Hi,
I have removed my basement bathtub to facilitate the installation of an interior french drain system. I plan to replace it with a shower of the same dimension. Shower pans are available in the correct size and with drains at the end like in a bathtub. I am told that I will need to increase the drainage piping to 2 inches. The bathtub drain and overflow piping were 1 and 1/2 inch and drained vertically to a 2 inch cast iron trap about 6 inches below the concrete slab. The existing trap drains to the main soil stack with a 2 inch vent pipe just 3 feet away from the trap.
1.) Do I need to install a second 2 inch P trap from the shower drain to the trap under the slab or can I just run 2 inch dwv pipe with appropriate slope to the existing trap.
2.) I have heard about a Fernco "donut" fitting that is designed to allow the attachment of PVC pipe to a cast iron hub. In this case it would be connecting the 2 inch PVC to the 2 inch cast iron trap,(actual internal diameter of the hub is 3 inches). Does anyone have pointers about installing a "donut" and whether they are acceptable by the plumbing codes.
Thanks,
John John

No self respecting plumber will run 1 1/2" for a shower but the new code allows it on the principle that the same faucet is used for both a tub and a shower.

no self respecting plumber will run 1 1/2" for a shower but the new code allows it on the principle that the same faucet is used for both a tub and a shower.
I will be using 2" pipe. The point of the question is whether or not I need 2 traps here. One directly attached to the new shower base as well as the current 2" cast iron trap that is 6" below the slab and 12" from the shower drain opening. Or can I just run 2" PVC pipe with appropriate slope from the new shower base, to the existing under slab trap?
I also asked about advice on using a Fernco donut to attach PVC pipe to the 2" cast iron hub. Any experience with this system?

Only one trap is legal, yes, you can use a fernco

1.) Do I need to install a second 2 inch P trap from the shower drain to the trap under the slab or can I just run 2 inch dwv pipe with appropriate slope to the existing Double trapping a fixture's illegal and against code. If you're replacing the old 1 1/2" drain line then replace with a 2" trap and raiser.

2.) I have heard about a Fernco "donut" fitting that is designed to allow the attachment of PVC pipe to a cast iron hub. In this case it would be connecting the 2 inch PVC to the 2 inch cast iron trap,(actual internal diameter of the hub is 3 inches). Does anyone have pointers about installing a "donut" and whether they are acceptable by the plumbing codes.

Yes you may use a "doughnut" (see image)to convert to PVC. Let me show you how. First clear and clean the cast iron hub. Then insert a Gasket, (see image) in the hub. Next take the piece of plastic pipe that you wish to convert to and file and bevel the sharp edges so it will slip into the gasket.
Next you will need Duel-Tite Lubricant to make the pipe slde in without friction. Apply the lubricant to both the pipe and the gasket. Now force the pipe into the gasket, KEEP IT SQUARE, do not cock it.
If you have a problem getting it started it's acceptable to take a razor knife and shave down the first few rows of ridges. You have just converted from cast iron to plastic. However, I'm confused! You should be converting to PVC at the main and not a cast iron "P" trap. This trap should be removed when you convert to PVC. What am I missing here? Tom

Thanks speedball1 and doug238.
To better explain, the existing 2" cast iron trap is currently close to where I need a trap for the new shower base because that's where the tub was that I had removed. The cast iron pipe from the trap continues on to the cast iron main under the concrete slab with a 2" vent teed into the line between the trap and the main just 3 feet from the trap.

I just need new piping from the hub of the trap to the new shower base. The drain opening of the shower base is ofset several inches from where the old tub's drain used to be, and I need 2" pipe instead of 1 1/2" pipe because it is a shower drain not a tub drain.

I was able to install the neoprene donut without too much of a problem, (just needed a few well place hammer blows through a scrap piece of 2x6 to get everything seated well). The manufacturer recommended using liquid soap to lubricate the donut and it worked out fine.