Plumbing – Seal a backup drain from sewer gas

The best option that I can think of would be to have the sewer lined. This company is simply the first Google search result I found:

http://www.dontdig.com/drainlining.php

The premise here is that a flexible liner is inserted into the pipe (usually using air - think of it as turning a sock inside out) and then the liner is hardened in place (water activated? resin?). The new liner will bridge any cracks or breaks in the pipe as well as any gaps in the joints.

I would also suggest actually getting the section of drain in question inspected...you will really want to know what the true problem is before deciding on the proper corrective action.

What size is that copper? You can get plastic p-trap kits in 1-1/2" and 1-1/4", and these can be easily connected to your copper via slip ring compression fittings. Basically they consist of a nut with a cone-shaped plastic washer, which when compressed by the nut squeezes against the (copper) pipe to create a seal. Usually these are only on the sink side of things and the wall side has glued-on plastic but you can use the compression fittings on both ends.

Here is a 1-1/2" Plastic p-trap

If you want to solder, a better solution than using a compression ring on the wall side would be to sweat on a copper x female adapter, followed by a male PVC adapter and a PVC trap adapter. Better yet if you can find one, just use a brass copper trap adapter like the following:

A less desirable but acceptable solution would be to use a fernco fitting to go from the copper to PVC, and then you can avoid soldering.