Wood SubFloor Flattening

Hdmstng · Jan 10, 2008

My floor has a bit of a slope to it and I'm trying to figure out the best way to flatten/level the floor prior to installing the tub. This is a 60" x 30" tub and the directions state to attach a 2x4 to the wall at 13 5/8" level. Problem is, from the upper left corner the floor drops ~1/4" to the front and back, and ~3/8" to the front right corner. (see pics, I've added arrows) You can also see how the floor bows by looking at the right and left side pictures.

My plan was to lay 1/2 cement board up to the tub for tiling, (the total bathroom is 5" x 10"), but that was before I pulled the tub and saw the sloping and bowing floor. Could I instead use some 1/4" plywood to flatten/level the floor, and then extend the cement board under where the tub is going to be placed. I'd also use some leveling compound to fill in the gaps that the 1/4 plywood would leave.

Any other suggestions?

First picture is the right side, second picture is the left side, and the third is the front view.

AZ Contractor · Jan 10, 2008

Find the highest point on the floor that the tub will be hitting.

Nail a ledger 13 5/8" up from the highest point to the back wall.

Slip your tub in. Check for level.

Now the tub should be hitting the floor at the front at one point.

Measure the height the tub is off the floor at the highest gap.

Measure the length from the highest gap to where the tub is hitting the floor.

Get a 2x4 and mark a triangle creating a piece of wood that will fill that gap.

Cut it and slip it under the tub. Check for level.

Mark the location.

Pull the tub off and nail that down. Put the tub back in.

Sponsor · 2022-01-29T12:20:58-0800

hj · Jan 11, 2008

shim

If he is going to shim the front of the tub, instead of wrestling a cast iron tub in and out, why not just calculate the size of the wedge required using a level and install it before placing the tub?

Statjunk · Jan 11, 2008

I'd take a different approach. I'd find the highest point and set your ledger board at the recommended height. Then pour a 2 bag mortor bed and set the tub in it.

The mortor will level the tub. Some guys will put a garbage bag on top of the motor to seperate the tub from the mortor.

Tom

AZ Contractor · Jan 11, 2008

statjunk said:
I'd take a different approach. I'd find the highest point and set your ledger board at the recommended height. Then pour a 2 bag mortor bed and set the tub in it.

The mortor will level the tub. Some guys will put a garbage bag on top of the motor to seperate the tub from the mortor.

Tom

We do that for Jacuzzis and whirlpools.

That will help a lot but you still should have a ripper (continuous support) under the leg at the front.

AZ Contractor · Jan 11, 2008

hj said:
If he is going to shim the front of the tub, instead of wrestling a cast iron tub in and out, why not just calculate the size of the wedge required using a level and install it before placing the tub?

I was going to say that but it sounds like a DIY'ers question and thought that might be a little advanced.

Statjunk · Jan 11, 2008

AZ Contractor said:
We do that for Jacuzzis and whirlpools.

That will help a lot but you still should have a ripper (continuous support) under the leg at the front.

Not sure what you're referring to with a ripper under the front leg. I've installed lots of tubs the way that I've described and not had a problem. The tubs typically will be supported by the mortor if I'm undstanding you correctly as in a support that reinforces the skirt of the tub.

Most of the tubs that folks buy at HD or Lowes are junk and if you don't mortor them in they sound hollow and flex under weight. So I think it's pretty much the only way to do it.

I even mortored in my cast iron tub.

Tom

Frenchie · Jan 11, 2008

Ledger supports the back edge.

Mortar supports the bottom / middle.

Floor or ripper supports the apron, which supports the front edge.

W/O a level surface to support the apron in the front, how do you ensure the tub squishes the mortar to level, and no further?

hdmstng - Only the front edge matters (the back gets supported by the ledger, not the floor). I can't read the measurement on the front left corner in your picture. How far off-level is the front?

Statjunk · Jan 11, 2008

Frenchie,

Unless I'm miss understanding you I just push on the back of the tub til it hits the ledger board. I then check to make sure the drain is clear. Tack it and done.

Have I been missing a step?

Tom

Jadnashua · Jan 11, 2008

A tub is not designed to hang from the ledger boards...that provides the edge with support, but the tub itself needs to rest ON something. If it can't directly be the floor, if it has feet, those need to be suported. If no feet, then something like a mortar bed; whatever is required to provide a firm foundation for the bottom of the tub.

I'd put up the ledger boards so that the tub can be leveled when inserted, plop a bunch of mortar on the floor so you get good coverage, then set the tub down into it. If you do things right, the tub will be able to be 'smushed' into the mortar to the point where it is level and the edges are on the ledger board. Then, leave it alone until the mortar or whatever you use sets up. At that point, you have a level tub with the edges supported by the ledger boards, the bottom by the mortar, and all is well with the world...a nice solid installation that doesn't rock, flex, squeak, and drains properly (at least it can if the drain is working) because the internal slope will be correct if the ledger is level and the bottom is supported without bowing or warping.

Frenchie · Jan 11, 2008

If there's no support for the front edge, or the front supporting surface is un-even, and you're depending on the ledger to make everything come out level...

- the tub can can sink too far: so that while you're level left-to-right, you're off level back-to-front.

- it can shift diagonally: matching the out-of-level postion of the front edge, by lifting one end of the back off the ledger.

Either of those can happen while it's drying, after you've set it properly & walked away.

Jadnashua · Jan 11, 2008

most of the tubs I've seen (and I don't claim many) have a ledger required on all sides except possibly the room side, so this is not a factor. Essentially, you want to ensure that the thing is supported once all work is done - if you can do it with ledgers and a mudbed (my preferred method) great, if not, then shim things solid first. If you don't catch at least the two ends or preferably at least three sides, then you'll have to do something to keep it from tipping while things set up.

Some have used foam for this...that can be problematic unless you can fill the tub with water, as when it expands, it can easily lift things out of whack. I'd worry about the stuff fracturing eventually and allowing the thing to sink, losing all of the support you originally got. Some of the better urethane foams remain resilient and that shouldn't happen for a very long time.

Frenchie · Jan 11, 2008

I haven't done that many tubs, either, but the ones I've done, only called for a ledger on the back edge. A ledger on 3 sides would take care of things, though.

Wouldn't it get in the way of the overflow?

Statjunk · Jan 11, 2008

I've done about 10-12 tubs in my time and I've always used one ledger at the back with 2 bags or mortor. Never had a problem.

Tom

Frenchie · Jan 11, 2008

New construction, or remodels?

I've had crooked floors interfere a few times... luckily I always dry-fit, first.

AZ Contractor · Jan 11, 2008

How do you put a ledger behind an overflow assembly???

Install the ledger (stringer) at the back.

Install the ripper. Tile and shower up.

Thats all you need to do.

Personally I like the feel of my soft acrylic floor. Adding mortar would make it hard. If I wanted a hard bottom tub, I would have a cast iron tub.

Some manufacturers specify that you are not to use mortar underneath their tubs. Check your installation manual.

Frenchie · Jan 11, 2008

Definitely check your instructions - if it is meant to get a mortar base, and you don't do it, in short order the bottom will crack (and leak).

Jadnashua · Jan 11, 2008

The last one specified just a 1x2 for the ledger and there was plenty of room for the overflow assembly. The ledger is only to support the edge, the bottom holds the weight. Whatever support the tub is specified to have underneath MUST make firm contact with the floor and it MUST be level to work right. How you achieve that is up to you, just don't hang it from the ledgers. A mortar bed is often the best choice unless the tub came with something that provided good support and the floor is made flat to support it.

Hdmstng · Jan 11, 2008

frenchie said:
Ledger supports the back edge.

Mortar supports the bottom / middle.

Floor or ripper supports the apron, which supports the front edge.

W/O a level surface to support the apron in the front, how do you ensure the tub squishes the mortar to level, and no further?

hdmstng - Only the front edge matters (the back gets supported by the ledger, not the floor). I can't read the measurement on the front left corner in your picture. How far off-level is the front?

The front left corner is off by 1/4", front right is off by 3/8, and back right is off by 1/4

Hdmstng · Jan 11, 2008

AZ Contractor said:
Some manufacturers specify that you are not to use mortar underneath their tubs. Check your installation manual.

Exactly. It's the American Standard Princeton from Home Depot. The directions state that no mortar bed is to be used, and it would void the warranty. In addition the directions show only one 2x4 stringer is to be used across the side of the tub.

Wood SubFloor Flattening

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