I have an existing horizontal 1-1/2" copper waste line that is soldered to a 90degree brass elbow that extends through the drywall under the bathroom sink which connects to the trap. i need to extend this waste line in order to replace the vanity and sink bowl. is it code legal to cut the copper pipe and use a rubber connector that will connect 1-1/2" copper pipe to 1-1/2" plastic pipe in order to extend the waste line. Can this then be concealed behind drywall. A concern is the connection leaking since it is on the second floor

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The answer is yes. Use the banded type of coupling as shown below.

I am not 100% sure on this but I don't believe you can use this type of fitting inside a closed wall area. Mechanical fastners and connects should be left open so they can be seen/serviced. Any chance you can give us a picture?