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The bathroom model saga continues. The old toilet flange was rusting, and the subfloor was rotting, so I pulled up the subfloor around the toilet, and replaced the drain pipe with ABS, and an elbow. The plan is to tile this bathroom.

So my question is- should the toilet flange sit on top of the backer board so that the top of the flange is close to flush with the top of the tile? Or should the flange sit on top of the tile (thus raising the top of the flange 3/8" or so over the top of the tile)? I've seen it recommended both ways, but have had some trouble finding the pros/cons of each.

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You also may want to put grout underneath the perimeter of the toilet to fill in any irregular surface areas, this will also reduce the chances of movement. –  chris Sep 25 at 16:47
Grout may cause the toilet to adhere to the floor in a way that's difficult to repair without destroying the toilet or floor. You want the floor level, the flange securely mounted, and then caulk the front and sides of the toilet. If your floor is not level, then plastic shims may be available to prevent rocking (wood may rot). –  BMitch Sep 25 at 17:52

5 Answers 5

up vote 5 down vote accepted

You want the flange to be secured to the floor as tightly as possible. If it's on top of the tile, I think you run the risk of cracking the tile when you're tightening the nuts on the flange, so I say attach it to the backer-board.

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You should always put the flange on top of the finished floor. Keep in mind toilets and tiles are often the same material. And, if your floor is level (as it should be), you are distributing the PSI weight of the toilet over an area, not 100% over the bolts. If you're getting it so tight, you are afraid of cracking tile, you should also be afraid of cracking toilet. Or, it's likely your tile is unstable / not a good install and would crack regardless. –  maplemale Sep 28 at 17:47

The toilet flange needs to be on top of the finished floor. Meaning the bottom edge of the flange needs to be on the same plane as the toilet. So if your toilet sits on the tile, the flange needs to be on top of the tile too. The spacing of the toilet exit "horn" and sealing surface is designed for this height.

almost all plumbers agree, here are some sources:

cutaway of toilet to flange spacing

if you have it lower than the finished floor you risk leaking toilet water (the water is the best part) into the floor or worse, in-between the tile, sub-floor, backer board. This is because the wax ring was designed only to take up the air gap, not to provide a "tunnel" or extension of the pipe/flange/toilet horn.

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I wouldn't recommend installing the flange on top of a finished floor like tile since that flooring may not be as stable as the subfloor (and you could crack the tile). But you can get a flange extension ring to bring the flange up to the finished floor level. –  BMitch Feb 28 '14 at 4:04
I disagree. It absolutely should be on top of the tile. There is zero reason your tile should "not be as stable". If anything, it should be more stable. If you have access to the joists, install 2x6 blocking around where the flange comes up to ensure even more stability. This is a good idea regardless as it gives your toilet a very stable spot to sit on. And NEVER install the toilet flange on top of the backer. It goes on the tile. I've installed floor and wall mount toilets right onto to the tile and never had an issue with cracking due to the flange being on top of finished / tiles. –  maplemale Sep 28 at 17:40

When I redid my bathroom (added backer board and tile to replace linoleum) I had to raise the flange too. All I did was cut a piece of plywood to fit around the flange and raise it about the same as the backer board. I agree with Niall C.'s answer not to put it over the tile since it could easily crack but I also think the backer board would break easily too considering the screws for the flange would be very close to the edge of the backer board.

But in my case, I didn't realize that I needed to raise the flange until after the tile was in and a piece of plywood worked really well in that situation. :)

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In the process of redoing my vinyl floor with backer board and tile. The flange is set on the finished vinyl floor as shown in the upper photograph. The result is a toilet that rocks,leaks at the wax gasket and has ruined the vinyl floor. It's not a cheap Home Depot toilet (can't remember the brand, but it was about $200 years ago...). Even after removing and replacing the wax gasket, then doubling it up with the plastic collared type and wedging the toilet front and back at 4 points, it still rocked and leaked. The same is true of the same model toilet in another bathroom. I will be mounting the flange flush with the new tile floor and using a rubber (foam) style gasket this time.

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You are doing something wrong. Sounds like maybe using too thick a gasket. Your flange always goes on top of the tile. Don't use the rubber (foam) style gaskets. If you don't want to use a wax gasket (I don't blame you), try using a "fernco waxless toilet seal." These are more reliable and easier to use in pretty much every application. –  maplemale Sep 28 at 17:43

If you are unable to put the flange on top of the tile, you NEED to use the Toilet Flange Tile Guide. It eliminates any radius cuts in the tile and provides never-before-seen protection to the sub-floor around the flange. Only like $8. WELL WORTH IT!!!

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