During your remodelling or repair projects, you will find some cases where you will want to transition from metal pipes to PVC pipes. Most sinks come with metal drain baskets, which you have to attach to plastic PVC drainpipes and traps. In other cases, you may have to switch from outdated metal drainpipes, like galvanised steel or corrugated black iron to PVC, which is easier and cheaper to transport, install, and maintain. Because you cannot chemically bond metal and PVC pipes, it requires special steps to create a watertight seal between the different plumbing materials.
Attaching PVC drainpipe to a sink’s metal drain basket
Locate the sink's metal drain basket underneath the counter or vanity. Remove the old drainpipe, saving any pieces that are still in good condition. Clean the threads of the drain basket with an old toothbrush or wire brush.
Repair or replace any worn gaskets or washers on the drain basket before continuing. You should have an assembled upper portion: a black rubber gasket, a plastic washer, and a metal locknut, on the pipe that connects the drain basket to the sink. Under that portion are smaller threads, or a tailpipe, that is connected directly to the old drainpipe.
Coat the smaller, exposed threads of the tail pipe with thread compound.
Slip a metal locknut over the end of the PVC drainpipe. The locknut should be the same diameter as the tailpipe on the sink's drain basket. You may be able to reuse the old one that originally came with the drain basket.
Insert a plastic washer inside the PVC drainpipe. The plastic washer should have a lip the same diameter as your PVC pipe and should have a flange that fits into the plastic pipe.
Position the drainpipe directly under the metal drain basket's smaller threads. The drain basket's tail pipe should fit snugly inside the PVC.
Twist the metal nut with your hands to tighten it onto the tail pipe's threads. Use a Stillson wrench or channel-locks to finish tightening the nut.
Transitioning from metal to plastic pipes
Wrap the threaded ends of the metal pipe with plumbers tape or thread compound. Attach a female double-threaded metal adaptor to the threaded end of the metal pipe. Female threaded adaptors are couplers or adaptors with the threads inside the fitting as opposed to the male, which has its thread outside.
Wrap the threads of a male PVC threaded adaptor in plumbers tape or coat them with thread compound. Twist the male PVC adaptor into the metal female adaptor and tighten it with your hands. Using a Stillson wrench, tighten the PVC adaptor further; however, do not overtighten since this will cause the plastic threads to snap.
Coat the inside of the male PVC adaptor with PVC primer. Coat the end of the PVC drainpipe you intend to install with the primer as well. Brush a layer of PVC cement onto both primed surfaces.
Push the PVC pipe into the male PVC adaptor with a turning motion to spread the glue over the entire fitting. Continue installing the rest of the PVC pipe in this manner until you have replaced the damaged metal pipe.
Take your old plumbing parts with you to the hardware shop to help you purchase the correctly sized fittings. Always tighten nuts and adaptors by hand first to avoid stripping threads.
Always check with your local building codes before attempting any plumbing updates. Any deviation from these codes could result in delayed sales or a decreased value in the home. PVC pipes are approved for use as drainpipes or ventilation pipes. They do not tolerate hot temperatures and can burst resulting in injury. Never use PVC pipes to run supply water through your home.