View Full Version : Bathroom remodel subfloor question

04-02-2007, 09:59 AM
I'm a DIY'er working on a bathroom remodel. I'm to the point where I tore up the old vinyl tile and found 3/4" particle board subfloor on top of the original plywood subfloor (I think it's 5/8", but not positive). At any rate, there was water damage at some point before I bought the place around the toilet flange (probably 1.5x2 feet). The particle board is basically dust around there and there is some damage to the original plywood underneath. It is fairly brittle in that area.

I've already bought slate tile to install (which is about 3/8" in thickness). Originally, I wanted to install 1/4" cement backer to the original plywood subfloor so the new tile would match up height-wise to the hardwoods (which are 3/4").

My question really is, should I replace that small area around the toilet? Or can I just install the cement backer right on top? Or, could I use some 1/4" plywood on top of the old subfloor to give it some added strength so I don't have to cut anything out? My main concern is I don't want to have a big lip between the bathroom and hallway and I didn't want to have to replace the toilet flange (it's metal piping, copper I think) as it was about flush with the floor with the 3/4" particle board installed.

On top of that, what's the best way to replace the subfloor if I have to? It seems to go beyond the bathroom into the adjacent kitchen, so I can't just replace a whole piece. Another note, the bathroom is really small, and there's hardly any floorspace, so the traffic around the toilet where the damage is would be nil. The only weight that would be on it would be the toilet and someone sitting on it.

Thanks in advance.

04-02-2007, 10:09 AM
The particle board must come out for tile. You saw what happens if it gets wet. The thinset moisture could cause it to swell as well, and you'd just ruin the tile quickly. Get a little water sloshed out of the shower, and that would end it.

Stone tile requires TWO layers of plywood. Before you do either thing, you must determine if the subfloor is strong enough to support stone. It must meet the deflection standard of L/720 which is twice as strong as for ceramic tile. 1/4" plywood is almost never considered structural, so it won't do - 3/8" minimum for a second layer and more may be called for. Don't worry about trying to match heights that much, a proper transition is fairly common and works. You can make it out of wood to match the existing hardwoods. To minimize buildup in the floor height, you can use a decoupling membrane rather than cbu - you'd end up at about 1/8" for one of them vs the 1/4", if that is important to you. Check out www.johnbridge.com (http://www.johnbridge.com) for tiling questions.

04-02-2007, 10:17 AM
The 3/4" particle board is DEFINITELY coming out. I'm in the process of ripping it out right now. I can't understand why they used it in a bathroom in the first place. I knew there was some damage to it when I replaced the toilet with a Toto Aquia after we moved in, I was just hoping it was limited to the particle board.

I'll have to check the joists and do the calculations. All I know is we have stone tile in the kitchen and entryway already, same thickness as the slate we're using, and it's flush with the hardwoods as well, so I figured I was ok and was taking the same route. But it's possible that was put in not to code.

04-02-2007, 10:30 AM
you have a number of decisions to make and they won't be clear cut or easy.

Since it's such a small space, and since you are not worried about the stone showing hairline cracks, you may end up deciding that your plan is fine the way you are describing it right now. Code does not specify ply thickness or joist size. AFAIK. I mean for tile.

It is true what Jim said. All of it.

F.Y.I. the Noble company makes a membrane even thinner than 1/8". You are absolutely right about ripping out OSB from the bathroom floor.


04-02-2007, 10:44 AM
Does anyone know if there's a place to buy the Noble membrane locally in the Seattle area? Before I start calling all the tile shops and plumbing stores?


04-02-2007, 10:59 AM
as a general rule, for any product, you look on the web site for dealer locator, or you call their 1-800 tech support number, and you get them to help you figure out which product you want while they tell you which stores can order it for you..


04-02-2007, 12:18 PM
I'm curious. How am I supposed to find out how long the joist length is for the deflection calculation? I know I've got 16" O.C and 2x10, but I don't know how to tell how long they are, since my basement is completely finished.

04-02-2007, 01:35 PM
A support is typically at the outside walls, any support walls that are perpendicular to the joists (although not all walls are support), or a beam. depending on the ceiling height in the basement, it may be obvious where a beam is.