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When a toilet is mounted to the drain pipe, the flange is the piece that fits between the base of the toilet and extends down into the pipe. Much of the time, the flange is a straight section of special pipe that has a flared opening at the upper end. The lower part of the pipe is lined up directly under the center of the top opening. An offset toilet flange is a connecting piece where the pipe that fits down into the drain pipe is not directly under the center of the top opening, but instead is angled so that the lower pipe is only under the back part of the flanged opening.
Although an offset toilet flange can be a bit tricky to install, in some cases it is the only realistic option due to the placement of the drain pipe. There are times when the construction of a bathroom results in a floor joist running directly under all or part of the preferred location for the toilet. This means that the drain pipe must come to the surface on one side of the joist, which limits placement options for the toilet.
When using a normal flange in such a situation, the toilet ends up being mounted in an awkward spot, typically either too close or too far from the wall. When the drain pipe is too close to the wall, using an offset toilet flange allows the location of the toilet to be shifted by several inches, moving it to a more appropriate placement in the bathroom. The offset toilet flange can also be used to move the location of a toilet back towards the wall if it is out too far, or to the left or right for better clearance around other bathroom fixtures.
Since the upper part of the pipe curves away from the bottom part, mounting an offset toilet flange requires a bit more effort because it is important to be sure the toilet is properly supported. In a standard flange the entire pipe is vertical and the lower part helps to support the upper section, since all of the weight is centered over the drain pipe. With the offset toilet flange, however, the upper part has no supporting pipe under the much of the flanged area. If it is not well-supported it is likely to break when used. Fitting a block under the front of the flange is essential to using this type of plumbing piece.