Richard Trethewey shows how to rough-in the drainpipes for a basement bathroom. (See below for steps.)
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Steps for How to Install a Basement Bathroom:
1. Lay out the 2x4 bottom wall plates to establish the perimeter of the bathroom walls.
2. Measure off the wall plates to locate the center of the shower drain and toilet flange.
3. Dig a trench extending from the existing drainpipe over to the new locations of the shower drain and toilet flange.
4. Use a right-angle grinder fitted with a diamond-impregnated cutting wheel to cut through the old cast-iron drainpipe.
5. Attach a flexible no-hub mechanical coupling onto the end of the just-cut drainpipe. Tighten hose clamp onto the cast-iron pipe with a torque wrench.
6. Cut and glue together 3-inch-diameter PVC pipes and fittings to form the drainpipe assembly that connects the old cast-iron pipe to the new shower and toilet drain.
7. Slip the end of the PVC drainpipe assembly into the mechanical coupling attached to the old cast-iron pipe. Tighten the hose clamp to secure the PVC pipe.
8. Next, run 2-inch-diameter PVC pipe from the drainpipe assembly over to the shower and vanity sink.
9. Install traps for each fixture, and be sure the pipes slope 1/8 inch per foot to drain properly.
10. Drive a 1/2-inch-diameter steel-reinforcing bar down into the soil beside each trap. Then, use duct tape to secure the bars to the traps.
11. Have the rough piping inspected before pouring the concrete floor.
12. Cover the open end of the toilet pipe with a foam cap. Wrap plastic around the shower-drain pipe.
13. Have the concrete contractor pour a new floor around the piping.
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How to Install a Basement Bathroom | Ask This Old House