I'm installing ceramic tile on top of backerboard and need to install a new PVC closet flange. One plumber friend told me to install the flange on top of the backerboard because it will provide a more stable surface since tile can break when tightening the toilet bolts. Another plumber friend said to install the flange on top of the tile so it seals closer to the toilet. Is there a consensus out there? Thanks

Either way is acceptable. However, depending on the thickness of the tile, you may need to add some additional expansion flanges. Some people put a thin bead of silicone between the extenders and some people use PVC cement to glue them together. Eliminates a possible leak.

I've seen it done both ways and both ways work. I would install the flange after the tiling. Makes tiling easier and it does make the flange sit closer to the toilet. You will have to drill through the tile to screw to the sub-flooring but that is not difficult.

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tile can break when tightening the toilet bolts.

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The screws pull the flange down toward the sub-flooring. Toilet bolts pull up from the flange toward the toilet, not down. So that logic is not correct.

Installing the flange on top of the tile will help keep the toilet centered, and, will prevent wax gasket erosion. The funnel shape of a closet flange will contact the toilet bowls horn if you try to install the bowl too far off center. Water should only come in contact with a gasket when there is a back - up in the piping connected to the flange...