A fibreglass bathtub is an affordable, flexible, and easy-to-install option that comes in many different sizes and style options. This is why many older households have these tubs installed. But when fitting a bath, ensuring that you provide the right level of support underneath the tub is essential. Bathtubs are generally designed round shaped with edges that remain higher off the ground to enable proper drainage, this leaves a void under the body of the tub which needs to be addressed and that’s why we are going to look at how to support a fibreglass bathtub.

Trap Underneath Bathtub

Why do You Need to Support a Fibreglass Bathtub?

If the fibreglass bath is not installed properly with support underneath you can expect to run into problems quite quickly. If the fibreglass bottom is left with a void underneath it will begin to show the strain of bearing the weight every time a person steps into it to bathe and fills it with water. You can expect to hear squeaks and groans of the material when you enter the tub and start to see cracks in the tub itself.

Supporting a bathtub will ensure that it remains safe, undamaged, and able to withstand the pressure of the extra weight of a person and bath water whenever you use it.

How to Support a Fibreglass Bathtub?

Supporting a fibreglass bathtub and filling in the underside of the bath doesn’t automatically mean contacting your local construction company. If you are handy with DIY and quick to learn you can get the job done yourself for a fraction of the price it would cost to hire someone to come in. If your bathtub has already been installed with a void beneath it the easiest and quickest way to provide some support is to use expanding foam spray. You won’t have to remove the tub and the procedure is very simple.

Below we will talk you through the simple steps you can take to support a fibreglass tub all on your own!

What You Need

Step 1 – Fill Up the Tub

The shape and size of your tub may vary slightly when it has the weight of water pushing down on it, so get started by filling up the tub with water so you’ll be able to fill in the shape it assumes when it is bearing the most weight.

Step 2 – Remove the Side Panel

If the tub has been installed with some small gaps between the bath and the wall you may be able to spray the foam from certain points at the top of the bath. Or else you can remove the bath panels and apply the foam from the side of the bath.

Identify which parts of the bathtub floor feel the most unsupported and flexible and then mark it out with some waterproof tape.

Step 3 – Fill the Void with Expanding Foam

Pop on your gloves and fill the underside of the tub with foam until the spray begins to push back through the hole or void you are using. Wipe away any then leave the tub for a few hours to let the foam solidify.

Note: Always refer to manufacturers’ instructions on how to direct the nozzle for the foam, and how long it takes to cure etc.

Step 4 – Tidy Up Your Work

Once the expanding foam has dried, you can replace the side panel and empty the bath again.

Note: If you get any foam on any hard surfaces by mistake, use a stanley or razor blade to gently scrape it off.


Supporting your bathtub is essential if you want to ensure that it lasts as long as possible and remains in a water-tight and safe condition. Whether you choose to use expanding foam or create a mortar base will depend upon the current state of your bath installation. If the bathtub has already been installed, then using expanding foam is probably your best bet. No matter which method you choose, you’ll find the process straightforward and achievable by following our simple steps on how to support a fibreglass bathtub.

How to Support a Fibreglass Bathtub Infographic

Plumbing Wizard Tips

“Once in use, do not touch in or around your eyes, expanding foam is extremely damaging to your eye health!”

“Always wear gloves when using expanding foam, expanding foam can cause significant irritation or allergic reactions!”

“Use the expanding foam under the base of the tub. Be careful not to foam all around the trap in case you need to get to it!”

“Be careful not to breathe in the fumes of the spray and ensure the windows are open. It is also a good idea to wear a mask and the fumes are toxic!”

Frequently Asked Questions

Can I use expanding foam under bathtub?

Yes, we recommend using expanding foam underneath a bathtub. Expanding foam will cure and become hard enough to provide support whilst also offering additional insulation to your tub.

Can you use expanding foam under a shower base?

Yes, expanding foam will work very well underneath your shower base. Not only will it provide support, but it will also be another source of insulation.

What do you put under a bathtub for support?

If you are in an apartment block with a concrete floor, you can use a base of mortar to support your bath. Alternately, if the floor is wooden, it is probably best to use expanding foam.

How to fill the void under a bathtub?

To fill the void under a bathtub, you’ll first want to fill the bathtub with water. This helps weigh the tub down and positions it in its natural state when in use. Once the bathtub is full, carefully spray expanding foam into the void or space beneath it. The foam will expand to fill the void, offering support to the tub and helping to prevent any future movement or shifting.


Lee Pearce is not just a master plumber; he’s a veritable Plumbing Wizard. With over 30 years of experience in the trenches of pipes and drains, Lee has become the go-to sage for DIY plumbing, saving homeowners thousands in potential call-out charges. As the founder of Plumbing Wizard, he’s dedicated to demystifying the complexities of home plumbing, offering easy-to-follow advice that stands the test of time and pressure. His online blog is a treasure trove of tips, tricks, and tutorials that empower everyday individuals to take charge of their home’s plumbing health. Lee’s practical wisdom is not just about fixing leaks; it’s about imparting confidence and self-reliance. When he’s not writing or elbow-deep in a plumbing project, Lee is passionate about educating the next generation of DIYers, ensuring that practical skills are passed down and preserved.

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