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.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1Mixing the mortar on a stiffer consistency and forming staggered mounds to support the tub without leveling on its ownDid this help you?
- 2Filling the base of the tub with water while the mortar setsDid this help you?
- 3Hiring a concrete specialist or having an insulation company foam the gapDid this help you?
- 4Using closed cell backer rod as filler before applying siliconeDid this help you?
- 5Using canned spray foamDid this help you?
- 6Shove it back up in to the hole the best you can, can try tape or flashing tape. Or shove some insulation in it the best you can.Did this help you?
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 ↗
“I am working on another plumbing issue under the house and I would like to seal this up while I'm down here but I don't know how to do that from under the house or what to use. I can't access it from above because there is an alcove tub in place.”source ↗
Where this came up
People with this problem also raised
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