How to Support a Fiberglass Bathtub: Dos and Don’ts

Having an unsupported fiberglass bathtub makes squeaky noise and leads to discomfort. In that case, creating a support base or having bathtub support is mandatory. So, how to support a fiberglass bathtub?

A fiberglass bathtub support can be of several types. However, the most commonly used support methods are creating a mortar bed or using expanding foam spray under the tub.

There are several more things that you need to keep in mind and work on. And that’s what my today’s article is all about.

What To Put Under A Bathtub For Support?

To ensure proper drainage systems, bathtubs are made round-shaped around the edges, and the edges stay a little higher off the ground.

As a result, the bathtub does not touch the ground completely. And that leaves a void under the tub, and it needs support. But, what do you put under a bathtub for support?

Usually, you can put two types of support under a bathtub:

  • Mortar Bed: A mortar bed, also known as a mud bed, is effective for perfect bathtub installation support. It is usually put under a bathtub while installing.
  • Foam Spray: Foam sprays are for bathtubs that are already installed and need support underneath. You can either spray it through the gaps on the floor or by creating a fixable hole in the tub.

How To Support A Fiberglass Bathtub?

How to Support a Fiberglass Bathtub

While a wooden bathtub support frame holds the upper part of a fiberglass bathtub, the bottom remains off the ground. And after some time, the tub starts to move and can even crack. That’s why using a support base underneath is essential. You can support a fiberglass bathtub using a mortar/mud bed or expandable foam spray.

Mortar Bed Support

how to make a mortar bed for a bath

Here is a step-by-step guideline on how to properly support a bathtub using a mortar or mud bed. Remember that the mortar bed turns out well put if laid to support a newly installed bathtub rather than supporting an existing one.

Step 1: Prepare The Floor

Take a plastic sheeting and cover the entire tub base area with it. Using the plastic sheet will prevent the mortar moisture from damaging the subfloor wood. To make sure the sheeting is of the right size, you can draw lines with a pencil according to your tub measure and cut it with a utility knife.

Here are some of the best plastic sheets for bathtubs that you can get a very reasonable price.

Step 2: Prepare The Mortar Mixture

Mix the concrete with an adequate amount of water. The mortar package will have instructions that you can follow. Then turn on the mixing machine and wait till the concrete and water are turned into mud. Use a rubber or plastic container to carry the mixture in.

Step 3: Lay The Mortar/Mud

If the bathtub is already installed, you can use a small shovel to lay the mud under the tub. If the tub isn’t installed yet, you can either pour or shovel the mud on the floor. The layer should be two inches high and 4-6 inches in diameter or enough to support the bathtub after drying.

Step 4: Final Touch

Once the layer is done, put another plastic sheeting on top of the mortar bed. This will prevent the mud from sticking to the tub and making it difficult to remove the tub in the future. 

And finally, place the tub on the bed and fill the tub with water. Let the filled tub sit on the mud bed for around 72 hours before you use the tub.

Check our latest guide about: How To Stop Bathtub Overflow Drain? Here are the Facts!

Also, read the related guide: How To Build A Frame For A Drop In Bathtub? Read this First!

Foam Spray Support

can you use spray foam to support a bathtub

I think expandable spray foam is a ‘tub-saver’ when it comes to existing fiberglass bathtub support. It also happens to be the easiest solution.

Step 1: Fill The Tub

Foam sprays expand for a certain amount of time after you spray them. And you never really know how far it can expand. So the best thing to do is fill the tub to give it some weight before you spray.

Step 2: Find An Access

You can make a hole in the tub to insert the spray nozzle. But the hole should be fixable. And if that seems like too much work, you can get access through the void under the bathtub.

Step 3: Spray Foam And Let It Expand

Insert the spray nozzle through the void. And try your best to spray around the middle rather than the edges. Because once the foam starts to expand, you will be surprised to see how much they do.

Step 4: Final Fix

After several hours you will notice the foam has expanded and created a strong base under your fiberglass bathtub. Finally, you can remove any externally expanded foam using a utility knife and drain the tub water.

How Much Weight Can A Fiberglass Tub Hold?

A fiberglass bathtub requires strong support because these bathtubs can crack easily because of extra weight. Depending on the size and weight, a fiberglass bathtub can support 350-400kg of weight, including water and body weight.

How Do You Support An Acrylic Bathtub?

If you know how to support the bottom of a fiberglass bathtub, supporting an acrylic bathtub won’t be a problem. Just like fiberglass bathtubs, acrylic bathtubs are not self-supported.

The tub is set on a wooden frame for top support, while the bottom may remain empty. To prevent the tub weight from rocking the tub when you climb in or make noise, you can use a mortar bed, foam spray, or even wooden shims.

Best Foam For Under Bathtub

If you want to use foam for securing your bathtub, you can use the following foam sprays:

Best Cleaner For Fiberglass Bathtub

Getting your bathtub dirty or having mold gathered here & there, is not uncommon. But they can lead to having an unclean tub if you do not clean them regularly. 

So I’ve found some incredible and effective for the best fiberglass bathtub cleaner

Best Fiberglass Bathtub

Of all the types of bathtubs available, fiberglass bathtubs are the least expensive. They are also lightweight, easy to clean and replace. Best fiberglass bathtubs tend to be strong enough to have excellent weight capacity, durability, and easy to clean and maintain. 

Best Fiberglass Tub

Repair KitIf your fiberglass bathtub is suffering from cracks, scratches, or any other kind of problem, it is time to repair them. Repairing in time allows the tub to last even longer, which is beneficial for you.

Users from Amazon recommend these kits for fiberglass bathtub repair:

However, do not mistake them or use them for fiberglass water tank repair kits. Both of them are different and require different types of repairing kits.

Frequently Asked Questions

Here are some of the frequently asked questions regarding “how to support a fiberglass bathtub” and other things.

1. How Do You Support A Bathtub?

You can support a bathtub using-

  • Mortar bed.
  • Foam spray.
  • Wooden shims.
  • Wooden frame.

2. How Do You Secure A Bathtub?

You can secure the tub to the studs by using roofing nails. You can use a hammer or drill to attach the tub with nails. But make sure you don’t hit the nail too hard and damage the tub.

3. What Is The Best Fiberglass Window?

If you are suddenly fond of fiberglass materials and want to have fiberglass windows, you can purchase them from stores or online. The best fiberglass windows are those that have the quality material, the right size, and come from a reliable brand.

4. How Do You Stabilize A Freestanding Tub?

Freestanding tubs are unique, beautiful and make your bathroom look more aesthetic. Since this type of tub stands on their own, they hardly require anything for stabilization. But in case your freestanding tub is not standing correctly or moving, try applying silicone sealant at the bottom to prevent the issue.

5. How To Support A Fiberglass Shower Floor?

If you want to find out how to support a fiberglass shower base or floor using an expanding foam spray, here is your answer:

  • You either cut a small hole in the tub bottom or reach through the gap below.
  • Spray the foam around the middle area under the shower floor. 
  • Wait for a few hours, and you will notice the expanding foam has created solid support.

6. Can A Fiberglass Bathtub Be Resurfaced?

Yes, a fiberglass bathtub can be resurfaced at any time. All the running water, dirt, mold, and cleaning take away the bathtub’s original touch over the years. Resurfacing means your bathtub will have a new gel coating and color, which will make it look fresh and help it last even longer.

7. Can You Install a Bathtub Without Mortar?

Yes, it is possible to install a bathtub without mortar but it depends on your bathtub placement.


Bathtub support comes both from the side panels and below. No matter how strong your bathtub material is, it will become squeaky and unstable over time if it does not have good support underneath. That’s where knowing how to support a fiberglass tub comes in handy. 

And now that you have learned how to support a fiberglass bathtub, I believe you will know what to do. But if you feel like you are not up for the job, you can always take professional help. And if you need further information regarding bathtub installation, WikiHow can help you learn more.

Good luck with the nice steps on how to install a fiberglass bathtub.

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