How to Add a Toilet Connection to a Sink

Locate the sink plumbing under the crawlspace of the house. Plan to fit the sink drain into the new main toilet drain pipe you will install. Picture all bathroom waste water traveling to the septic system via the toilet drain. Use a totem pole as an analogy -- with the toilet drain hooking on lower on that totem pole. Connect the toilet drain lower than the sink drain on a single, vertical drain pipe carrying waste out of the house.

Decide where the toilet will sit in the bathroom floor plan. Cut a 4-inch hole with a hole saw to accommodate the toilet drain to be covered by the toilet flange and wax ring to seat the commode. Measure 12 inches from the wall to the center of the toilet flange. Use 3-inch stainless screws to secure the toilet flange to the bath subflooring.

Follow building codes in running the toilet drain horizontally over to septic or sewer lines. Use metal strapping to secure a long drain at different points to various floor joists. Ensure that the drain drops down approximately one-quarter inch for every linear foot to keep the waste flowing.

Fit the sink drain, which will likely be only 1.5 inches in diameter to the main drain either inside the wall or under the house. Refer to local building codes to determine the type of connector you are required to use. Check the sink P-trap to ensure the trap is installed properly to keep sewer gas from the sink area.

Install the toilet itself with a wax ring after the toilet drain and sink drains are in place. Don’t create a different vent for the toilet back inside the wall at any point, because the existing sink vent will serve the toilet, as well. Don’t worry about installing any kind of curved P-trap for the toilet, because toilets have an integral trap as part of their design.