New install of acrylic Kohler 1121 seems unstable on its mortar

[Jim Bassuk]

I am building a new undermount tub and have followed the relevant threads on this forum, which correctly point out that there are no specific guidelines about how to go about this. So I built a nice 2x6 frame such that the tub's lip will hover about 1 inch over the 2x6 frame. My thinking here was that once the tub was installed I would slide in various strips of plywood (1/8, 1/4, 1/2) to further support the tub by the lip -- and continue to build it up so that the final picture frame deck (made from either solid surface or quartz) would be super strong.

I brought in a licensed plumber who fabricated the drain connections and embedded the new tub with mortar, which was poured on a sheet of 10 mil plastic. It was all level and we filled it up, squishing the mortar between the bottom of the tub and a sheet of 10 mil plastic. After leaving the filled tub alone for a couple of days to allow it to set, I was surprised upon draining as to how unstable it was. I can make it rock back and forth a tad and the tub+mortar+plastic was easily moved about a 1/4 inch so as to better align my center marks.

So what gives? Is this common? Should I mix up some more mortar into small bags and slip them in so it doesn't rock? The 4 tiny feet rest on my level subfloor OK.

Do I ask for a redo or should I continue on with my plan to beef up the underside of the lip with strips of fir or a good grade of plywood. Do I screw the 2.75 inch wide lips into my 2x6? I haven't seen any mention of screws in forums or in the Kohler instructions. Using screws seems reasonable; if this was an alcove mount you would screw the vertical lip into the studs.

Many thanks in advance!

 

[Terry]

A flat bed of mortar? I don't normally get so lucky that I can lay it out flat so that it works. I have gotten by with ridges or piles where it can have some room to move sideways and fit to the form.
If the tub sat on the four legs out of the box and was level, then sometimes I just go with that. I have not installed using a solid surface above the tub yet. Mine have been a tub dropped into the decking from above.

 

[Sponsor]

 

[Breplum]

You can try injecting mortar but unless you have access from all sides, you may need to pull it up and start over.
We normally do exactly as TL describes. Or in a case like yours with legs, I would use a non-shrink grout or good dryish mortar mix and shove it in from the sides while it sits on the legs.
Tubs with legs often are not required to have mortar.
In 45 years we have never filled a tub during installation with water because mortar shrinks and you may have slightly deformed the underside from the water fill. Just get inside to settle it, but with legs it should be on the ground in the first place, if it has adjustable feet, then it is designed to set on those feet directly on ground.
Tubs set in an alcove should have a vertical flange which is water proof. Those do get predrilled and screwed to studs with blocking between the studs to pick up drywall or mortarboard edges.
If there is no vertical flange with an alcove installation, then we have an aluminum flange fab'd to create a caulked and screwed water barrier flange.
Never rely on but joint wall material to be the waterproofing.

 

[Jim Bassuk]

Update: My issue for this undermount tub install was solved by mixing up another bag of mortar and stuffing it under my tub, making sure that the four feet remained rested on the level floor. It would have been great if my plumber had used 2 bags instead of one. The use of one bag wasn't enough to support the basin.

Since I intentionally built my deck 1" short of the tub's lip, all I had to do was to slip some strips of 1" plywood (cabinet grade) under the tub's lip. I used my table saw's dado bit to carve out a 1/8" deep channel in the 1" plywood. Then all I had to due was to generously apply GE advanced silicone sealant into this channel to glue the tub's lip firmly to my deck.

I am happy to report that my undermount tub + deck was built like a tank. I can sit or stand on my deck and nothing moves! So my project rolls on...