How to fill a 4-inch gap under an acrylic tub
Standard tutorials recommend one to two inches of mortar, but a four-inch gap requires an excessive amount of material that is heavy and likely to crack. Because plumbers often refuse to install a mortar base, DIYers are forced to bridge this deep space themselves without proper guidance, leading to structural sagging or failed installations.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1Mixing the mortar on a stiffer consistency and forming staggered mounds to support the tub without leveling on its own
- 2Filling the base of the tub with water while the mortar sets
- 3Hiring a concrete specialist or having an insulation company foam the gap
- 4Using closed cell backer rod as filler before applying silicone
- 5Using canned spray foam
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“I need to redo mine because someone didn’t do this, and it looks like trash now.”source ↗
“Wish I had seen this comment five years ago. I had the same problem and did not solve it correctly.”source ↗
“No plumber in my area was willing to do a mortar base during the installation of our acrylic tub so I’m having to do it myself.”source ↗
“Can anyone please advise on the best way to fill the gap under the tub? Most online tutorials recommend 1-2” of mortar which should touch the belly of the tub, but mine sits ~4” off the ground and would require a ridiculous amount of mortar that would probably crack.”source ↗
Where this came up
People with this problem also raised
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- 6Second-floor bathtub is cracking and flexing with no backup bathroom
- 4Gaps between shower wall and drywall
- 2How to keep a tub spout drop ear from wiggling