Fiberglass and acrylic tubs are lightweight, which is one reason they’re so popular. But depending on the tub and installation, you may feel a little flex or movement under your feet.
Personally, I don’t want my brand-new tub feeling squishy every time I step into it.
Ask About a Mortar Bed
One common way to give the bottom of the tub more support is to set it into a mortar bed.
The mortar fills the space underneath the tub and creates a solid base for it to rest on.
Instead of portions of the tub bottom spanning small gaps or voids, you’re giving it more consistent support underneath.
Less flex. Less movement. More of that solid feeling under your tootsies.
What If the Tub Already Has Supports?
Your tub may already have built-in feet, supports, or a foam base underneath it.
That doesn’t automatically mean you should skip the mortar.
Depending on the manufacturer’s installation instructions, a mortar bed may still be allowed or recommended to fill voids and provide additional support.
This is where I’d ask the installer before the tub goes in rather than discovering afterward that it moves more than you expected.
What About Spray Foam?
You may also see installers use low-expansion foam underneath a tub, particularly when there’s a larger gap they’re trying to fill.
I’d be careful here.
You don’t want someone grabbing a can of expanding foam and assuming more is better. The wrong foam or too much expansion could put pressure on the tub instead of properly supporting it.
If foam is being used as part of the installation, I’d want to know it’s appropriate for that specific tub and being installed according to the manufacturer’s requirements.
Check It Before Everything Gets Finished
This is the perfect time to catch movement.
Once the tub is installed, I’d carefully check how it feels underfoot before the surrounding walls, tile, or other finishes make access underneath more difficult.
If it noticeably flexes or shifts, ask about it now.
I’d much rather have the installer address the support while everything is accessible than try to fix a moving tub after the bathroom is finished.
The Manufacturer Gets the Final Say
Most importantly, pull up the installation instructions for your exact tub.
Some manufacturers specify mortar or another bedding material. Others have their own support requirements.
It’s little construction details like this that are easy to miss because they disappear once your house is finished. That’s exactly why I created the Ultimate Home Building Checklist. It helps you know what to look for and what to ask your builder while you can still do something about it.
