I uncovered a really old clay or cast iron house trap pipe(Im told its called)below lavatory and tub drain in a really old houseIm not a plumber at all,but all of this looks really wrong to me even.Of course commode hooked into it also.Crawl space so shallow and floor was only unfinished plywood soI just pulled it up and there it was.. no digging.I still don't know where to start.Originally was a non draining tub,driving me crazy.So Im all the way down to this... end of the line.Can all this be replaced with pvc?I hope!Im even havig a hard time imagining the wrench it would take to loosen this trap pipe... Im good with my hands,just need some advice... Thanx

If your drains are clay and cast they are really ready for replacement and PVC would be great. I would break the pipe rather than trying to unscrew it but disconnecting at the tub will involve a wrench or two. Is this septic system or city sewer?

Its septic tank,and its only about 8feet from where the commode flushes.All is clay or iron,disconnected from tub... that did take some doing.Standing on pipe wrench,cant believe it didn't break.The square nut on end of old house trap DID break, so now I know I hVE ISSUE.Do I use some kind of pvc p-trap to replace this ''house trap''?and then work my way back with sink and tub drains?what kind of connection for commode line?

I uncovered a really old clay or cast iron house trap pipe(Im told its called)below lavatory and tub If these are pipes inside the house they aren't clay, it's cast iron. Could you take a picture and show us?

Can all this be replaced with pvc? It sure can, but before we give you instructions we have to know what we're dealing with. We really need pictures of your drainage.
Roughly speaking. You will have to convert to PVC at the stack and terminate with a "P" trap and raiser directly under the tub waste.
Lets have more details such as the distance between the tub drain and the stack and the pictures sure would help. Back to you, Tom