect the leveling and slope of the bathtub.

Will Concrete Shrink When Used Under the Bathtub?

When concrete dries, it is expected to shrink by a small margin. To answer your question on whether concrete will shrink when used under a bathtub. It’s true that will happen. However, it will not affect the floor and bathtub as long as you use a mortar bed.

We added the extra step of laying a mortar bed to ensure that the shrinkage of the concrete floor does not affect your tub. The main purpose of the concrete floor is to reinforce tub support.

Can Concrete Rot Lead to a Void Under the Bathtub?

Concrete does rot over time, especially if the reinforcement bars are corroded. But it’s difficult for concrete to rot to the extent that it creates a void under your bathtub. Concrete may lose its structural strength and sink when it rots, but it will not leave a void.

If there is a void under your bathtub, it was intentionally left there. The contractor or plumber could have left the void to create room to fit the bathtub.

This was probably done long ago because modern construction standards are very strict on flooring and bathroom installations.

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