Mortar bed for acrylic tub

[Mrosenb]

I know this has been discussed here before but I’m still having a hard time. I’m installing the American Standard Studio alcove tub with an apron. What’s throwing me off are the 2 rails running down the length of the tub. Do I want to put the mortar in piles in between these rails to support the osb underbelly or do I lay a thin layer for supporting the 2 rails? If I put mortar down the center, the rails won’t be touching the floor and I have to be careful not to use too much mortar because it will just get trapped between the rails. My understanding of the manufacturer’s instructions are to make sure the rails are supported but it’s not really clear. They provide video install instructions but they don’t even use a mortar bed.

 

[Terry]

If the floor is perfectly flat, then you can slide the tub in and it will be good.

Sometimes in older homes, flooring settles and gets a little out of level. In those cases shimming of some sort needs to happen otherwise it shows up when tiling.

If it does need shimming, depending on the tub, it makes a difference in what needs to happen. With a tub like the one above, if the rails were flat to the floor, nothing needs to happen. Once that changes though, and it becomes a shimmed tub, then how does that happen. If you had access to place shims in there, that could work. With no access, then piles of mortar that squish out and conform can be a good way.

You can't really lay out a flat bed and expect it to be perfect to the bottom of the tub. Adding just a bit to change level is not normally successful. That's why piles of something spaced apart, allowed to move and shift outward to allow the tub to drop down to preplaced shimming is how many of us are doing it.

 

[Sponsor]

 

[Mrosenb]

My floor isn’t 100% level so I’m going to use mortar. This is actually a reinstall for me. I installed the first time using piles between the rails. It felt solid but I noticed a minute crack on the top of the tub. I put a scope under the tub and could see some gaps where I thought the tub was being supported. I was able to slide some shims under the rail but then tub started creaking a little when I stood in it. I figured it best I redo the install since the walls aren’t up yet and tub is not difficult to remove. Do I want to put the mortar under the rails or under the center where it would be supporting the osb?

 

[Terry]

Do I want to put the mortar under the rails or under the center where it would be supporting the osb?

I would place outside the rails, mainly because you're just tweaking the angle just a hair.

 

[Mrosenb]

I would place outside the rails, mainly because you're just tweaking the angle just a hair.

Ok I’ll give that a try. Thanks!

 

[Mrosenb]

Ok so I have the tub out and see the subfloor is pretty close to level. I’m going lay down 2 sheets of tar paper as a shim which will make it perfect. My problem now is with the stringers. If I get the tub to rest perfectly on the floor, the manufacturer’s instructions for the height if stringers are too high and will raise tub off the floor slightly. I would have a hard time getting the stringers at just the right height so the tub rests on the stringers while also being supported on the floor.

And now that I’m confident I can get the tub supported on a level floor, I can use mortar down the center between the rails for added support of the osb bottom.

I also just wrapped the tub and bottom walls of the alcove with some R13 insulation.

 

[Mrosenb]

I spoke with a representative from American Standard this morning. He said the tub does require mortar. Don’t worry about the rails. Support the center of the tub instead. No need for separate piles of mortar. Just a bed of mortar for the center for the length of the tub. He said he’s been trying to get the company to revise their instructional video because it doesn’t show the tub being supported with mortar.

 

[Terry]

Wow! The rep said to place a solid bed under there? I guess he's never done a tub install. It's impossible to perfectly match up the bottom of the tub to the mortar that is placed on the floor. We have learned over decades of time, that the small piles that squish out and conform to the bottom of the tub is what works. We have tried it both ways. One way doesn't work, and the other way works "every" time.

Small piles of mortar that squish out and conform.

And by the way, Terry Love the plumbing wizard, was the first one on the Internet to suggest this method. I had plenty of people that didn't understand it at first, but yes, now it's the industry standard that the plumbers follow. They all do it, I started it. That's how frigging old I am. I figure things out, post what I know on the Internet, and then it becomes the standard.
Just saying.

 

[Mrosenb]

Wow! The rep said to place a solid bed under there? I guess he's never done a tub install. It's impossible to perfectly match up the bottom of the tub to the mortar that is placed on the floor. We have learned over decades of time, that the small piles that squish out and conform to the bottom of the tub is what works. We have tried it both ways. One way doesn't work, and the other way works "every" time.

Small piles of mortar that squish out and conform.

And by the way, Terry Love the plumbing wizard, was the first one on the Internet to suggest this method. I had plenty of people that didn't understand it at first, but yes, now it's the industry standard that the plumbers follow. They all do it, I started it. That's how frigging old I am. I figure things out, post what I know on the Internet, and then it becomes the standard.
Just saying.

I will go with the piles and report back.

 

[Terry]

The tile trowel is sort of the same thing. Not solid grout, but with ridges that allow things to conform.

 

[Jeff H Young]

I usually look at instructions and then adapt my own ideas . I like piles supporting bottom of tub but always support under the feet. if the center carries some weight ok , but most work is on feet. EG your tub I may not use a ledger board a cast iron I dont use ledger a press steel gets ledger

 

[Weekend Handyman]

Just out of curiosity. How many piles do people usually use? I kind of envisioned one up front, one in the middle, and 1 at the rear. I would see them being squished down to like 1 foot diameter circles?

 

[Terry]

Just out of curiosity. How many piles do people usually use? I kind of envisioned one up front, one in the middle, and 1 at the rear. I would see them being squished down to like 1 foot diameter circles?

It's not just a few, I do many piles. The idea is to leave enough space between that when the tub is being let down, it pushes them down and moves them into position supporting a good portion of the tub. You do need some space though so that spreading can happen.

 

[Jeff H Young]

Now that I look at it it looks like a builder 30x60 or possibly 32 inch should have the flange along the ends and side wall I put a screw through to each stud plus its solid on the floor really shouldn't even need to put mud. but wanting quality it can help Don't know how many piles are going to fit between those legs / stringers . but totally get the multi pile strategy you can mix thick and it wont slump but will squish