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Replace floor under tub?

jmac's picture

Hey all. I am renovating our upsatirs bathroom.

In the process of prepping the floor for tile, I removed the old vinyl to find 3/4" TG pine floors nailed directly to the joists (no subfloor). Some of the boards have some water damage so I am going to pull them up and put down 3/4 CDX. (plus 1/2 CBU for the tile.)

Under the tub there is also TG pine, but it is in great shape. Since I am not tiling under the tub, the question is can I leave the TG as is under there, or should I replace all of the floor with CDX?

If answer is replace everywhere [which is my instinct, but I am often wrong:)] what is the minimum acceptable size of the 3/4 CDX for subfloor? (i.e. can I have some smaller sections like 2' x 4'etc.) Thanks.

(post #95947, reply #1 of 14)

Or...maybe you could leave the T&G (This was standard subfloor in the olden days.), screw it securly to the joists and put your plywood over this. Personally, I'd use AC plywood and tile directly over that, but there are differing opinions about that around here. If you're moving the tub anyway, I suggest putting plywood under it, too. Not only will it be stronger, but it won't look like the tub is sinking into the floor.

Al Mollitor, Sharon MA

(post #95947, reply #3 of 14)

Thanks for the reply. Your suggestion gave me this thought...

A lot of the exposed floor not under the tub needs to go, but I could probably save a good 4 x 4 area by the door though. Since that area is much less likely to get water on it, would 1/2 AC overlay be sufficient for that section? (The wife does not want too much of a step up) or would 3/4 be what you suggest? [Also what would be your method for securing the AC-- e.g what size nails, etc.]

If so I could also lay down 1/2 AC over the floor under the tub too which would make it flush (pre-tile). And save the effort of removing all of those boards.

Also would there be any problems with part AC, part CBU for the undelayment?

(post #95947, reply #5 of 14)

I usually hope for as few joints as possible and try to place them where there is a minimum of traffic. No joints under the toilet. In a typical bathroom that I get to work on, there may be only one joint, since most baths are smaller than 8'x8' and pywood is 4'x8'.

I think the tile folks like at least 1" of plywood for rigidity. Cement board doesn't count since it's not very stiff. It's used as something for the tile to stick to, but it's not stiff enough for floors. If floor thickness is limited, skip the cement board. If you're worried about the tile sticking to the plywood, there are membranes (Ditra?) for that.

This is the time to replace any old plumbing under the floor.

I'd consider cutting out badly rotten pine and replacing it with 3/4 CDX glued and screwed to the joists. Screw all remaining pine with 2-1/2" screws. Glue and screw 3/4 AC over everything. Place the tub, set in Structolite or similar bed. Tile the floor with thinset. Set sink and toilet.

Al Mollitor, Sharon MA

(post #95947, reply #6 of 14)

Thanks. That seems like a better plan than I had. Wish me luck.

(post #95947, reply #8 of 14)

> (The wife does not want too much of a step up)

This is a whole lot more work, but if you want to get the bath floor as close as possible to the level of the adjacent hallway, you can nail cleats to the sides of the joists, and drop your subfloor between them.  You find this a lot on old mortar bed jobs.


-- J.S.




-- J.S.


(post #95947, reply #9 of 14)

I'm not going to let her know that this is possible.  Thanks.

(post #95947, reply #10 of 14)

I'm not going to let her know that this is possible  Does the DW have email at work?  What is it, so we can invite her to join the discussion ...   Anything for a friend.

Whenever you are asked if you can do a job, tell'em "Certainly, I can!"  Then get busy and find out how to do it.  T. Roosevelt

I'm sorry, I thought you wanted it done the right way.

(post #95947, reply #11 of 14)

The best way to do it is to remove the tub, toilet and everything else and then cut out the entire floor. Expose the joists, check them for rot. I'd be surprised if there is none at all, and expect to see it mostly around the toilet and the permiter of the tub.

Scrape, poison, sister and/or consolidate joists as necessary.

Install doubled solid structural blocking around the toilet chase and also double up joists under the tub feet. Chances are fair this was not done when the house was built. Consider yourself the owner of a well-built house if it was....

Lay on ¾" ply screwed and PL Premium glued to the joists. Make no unsupported joints between sheets of ply anywhere. Every joint must land on solid 2x blocking.

This brings you back up to the previous floor level. If you want to stay as close to that level as possible, you'll need a special membrane from Kerdi which is designed to compensate for insufficient floor stiffness. Check with your tile supplier for that.

If you don't mind coming up a bit more, plus your tile thickness, glue and screw another layer of ½" (for tile up to 6"x6")  or ¾" (for big tiles) ply on top of the first layer. Run the ply 90 degrees to the first set of sheets. Use lots of glue and screw on 6" centers with 2½" flooring nails--the kind with a ¾" naked shank under the head to avoid bridging.

Some people here will tell you you need cementitious backer board on top of that because this is a wet area by definition. I won't deny it makes a better job, but I have done installations in bathrooms without it a few times for various reasons, and there have been no problems on those installations. (I know I'm gonna get flamed for that, but there were 'circumstances'....) In your case, it will mean another quarter- or half-inch of floor height, depending on what type of cement board you use. (I no longer use ¼" cement board after two bad experiences with it.)

Why don't you want to tile underneath the tub itself? Is this a footed cast-iron tub, or (yuck!) an enameled-steel skirted type?





'Y-a-tu de la justice dans ce maudit monde?


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #95947, reply #12 of 14)

Thanks.  I ended up pulling up all of the boards (more rot than I first noticed) and put down CDX like you suggest.  All joists were surprising in good shape.  One had minor surface damage but otherwise was fine.  Some of the old toilet blocking was rotted and I replaced it.  Adding blocking elsewhere [2x4 had to be used for a couple of the blocks. Is that OK?-- too late now if not:(]  Haven't had time for the next steps yet-- real job is getting in the way.

Tiles are less than 6x6 so 1/2 would work... thats good news.  Curious as to why you put the second layer of ply on 90?

Tub is the second type-- wasn't planning on tiling under it.  What is the benefit of doing it?


(post #95947, reply #13 of 14)

For the 2x4 blocking: as long as the blocking transfers the load down to a structural member in the floor frame, it's okay. If it's just acting as a nailer, it won't help hold up your tub or toilet. But from the way you worded your question, I get the feeling you put in 2x4 because you had no room for anything else; so that is probably okay. Moot, though, since you closed the floor.

Put the second layer of ply on at 90 degrees to the first so the grain directions in the face plies of the two sets of sheets are not parallel to each other. Plywood gets a lot of its strength and stiffness from the varying angles at which the grain of each succeeding ply is set.

Doing this will also help insure (to some extent; you may have to trim the 4x8s in the second layer by a foot anyway) that you don't have any 'through' joints--in other words, avoid having a joint between two sheets in the top layer lie right on top of a joint between two plies in the lower layer. Make a sketch layout of each layer of plywood so you won't have to figure it out while you're standing there with glue all over the floor, LOL.

As to tiling under the tub, I would do it for three reasons:

(1) It's a small area, so the material used under there won't cost that much;

(2) tile will provide better protection from water than just plain plywood, even if you lay felt on it--and there is always some water spilled on a bathroom floor near a tub; with the type of tub you propose to use, small amounts of water can easily weep underneath the skirt and then soak into the naked subfloor, causing rot and structural damage over the years (you've already seen that); and

(3) if, some years down the road, you (or some future owner) buy a real bathtub (just my personal design prejudices showing here, LOL), one with feet, you won't have to do anything but put a trim flange around the waste pipe where it goes through the floor and you'll be able to install the new tub. Otherwise, you'll be out there running around looking for tile to match exactly what you're installing now--and the tile industry is so constituted as to make it virtually certain you won't find it, unless you choose now from a very short list of 'permanent' classics.... Tile is worse than clothing fashion. What's 'In' this season is 'Out' in six months....


'Y-a-tu de la justice dans ce maudit monde?


How now, Mighty Sauron, that thou art not brought
low by this? For thine evil pales before that which
foolish men call Justice....

(post #95947, reply #14 of 14)

Thanks.  Makes sense.  I appreciate the help. 

As long as the boss doesn't ask me to work this weekend,  I might actually make some progress on the bathroom...


(post #95947, reply #2 of 14)

Kinda depends on how much work you had planned ... or have already ... done.  Is the tub still in place?  Do you intend to remove & reinstall it?  If yes, then leave the pine boards and add a layer of 3/4 ply.

If you want to leave the tub, then try to get as few pieces of ply as possible.  Pieces nailed to three joists would be a minimum.


Whenever you are asked if you can do a job, tell'em "Certainly, I can!"  Then get busy and find out how to do it.  T. Roosevelt

I'm sorry, I thought you wanted it done the right way.

(post #95947, reply #4 of 14)

Thanks. If three joists is the minium length of the board, is there also a minimum width? (eg. would ply across 5 joists be OK even if it were only 1 1/2 wide? Just curious.

(post #95947, reply #7 of 14)

I just finished the exact same job two weeks ago.  The home was a former church, about 100 years old.  The floor was a mess near the tub, with some holes rotted through the pine plank flooring, which was underneath a layer of linoleum.  The previous owners had leaked water out behind the shower curtain for some time.  I pulled all the baseboard, cut out the section of floor from the tub to one foot out, and removed the drywall that had been damaged at the end of the tub.  I scraped the floor joists clean - true 2" lumber - and found them to be in good shape.  The wood rot had acted mainly on the flooring.  I inspected under the tub and found no damage.  The I installed a 1/2" patch in the floor and custom cut a cedar shim for the edge of the tub.  I then replaced the drywall with backer board.  Then I put a new 3/4" subfloor over the entire old floor in the bathroom, replacing the toilet in the process.  I don't reinstall old toilets.  I put in a brand new Kohler for what a plumber would have charged to pull the old one out, repair the mounting ring, and install extenders and a new gasket.I sealed the area near the tub with a generous amount of Silicon II before installing a new vinyl floor and baseboard.  Then I installed a glass shower door to prevent recurrence.  New baseboard and a few other minor details completed the job.

Les Barrett Quality Construction