Confused about mortar under tubs...

The reason I was thinking the weight of the empty tub would rest on the ledgers is because I'm assuming the tub will sink in the mortar before it cures to the point that the ledgers take on that weight and stop it from sinking. Does the mortar not have that much give to it or should I be propping up the tub until it cures to stop it from sinking too much?

Thanks for all your help Jim!

If you used concrete, it might sink...if you use deck mud, probably not. If you settle the thing down until it just is touching the ledgers all around, you shouldn't need to prop it up while it cures. In reality, think of the tub as a boat. It will 'float' on the mortar...you want to ensure good contact and coverage, so you put enough in so you can mush it down to the proper level. The prepackaged deck mud or sand mix is a 3:1 sand/cement mix (usually). If you were going to be building a shower, you'd probably use closer to a 4-5:1 mix. The higher amount of cement in the package means it will smush more than that used in a shower. Deck mud is much closer to wet beach sand than concrete...you don't pour it, you place it (and it stays!). There's no worry about deck mud running; just mix it well, and not too wet.

Deck mount tubs and tubs not equip'd with showers still need waterproofing measures. This is often skipped in renovation projects and can lead to costly repairs.

Installing waterproofing to the tub deck before installing the tub is ideal but often it is not practical in new construction. Installing a sheet membrane like Kerdi, Dural, Protegga or Noble can be a challenge and you will find that a liquid membrane and a small brush will work well.

Often these tubs are set on little stand offs or scrap pieces of plywood. I like to leave them in as long as possible and then catch the waterproofing up once most of the tile is installed and the blocks are removed.

Since the water has no place to go once spilled on a tub deck an epoxy or urethane grout makes perfect sense. If that is not practical then building a small lip into the tub deck can help.

Sheet membranes tend to wick water a few inches. All of them. So again if you are using this tub for your kids you might consider a liquid as your best approach.

Red Guard,
Aqua Defence
HPG
Hydro Ban

jadnashua said:

If you used concrete, it might sink...if you use deck mud, probably not. If you settle the thing down until it just is touching the ledgers all around, you shouldn't need to prop it up while it cures. In reality, think of the tub as a boat. It will 'float' on the mortar...you want to ensure good contact and coverage, so you put enough in so you can mush it down to the proper level. The prepackaged deck mud or sand mix is a 3:1 sand/cement mix (usually). If you were going to be building a shower, you'd probably use closer to a 4-5:1 mix. The higher amount of cement in the package means it will smush more than that used in a shower. Deck mud is much closer to wet beach sand than concrete...you don't pour it, you place it (and it stays!). There's no worry about deck mud running; just mix it well, and not too wet.

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This is the correct way. Wet mortar is futile because of high shrinkage. Must be damp beach sand to stop over shrinkage. The guys that get a bag of plaster really have it right because it EXPANDS when setting.

Well, my new house has half filled concrete bags under the corners of the garden tub. Is this ok?

I set my Sterling Ensemble tub without mortar. It has been set for a few weeks and now it has a small crack on the top edge close to the drain end. Is this caused because of no mortar? These tubs have good structure and feet under them. What can I do now to remedy this? Without removing the tub. All tile is set.

ballvalve said:

This is the correct way. Wet mortar is futile because of high shrinkage. Must be damp beach sand to stop over shrinkage. The guys that get a bag of plaster really have it right because it EXPANDS when setting.

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I have a HydroSystems Sydney bathtub and it clearly states in the instructions that only mortar is to be used.

Click on the pic for the full manual.

I know this is an old post, but I ended up installing one of these platform tubs from Maax. The instructions were terrible, hinting at either mortar or wood to support, but not specific on how to do it. I searched for hours on the right way, and believe I came up with a good method for anyone researching in the future. What I did for the drop in tub was build wooden mortar pockets I'll call them, tacked to the floor with a 3/4" gap between the platform for mortar to ooze. The base of the platform was 3.5" off the ground, so I wasn't comfortable just piling 4 inches of mortar. My picture is apparently too large to upload, so sorry about that. I framed 5 large areas under the tub platform and around legs with a gap for the drain pipe coming out one end. Filled all pockets with 2 bags of mortar (since the gap is large, more was required). It is as solid as a rock. If anyone runs into this again and wants more details, let me know!

PegCityGary said:

I know this is an old post, but I ended up installing one of these platform tubs from Maax. The instructions were terrible, hinting at either mortar or wood to support, but not specific on how to do it. I searched for hours on the right way, and believe I came up with a good method for anyone researching in the future. What I did for the drop in tub was build wooden mortar pockets I'll call them, tacked to the floor with a 3/4" gap between the platform for mortar to ooze. The base of the platform was 3.5" off the ground, so I wasn't comfortable just piling 4 inches of mortar. My picture is apparently too large to upload, so sorry about that. I framed 5 large areas under the tub platform and around legs with a gap for the drain pipe coming out one end. Filled all pockets with 2 bags of mortar (since the gap is large, more was required). It is as solid as a rock. If anyone runs into this again and wants more details, let me know!

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Wow funny you did this as I also live in Winnipeg and also just installed a Maax tub last night. Mine has an integrated particle board platform under it, with two 4x4" 'feet' in the back, and the tub flange in the front. I mixed up mortar and made a bed between two sheets of poly, but mixed it too dry. I was able to check it the day after via an inspection hole in my basement ceiling, and it was dry and crumbly. Pulled it out, mixed up fresh mortar properly, dumped about 3/4 of a 55lb bag out roughly 2.5" deep and set my tub in.

My tub is a Maax Bosca, so the tub is at most 2" away from the subfloor. I've got a nice solid bed covering the entire floor of the tub. Definitely the least fun part of the reno.

I see this is an old thread but I am planning for a whirlpool tub, so far I have made a 2x4 frame with the 2x4s running across the floor joists ( opposite direction to spread the weight ) and then I put small pieces on top of the joists, with a 5/8 CDX top, not nailed down yet , so I am planning to use some mortar but i wonder about it, since it will be on top of the 5/8 cdx subfloor, which isn't that strong by itself, and also, if the piece that comes with the tub is not for support, as some say, then what is the point to begin with if there is basically nothing under the tub other than that piece of chip board?

I was thinking of adding some more joists underneath for better support and then putting 2x4 on top of those, so roughly wood support at 8" on center, otherwise the mortar is only held up by the subfloor

You can also use a good expanding foam under tubs

Some tubs come with integrated support, but that only works well IF the floor is level. You really want the tub's upper surfaces to be level so that the designed interior slopes properly, and you don't get pooling of water on the horizontal surfaces, or it being directed out into the room. If it's not level, it also makes adding a sliding glass door messy or tiling the walls.

So, the mortar underneath can solve that problem for you by first leveling the tub and second, providing support. Note, if the tub is CI, it's strong enough to support itself, but it still needs to be leveled, if the floor isn't right. Most of the acrylic, steel, and fiberglass tubs out there will deflect when you're standing in them unless you've provided good support. That flexure can cause stress cracking, and depending on the drain, eventually cause that to leak.

Some expanding foam gets brittle when it cures...some is more resilient. The stuff that ends up brittle can end up dust over time from movement that might be from weight or thermal expansion.

Like almost impossible to fully have concrete fill all of the void. Most of the time, it's either too low to matter, or too high to allow the tub to drop all the way down.

This tub is seven feet long, with legs. How high did you install your deck? Was the deck level to the floor? Is the floor leaning? Was the fiberglass tub perfect out of the mold from the factory?

I've been installing tubs since the 70's and jobsites and tubs are not perfect. Hopefully the supplied tub is not garbage but has some structure. This tub above has four points of contact. Adding mortar makes it better, not worse.

I have used concrete in a contained plastic bags before. Usually on cement floors but wood floor is fine as well. Does not shrink and is really firm after setting-the bag keeps it from sticking to either surface. This is for plastic (fiberglass) one piece showers and tubs.I learned it from another plumber. The wood is best but its pain to get right.

Terry said:

How high did you install your deck? Was the deck level to the floor? Is the floor leaning? Was the fiberglass tub perfect out of the mold from the factory?

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I don't remember all of the details. The deck was built about 1 inch higher than the depth of the tub. Maybe it cheap construction?

Old thread here I know. Read this thread and many others on setting tubs, still have a couple questions I'm hoping someone can help with.

Will be installing new 60x32 maxx acrylic tub in alcove on 3/4" plywood subfloor. Floor slopes slightly (~1/4") from center of alcove down/out towards main part of bathroom floor. I just dry fit the tub and surprisingly apron sits flush against subfloor for all except last ~8" at right where there is at most a 3/16" gap. Tub rim is level both ways.

Standing in tub I feel just a very little bit of flex which is more than I'd like, having lived with a fiberglass tub for years that flexed like crazy. Plus looking through an access panel behind the alcove and up from basement through hole in subfloor for drain shoe...I can see that the factory base of the tub (osb looking panel with grooves in it and 4 feet at corners) fully contacts subfloor towards apron side but is well off the floor on the opposite side (long wall of tub).

I plan to staple plastic sheeting to subfloor, and lay deck mud or plaster in piles or rows ~1-2" high then cover with plastic.

Questions:
1) install manual calls for a ledger under tub rim, doesnt speak to mud/mortar etc. If going w/ mud or plaster should I even bother with installing a ledger? At manufacturer spec'd install height?

2) my understanding is I shouldn't stand in tub after setting it to get it to squish down, just get it level and then screw flanges to studs, correct?

3) Which is easier to mix & work with...deck mud or plaster? I'm leaning towards the mud but am open.

Thanks

seldomright said:

2) my understanding is I shouldn't stand in tub after setting it to get it to squish down, just get it level and then screw flanges to studs, correct?

3) Which is easier to mix & work with...deck mud or plaster? I'm leaning towards the mud but am open.

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2. I think I have read adding water into the tub for weight is good, but I don't find that at at the moment.

3. You want your piles to squish. Deck mud may be more resistant to squishing. I have not done a tub install, but it sounds to me as if you would not want a dry deck mud.

An alternative to sheets of plastic is to put your mortar blobs into non-airtight loose sandwich bags.

Use deck mud not plaster-plaster becomes brittle over time
I would just put it heavy duty garbage bags. But you can just make lines as you said for smaller paces
Do not add water weight or stand in it as you want contact with fiberglass in a normal condition not just with weight.

Thanks Onokai, got it