Bath Remodel - Plumbing Problems:

Installing An Offset Toilet Flange

In This Article:

A complex-shaped cut-out is made in the sub-floor, a piece of treated lumber installed for a spacer, and the offset flange is glued to the 3" drain pipe.

Related Articles:
Skill Level: 3 (Moderate) Time Taken: 1 Hour

By , Editor

The previous owner had installed a cheap conventional plastic toilet flange and had hacked out a huge notch in the side of the 2x8 floor joist to accommodate the plumbing. Our room improvements included adding a layer of 3/4" foam insulation, followed by " drywall. In the old installation the toilet was tight against the wall. We added at least " to the wall thickness (the old plaster-and-lath was about " thick), so we knew that a conventional toilet flange would only amplify the damage to the floor structure. 

The PVC drain plumbing was installed a month earlier, and the toilet flanges had been put in place but not glued (as is common practice) until the finish flooring installation was complete. The toilet flange can rest on the finished floor. The toilet horn, which fits into the flange, is recessed into the toilet body and lies about 1 inch above the floor level

Toilet drain pipe before installing toilet flange. The 3" drain pipe lies just below the floor level. We had covered the pipe opening with duct tape to prevent tile mortar from getting on the pipe end (and to keep sewer gases from entering the house during remodeling).


The offset flange. Note how the pipe is not centered directly underneath the red mounting ring. This is the only sensible choice when a toilet must be mounted directly over a floor joist. Offset toilet flange.


During tile installation we had left this unusual shaped opening in the finish floor. We arrived at this shape by test-fitting the flange and drawing lines around the red mounting ring.

We made the complicated angle cuts (instead of making a simple square cut-out) because it appeared that the back corners of a square cut-out might not be covered by the toilet.


We cut a 3/4" thick piece of treated lumber (1x8 stock, actually) to fit in the tile cut-out. Then we used a 4-1/2" hole saw to cut the opening.


We tested the fit of the wood filler block. It took several tries to get the shape just right, because of the complex shape of the offset flange.

In fact, we determined from the test fittings that the only way to attach the flange to the pipe was to slip the wood filler block over the flange's pipe fitting and then slip the fitting on to the pipe. There was no way we could install the filler first and then connect the flange.

We applied purple PVC primer to the pipe and the fitting. We also raised the drain pipe up and secured it with a temporary support, making sure that the pipe had a suitable slope (1/8" to 1/4" per foot) when the whole job was done. 


We applied PVC cement to the fittings and stuck them together. At this point the entire flange is elevated from the floor by the temporary support.


We re-positioned the pipe support and drove long deck screws into the flange. Some of the holes at the back could not be used because the screws would penetrate the pipe.


We filled the gaps with caulking. It is very likely that the wax toilet seal will leak someday, and this caulking may prevent water damage to the wood floor structure.

Continue to the toilet installation.


Tools Used:

  • Cordless Drill/Driver
  • Heavy Duty " Drill
  • 4-1/2" Hole Saw


Materials Used:

  • Offset Toilet Flange
  • PVC Cement, Primer
  • Deck Screws
  • Caulk


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Copyright 2000, 2005

Written August 23, 2000
Revised January 3, 2005