PVC to Cast Iron Toilet drain... which Fernco?

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Old 03-12-17, 11:29 AM
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PVC to Cast Iron Toilet drain... which Fernco?

I had a broken toilet flange. I cut off the old lead drain pipe, leaving a 1.5" stub before the cast iron fitting. How should I connect the new PVC pipe? I tried to link pics of the possible solutions.
  1. Use a shielded 4" Fernco coupling to a PVC 4" to 3" reducer, then 3" PVC pipe. Pic#2
  2. Pass 3" PVC through a Fernco 3" to 4" reducing coupling, with PVC extending deeper into the Cast Iron. Pic#3
  3. Your incredible solution

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Old 03-12-17, 11:55 AM
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3) 4" shielded Fernco to 4" PVC to 4" elbow to 4" flange.
 
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Old 03-12-17, 06:01 PM
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Based on the 4" pipe to street elbow to flange being too high, I plan to reduce the pipe size to 3"... see below. I've also been told that 3" is better because it flushes at a higher velocity.

The distance from the top of the stub to the top of the rafter is 1.75". The rise for the 4" PVC Street elbow is 3.25". 3.25 - 1.75 = 1.5" above the rafter.

I planned to use 0.75" plywood + 0.5" backboard = 1.25"

That leaves me with a street elbow 0.25" above the backboard before I attach the flange. As I understand it, the flange should sit flush with the backboard, not above it. Am I wrong - should the flange sit atop the level of the floor tile? How would I screw down the flange if it is sitting on the tile?
 
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