plumbing a additional bathroom -need some information/advice?

Adding a bathroom/ mudroom to first floor in existing screen porch with cement floor. Sewer exiting in 3 1/2 PVC through concrete into yard on its way to sewer connection. How deep does the pipe need to be underground as it exits the house? and would cast iron be a better choice? Thanks!

Answers:
since its not commercial dont put cast iron below ground. i know the original prob is but back in the day thats all we were allowed to use. pipe according to code in North America has to be 4" leaving the house (this is inside measurement)

below the frost connection? (inside the house?) it has to be as deep as the drain your tying into with a grade of 1/4" to foot( 1:50)
so if its at 24" thats where you tie in you will use a wye connection not a T and have a pipe comming off the drain to serve as a vent all drain plumbing needs to balance pressures both negative and positive PVC will work as will ABS.
if your cutting into Cast iron in the ground you will need snap cutters you cant saw cast very easily it takes a good man with a lot of time and patience and a damn good sawsall blade to get thru it or a zip cutting wheel on a grinder. then you would connect with Stainless Steel MJ couplings so they dont rust.
cast takes forever to rust thru in a house and i havent replaced cast newer than 50 years old underground.. but there again i dont know where you are in the world but i have worked in North America and UK

Other answers:
Below frost depth (and enough for water to flow downhill)
The frost depth in the midwest is around 34"+-

Cast Iron is never a better choice.

PVC is your best bet.
Below frost depth (and enough for water to flow downhill)
The frost depth in the midwest is around 34"+-

Cast Iron is never a better choice.

PVC is your best bet.
No cast iron is only used above ground,it will corrode in the ground.Use PVC and I'm pretty sure your existing pipe is 4 in.If its a traffic able area 600mm if not 450mm.But you must lay the pipe on at least 50mm of blue metal(crushed stone) and 100mm above pipe then soil.But make sure you will have enough fall(grade) to the main sewer connection.To deep and you will be below the connection,and sewer does not run up hill
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