Cast iron p trap under slab removal

[Mtnj31]

Hello guys. I’m working on a bathroom remodel and I could use some advice. We were smelling sewer gas pretty strong and it appeared to be coming from the tub drain. I removed the old tub and saw the old brass drain assembly was cracked just after the connection where it met the cast iron p trap. The trap is cracked at the top and I want to replace it with PVC. I’ve read plenty on which couplings to use but the cast iron p trap OD is 2.5 in. I’m wondering what size coupling I could use to attach the 2.5 in cast to PVC once I remove the cast p trap. And can I attach a 1.5 in PVC trap for the tub drain to this? This size pipe isn’t too common from what I’m reading. Any advice is greatly appreciated.

 

[Jeff H Young]

Are you sure its not aproxx 2.375 " ? standard 2 inch

 

[Sponsor]

 

[Jeff H Young]

btw Ive never seen a 2 1/2 " trap

 

[Mtnj31]

btw Ive never seen a 2 1/2 " trap

Hey Jeff. I measured the circumference of the pipe and it was approximately 7.875 inches. I then divided that by pi. Is that the right way to get the size? Thanks.

 

[Jeff H Young]

2 1/2" inch pipe has a diametor of 2 7/8 inch approx 2.875 with circumfrence just over 9 inches. sometimes the od on old castiron is out of round .
but according to your measurements your od is almost exactly 2.5 inches just about an 1/8th inch oversize for 2 inch, perhaps rust and crud?

 

[Mtnj31]

2 1/2" inch pipe has a diametor of 2 7/8 inch approx 2.875 with circumfrence just over 9 inches. sometimes the od on old castiron is out of round .
but according to your measurements your od is almost exactly 2.5 inches just about an 1/8th inch oversize for 2 inch, perhaps rust and crud?

There is definitely some rust and crud around that pipe. I guess I should see if I could scrape off some of that before I attempt to attach the PVC to it. Do you think I could use a transition coupling to attach that 2in cast to 1.5 in PVC trap? Or do 2 in PVC and transition to 1.5 at the drain. Thanks again.

 

[Jeff H Young]

There is definitely some rust and crud around that pipe. I guess I should see if I could scrape off some of that before I attempt to attach the PVC to it. Do you think I could use a transition coupling to attach that 2in cast to 1.5 in PVC trap? Or do 2 in PVC and transition to 1.5 at the drain. Thanks again.

I like my p trap to match trap arm. So Id use a 2 inch trap if not too much trouble. Im thinking about whether to use a fernco rather than a shielded coupling. leaning toward the fernco!

 

[Mtnj31]

I like my p trap to match trap arm. So Id use a 2 inch trap if not too much trouble. Im thinking about whether to use a fernco rather than a shielded coupling. leaning toward the fernco!

Thank you so much for the help. I’m thinking the Fernco is the way to go too. Thank you again for the help, I truly appreciate it.

 

[Jeff H Young]

you bet ! try to clean that up (outside of pipe) and I hope the coupling fits well. run a hose down and blast water good to clean it out and verify its clear , plus that the coupling seals. if there is a blockage adress befor building a shower

 

[Mtnj31]

you bet ! try to clean that up (outside of pipe) and I hope the coupling fits well. run a hose down and blast water good to clean it out and verify its clear , plus that the coupling seals. if there is a blockage adress befor building a shower

Will do!

 

[wwhitney]

In re the coupling type, Fernco makes both unshielded and shielded couplings. To my understanding if the coupling is going to be buried in the soil, you're supposed to use the ~4" long rubber type where the shield is optional, rather than the ~2" long shielded type that's required above ground. That means you'll need 2" of pipe stub on each side for the coupling to connect to.

Cheers, Wayne

 

[Mtnj31]

In re the coupling type, Fernco makes both unshielded and shielded couplings. To my understanding if the coupling is going to be buried in the soil, you're supposed to use the ~4" long rubber type where the shield is optional, rather than the ~2" long shielded type that's required above ground. That means you'll need 2" of pipe stub on each side for the coupling to connect to.

Cheers, Wayne

Thanks Wayne for the heads up on that. That info will help me out tremendously. I definitely don’t want any future leaks. Much appreciated.

 

[alex rodriguez]

I’m dealing with this same situation right now. How did you end up converting the trap to PVC? I can’t reach far enough in my situation to cut past the trap

 

[Jeff H Young]

I’m dealing with this same situation right now. How did you end up converting the trap to PVC? I can’t reach far enough in my situation to cut past the trap

why not start a thread this one is years old good luck with project