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CharlieR 02-23-2013 12:26 PM

Is mortar under bathtub necessary?
Hey all,

Question. For a acrylic/fiber glass bath tub, not sure what exactly it is, how recommended is the mortar underneath for support? It has 4 feet, but I'm more concerned about the rest of the area where there isn't feet. It is a Kohler Archer tub if that is any help. I really want to do it, but a plumber has told me not to bother, should I see if he'll do it anyhow?

THanks in advance for any advice.


Alan 02-23-2013 12:52 PM

While the base may be strong enough to support the weight of the tub, we usually put mortar around where the legs sit, to prevent the tub from shifting horizontally after installation.

If it's your tub, and you want mortar under it, he should do it.

jaydevries 02-23-2013 01:22 PM

unless it is full contact bottom and sits level . i always set in piles or full bed set of mortar or speed set just due to the amount of units i have seen crack due to not enough support or over weight persons

joecaption 02-23-2013 06:34 PM

What's the directions say? Go online and check. If it says morter there's a reason for it.

funfool 02-23-2013 06:43 PM

When the plumber says do not bother with it, that skeers me and want to know more about the plumber. :huh:

For the couple of dollars and few minutes it takes, I would.
And once the tub is sitting in the wet mortar, I fill the tub with water for the weight.
This means dragging a hose through a window somewhere, or fetch it with 5 gallon buckets if needed.

But yeah, if I set that tub, is getting mortar and then filled with water while the mortar cures.

CraftsmanConnec 02-23-2013 08:51 PM

Craftsman Connection - Do it right.
I'm a kitchen and bath remodeling contractor for nearly 15 years. I've been a former building inspector and home inspector. Licensed General contractor in California and Texas, Angie's List Super Service Award winner in 2010, 2011, 2012.

The reason for the mortar bed is two fold:First the feet (aka shipping blocks) are not going to make the tub sit level, so the mortar also may be needed under the feet. Sometimes you won't see feet, there will just be a piece of OSB plywood.
Secondly, the vinyl/fiberglass will dry out and crack over time if it it stressed without support. I've seen hairline cracks in these types of tubs that are 10-20 years old. I don't think I remember seeing a mortar bed in all the tubs I've pulled out. Just because some builder or plumber is being or was lazy, doesn't mean that you should be. The mortar bed isn't easy for a beginner, but start by putting blocks under it to prop it up to get it level so you have an idea of how much mortar you will need. Don't make the mortar too wet or it will sag away from the tub as your setting it.

jaydevries 02-23-2013 09:32 PM

i do not recommend filling tub with water after setting in mortar bed due to the weight of the water will flex the tub pushing mortar down. which when water is drained will flex up causing a slight gap which causes a flex point in tub.
i am sorry to disagree with another post but this is what i have seen.
also if it is a full contact bottom ridgidized foam tub i use about 2 tubes of silicone underneath to prevent squeaking

funfool 02-23-2013 11:35 PM

Is why we have forums and discussions.
How much do we think that new tub deflects when adding water?
I think a brand new tub is not much, adding mortar under it and filling it with water, is only going to apply pressure in the same way applying pressure to wood glue allows it to have a stronger bond.

water is not required, I do not see a reason to quit adding it any time soon.

Alan 02-24-2013 12:34 AM

The other reason for not adding water when setting a tub :

Typically we build our decks taller than the tub itself. This means using something to take up the space. It's never going to be just perfect.

Get it within an inch, and then float the tub in a bed of mortar.

Use shims or chunks of 1/8 plywood between the tub and the deck. This helps keep a uniform gap for caulking.

Now the reasoning : If you fill the tub with water, it will pull all the tub weight down from the rim of the tub. NOT the strongest part of the tub and it can crack.

If you let the mortar set up, then you can pull the shims, and the tub will always maintain the 1/8" gap and then you caulk the tub to the deck.

For alcove installations it's still a good idea, but usually alcove tubs have better support under them. If it was my own, i'd do it.

747 02-24-2013 01:26 AM

yes for stability

CharlieR 02-24-2013 12:23 PM

Thanks everyone. I had planned to do most of this myself, but I am way behind schedule! Finding out DIY with little kids is not easy!
Anyhow, appreciate all the feedback. I guess I can ask if he'll do it if I want and if not, get someone else, or someone to just do that part with me later or something.
THe instructions mention an either or, mortar, or gluing the feet.... seems a bit difference to me, but what do I know! :wink:

It is an alcove installation. Does the rim still need support? I need to double check the instructions, but for some reason, I don't recall it mentioning that. I know the tub I took out had it. Seems like a good idea regardless.

Do any of you put any plastic sheeting between the mortar and the tub? I've read that is not a bad idea in case you need to remove it later, that it's not stuck to the mortar, just supported by it....

CharlieR 03-15-2013 08:28 AM

So follow up question. Can you put in Mortar support, after the tub is already installed? Kind of push it under for some added support? Once the tub is level already?

Alan 03-15-2013 10:12 AM


Originally Posted by CharlieR (Post 1137678)
So follow up question. Can you put in Mortar support, after the tub is already installed? Kind of push it under for some added support? Once the tub is level already?

Kinda depends on what the bottom of the tub looks like. Might be easy in some cases, but more difficult in others. I'm assuming you're asking because he refused to set it in mortar? :whistling2:

CharlieR 03-15-2013 10:24 AM

Wouldn't say refused... lets just say strongly didn't think it was necessary and I'm unfortunately easily swayed / like to avoid confrontation and am behind schedule so the wife just wants things done :jester:

I can take a picture. Essentially there are 4 feet... integral skirt. However, I have access both from the foot of the tub, and slightly lesser access from the head of the tub... so I could scoop some in there and push it in... at least add some support in places other than the feet... provided the consistency of the mortar is such that this is a possible task?

retired guy 60 03-16-2013 03:53 PM

I agree with joecaption. Go online and see what the installation instructions call for with regard to your particular model tub. I came up with this from the Ask Me Help Desk:
"If the manufacture does not provide support ... then the installer must bed the tub himself.
If the tub isn't bedded with mortar, cement or Structolite then it will 'flex and give' a bit every time someone steps into it. In time this can rupture the drain seal and develop a leak. It can also put a strain of the fittings below the drain. And now you know the rest of the story."

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