How do you replace old plumbing lines under the house?

Supply or drain?

Supply: Start by mapping out where the lines go and in what order. Once you have a clear plan then consider what materials you want to use. If you're replacing galvanized with copper - you will have to be able to access every joint without running the risk of starting a fire because copper needs to be soldered (using SILVER solder - made for plumbing, NOT Lead based solder - poison issues).

If you choose PEX (which is what I have done) you'll find that much easier. But remember that you have to get into some narrow places and you won't have room for the crimp tool. In those cases you will have to pre-assemble some parts, simply because once installed you can't crimp the connections. ALSO consider that using PEX means you will have to run some electrical ground lines. PEX is not considered a good ground source because of the plastic nature of the pipes.

Now: If you are doing drain lines - you can use ABS. It's easy to cut and all the parts get glued together. Just be sure you know how to glue them together before you start. Yes, you have to KNOW how to glue them otherwise they will not hold and will collapse and leak. Again, you'll have to start with a good map of what's already there. If some of the lines are cast iron, you will need a good saw to cut through them. They have cast iron pipe cutters you can rent, which I've done in the past. If you have galvanized drain pipes - those can be cut more easily with a hack saw. Just have plenty of blades handy. Even if you're using a reciprocating saw, have plenty of blades (depending on how many pipes you need to cut). It's likely you may have to thread some connectors together, so you'll need a good set of pipe wrenches. Always use two wrenches when putting a joint together or taking it apart. Failure to do so may cause something else to come loose somewhere else OR can break a pipe you don't want to disturb.

If any of this is uncertain to you then call a professional plumber. It's worth the investment.

If you're talking about replacing the main drain line that goes out to the street, there's a process called "Rhino Busting" where they use a steel cable to pull a new drain line into the old one bursting the old pipe as it goes. I've had that done. Cost me little less than $5,000, but well worth the investment because I won't have any drain issues for another hundred years. So by the time I'm old(er) I won't have to face this issue again.

Hope this helps.

'av'a g'day mate.

'')

As usual the asker fails to provide sufficient information. Really basic stuff like is it a slab or does it have a crawlspace? Like are you talking waste lines or just clean supply lines? What material are the existing pipes made of? Nobody can answer intelligently when the asker

DOES NOT PROVIDE SUFFICIENT INFORMATION.

It's the 101: who where when why what and how.

We can't answer until we have more facts. You should have thought it through before posting.

Tony is wrong. Silver solder is not used on copper as it has a hight melting point and it is used where joints are exposed to high temperatures. Silver solder is very expensive too. Why not by pass the old lines and put in new lines. You can have them above ground if you wish.

Are they for WATER ONLY? If so, just install new lines above ground.

Hire a professional plumber with a lot of experience.