subfloor/underlayment bathroom floor replacement - Ceramic Tile Advice Forums

ericinmichigan · 05-26-2004, 09:33 PM

Hello everyone,

OK, here is my latest DIY project on my house. I'm doing a complete remodel on one of the bathrooms. I ripped everything out down to the studs. There was some water damage/(dry rot?) on the subfloor under the tub encompassing an area roughly 12" x 14" square. The house is a 50 year old ranch house with a basement, bathroom is upstairs, floor joists are 2" x 8" on 16" centers, 3/4" thick lumber subfloor laid at a diagonal to the floor joists. The old bathtub rested directly on the subfloor. The rest of the room, (roughly 6.5' x 4') has a 3/4" plywood underlayment laid over the subfloor. 1"in square ceramic tile was laid directly on the plywood underlayment. The tile was in great shape after 50 years, no cracks, etc which leads me to believe the floor was sound.

Here is what I am thinking of doing, but I want some input first before I do it. Obviously I have to cut out the rotted area of the subfloor, I was thinking of replacing the original 3/4 " lumber subfloor in this area with 3/4" plywood. As for the original 3/4" plywood underlayment where the old tile use to be here is what I was thinking of doing. Currently the plywood underlayment is level with the recently refinished red oak hardwood floor in the hallway directly outside of the bathroom door, the old tile surface was approx 1/8" above the floor in the hall. I was thinnking of ripping out the old underlayment, replacing it with 1/2" plywood, then installing 1/4 " Hardibacker board on top of the plywood underlayment. This would give me a total floor thickness of approx. 1.5 ", (subfloor 0.75", underlayment 0.5", hardibacker 0.25"), which would make the new subfloor/underlayment layers equal to the current height, I would then install a 6" square tile on top of the Hardibacker board. If I am correct in my thinking the new finished tile floor will be approx. the same height as the old floor. What grade of plywood should I use, and is the floor likely to be rigid enough to get the job done? Please let me know if I have left out some info which would be of use to you.

Thanks in advance.

Eric

doitright · 05-26-2004, 10:39 PM

Hi Eric, Welcome!

Everything that you're proposing sounds good.

One thing we don't know is the span of the floor joists.

Instead of hardibacker board, why not try Schluter Ditra. You'll gain another 1/8".

For the plywood, you'll need exterior grade ccx or better. Make sure they are not butted together or nailed into the joists.

muskymike · 05-26-2004, 11:15 PM

Hi Eric,
John's right if you tear out the old ply and add 1/2 ac ply then the ditra your tile should be close to the height of the wood or maybe a hair higher.

John Bridge · 05-27-2004, 06:16 AM

Welcome aboard, Eric.

ericinmichigan · 05-27-2004, 09:55 AM

Thank you to everyone for the speedy replies.

The floor joist span is 11 feet 6 inches.

I like the idea of using Ditra in lieu of the Hardibacker board as it will enable me to gain 1/8" which will help make the finished bathroom floor height more closely match up to the outside hall floor. I googled Ditra and it looks as though ease of installation is similar to that of Hardibacker board. Can I purchase Ditra at one of the big box stores like Home Depot or Lowes, or is this a specialty item carried only at flooring stores? I searched Lowes and Home Depot's web sites and found no mention of Ditra.

One more question guys. Musky Mike, in your reply you referenced "1/2 ac ply". Now I understand ply is plywood and 1/2 is thickness in inches, what does ac mean? A newbie question I'm sure, but I don't want to leave any stone unturned while I am gathering the data necesary to ensure I do the project right the first time.

Thanks again for your help everyone!

Eric

muskymike · 05-27-2004, 10:19 AM

Hi Eric,
ac is just the grade of plywood, that means one side is grade a which is smooth, that side goes up and the other is grade c which of course goes down. I usually glue and screw this but some my say not to glue it. Make sure like John said leave an 1/8" gap between the joints and dont screw or nail it the joists. Then you will have a nice smooth surface to apply the ditra to.

MichaelJ · 05-27-2004, 04:32 PM

Why would you not want to screw the subfloor into the joists as well as in between the joists? I put 3in screws when I layed 3/8 ply over 5/8 along with BullgripPL400 and 1.5in screws between the joists and lots of them, then I tiled on top of the ply. I have a solid 13by13 porcelian tile kitchen floor.

Thx Michael

John Bridge · 05-27-2004, 06:59 PM

Michael,

I'm sure your floor is fine. At ease.

Some of the college boys in the industry have determined that if the second layer of wood is not attached to the joists that it can't be affected by the movement of the framing. And the layer is still held down well by attaching it to the first layer. It's the way we're going with it now. Again, I'm not worried about your floor.

MichaelJ · 05-27-2004, 07:51 PM

OK, I'm going to play the devils advocate here. So what the boys are saying is that the 3/8 plywood on top of the 5/8 ply is also acting as an isolation membrane

Indeed some things have become better with time but sometimes we need to look back and see that the castles of old are still standing strong today. I worked in Denmark with my cousin who was a bricklayer,tilesetter,a real old fashion mudman, I was with him on one job at Helsingor castle and when we put the mud on the wall which by the way was straw instead of chicken wire we were not aloud to use cement in the mud. Things had to be as it had been for 100's of yrs. The walls held up just fine.

Another job we layed the tile right on top of the 3 to 1 mix of sand/cement ,no thinset,no acrylics,no sealers, so I wonder if things just haven't become cover your ass,overkill bulldung. Just me thinking outloud