Drain and vent lines must be positioned precisely, so you should install them before the supply pipes. It might be possible to simplify supply runs by moving a vent pipe over a few inches. If you must run drainpipes across a floor, carefully calculate the amount of vertical space available; you must slope the drainpipe at ⅛- to ¼-inch drop per foot. Sometimes it’s difficult to visualize how drainpipes will travel through walls and floors. Once you start assembling the pieces and testing them for fit, you might need to modify your plans. Some inspectors prefer horizontal vent pipes to be sloped so condensation can run back to the drainpipes; others don’t consider this important. To be on the safe side, slope the vents.
Check your plumbing setup first to ensure the project goes smoothly. You might find that a joist is in the way of a toilet bend, for example. If so, remove as much flooring as necessary to get at the framing. Cut the joist, install a blocking piece, and attach 2x4 cleats around the opening. If the toilet drain does not connect directly to a vent, you must find another way to vent it. If the drain line runs away from the wall where you want the vent, use a reducing Y and a 45-degree street elbow to point the vent line toward the wall. The horizontal vent pipe runs right next to the closet bend. Toilet Vent Option 2: Parallel If the vent wall is parallel to the drain pipe, install a 45-degree reducing Y and a street elbow to point toward the wall. You might need another elbow (of any degree) to position the vertical vent where you want it. Toilet Vent Option 3: Opposite Side If the vent wall is opposite the drain line, use a reducing Y and a street elbow. The fittings can be pointed straight at the wall or at an angle, as needed. Editor’s tip: To run a vent pipe through the ceiling, first drill a test hole to make sure you won’t bump into any joists in the attic (illustration above, right). You might need to move the hole over a few inches. The top plate can be doubled, meaning you have to drill through 3 inches. You might need to drill with a hole saw first from below (shown), then from above. How to Run a Shower Drain: A 1½-inch drain trap is often permitted for a shower, but a 2-inch trap will ensure quick flow of water and will be less likely to clog (image above, right). A shower has no waste-and-overflow assembly, so the rough plumbing consists of a cemented trap that rises to the correct height for the shower base.
