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Old 05-16-2012, 06:52 PM   #1
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Plywood Platform Under Bathtub?

I'm installing my first acrylic bathtub (in an upstairs bathroom) and want to do it right.

I'm cool with everything except the required mortar bed.

There is a 2 1/4" space between the subfloor and the bottom of my tub. Mortar is heavy. I'm worried about the weight of all this mortar, a tub full of water, and my fat butt.

Would there be anything wrong with screwing a piece of 3/4" plywood under the tub and placing the mortar on that? Heck, I'm even thinking of sandwiching two sheets of plywood under it. Plywood is a lot lighter than mortar. What do you think?
Thanks for the advice!


Last edited by oh'mike; 05-16-2012 at 07:13 PM. Reason: Added spaces
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Old 05-16-2012, 07:14 PM   #2
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Your plywood riser under the mud bed is a fine idea---go for it--Mike--


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Old 05-16-2012, 08:15 PM   #3
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Thanks Oh'Mike!!
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Old 05-27-2012, 05:07 PM   #4
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ive got a question about this , how can you tell if there is enough morter under the tub , mine has a lip on the side of the tub that sits on the floor and you cant see under it once the tub is inplace .

i was debating cutting strips of wood on the floor then putting morter in between the strips then lay the tub on that . of course the strips of wood would need to be cut individually to compensate for the tub draining .

Last edited by oldrivers; 05-27-2012 at 05:19 PM.
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Old 06-03-2012, 10:09 PM   #5
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Here's how I finally did mine, after putting way too much research into it and taking every good tip I could find:
I took the measurement of the entire inside bottom of the tub (careful to leave room away from the drain, and cut plywood squares of the same size. Like you, I didn't want too little mortar, so I settled on about 1" thickness. In order to make sure my platform was high enough, I made 4 small batches of "bread dough" (really just flour and water), rolled them into balls, placed them in sealed plastic bags, placed the bags on top of the platform, and lowered the tub on the dough. I then lifted the tub, and could easily see if I had my 1" thickness.

It took a 3/4" piece, and a 3/8" piece to get the thickness I needed. I pre-drilled holes in both plywood sheets, put wood glue between them, and screwed them in place under the tub. I then covered the platform with roofing felt, and nailed metal lath on top of the felt. I was told that this would help keep the mortar from shifting. Makes sense to me. Then you lay the mortar, cover it with plastic (so the tub doesn't stick to it), and finally drop the tub down.

The mortar part is where I screwed up. The tub instructions say to lay a mortar bed under the ENTIRE bottom of the tub. On my first try, I simply put the mortar in one continuous layer on top of the lath. When I laid my tub down, I couldn't get it to sink to the floor. So I had to lift the tub back up, and taking a cue from tile setters, made three long strips of mortar down the length of the platform with about a 1" gap between them. Then, when I lowered the tub, the squished mortar had a place to go.

I let the mortar cure for 5 days before lifting the tub to inspect the damage. Everything was perfect. The mortar squished to fill the voids I made, and the tub is rock solid.

This is way more information than you wanted, but I'm so pleased that I had to tell someone!!!
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bathtub , mortar bed

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