How to Vent a Bathtub

Consult the bathtub manufacturer’s installation instructions to learn where the drain-overflow combination must be located. Some manufacturers print an installation template on the inside of their packaging. If no template is available, measure where the drain hole is in the tub and use a broad-tipped marker pen to mark that point on the floor, immediately below where it is to be situated. Next, draw a straight line between that mark and the point at which the drain will either exit the building or join a pre-existing sewer feeder, properly called a “soil stack.” Cut a 12-inch-long by 9-inch-wide hole in the floor, starting at the original mark and following the line; this is to accommodate the P-trap.

Install a 1-1/2-inch P-trap and waste-pipe line and route the pipe, with a 1/8- to 1/4-inch fall, along the line drawn toward the building exit or the junction with the soil stack that serves the bathroom. In all but new installations where a stand-alone tub -- such as a Jacuzzi -- is not a part of a bathroom system, all the fixtures typically share a common waste: The sink enters the waste at the highest point, the bathtub slightly below floor level and the toilet below that. All are typically served by a single 4-inch downpipe that feeds directly into the sewer line. In all configurations, the P-trap must precede the vent in the direction of waste-water flow.

Connect the waste from the bath to the downpipe using proprietary fittings. If no such downpipe is available for connection, one must be provided before the vent can be installed. Use a reducing T fitting to make the connection: The 4-inch orifice points downward and fits over the soil stack downpipe to the sewer; the 1-1/2-inch orifice points toward the tub and fits over its waste pipe; the 2-inch orifice points upward to accept the vent pipe.

Create a stand-alone vent for a tub that is not part of a bathroom suite by installing a T fitting instead of an elbow where the waste from the tub right-angles downward toward the sewer. In this case, with no toilet as part of the installation, a 2-inch downpipe is adequate. Both the up-facing and down-facing orifices need to be 2 inches, and the tub-facing orifice should again be 1-1/2 inches.

Route the vent from either installation to a point above the roof of the building, as determined by local code; such codes vary widely from place to place. The Plumbing Code, a non-regulatory advisement standard, notes a typical height as being between 6 and 12 inches above the roof. Vent stacks exist only to exhaust non-pressurized sewer gases and to admit circulatory atmospheric air into the system, so several restrictions in the route do not typically cause problems. Design the route to be as visually unobtrusive as possible; use 45-degree elbows to create offsets around obstructions such as water tanks, then simply route the stack back to its intended termination point using more fittings. At no point, though, can any extent of the stack point downward.

Attach the vent pipe to solid supports at intervals as advised by the manufacturer; these advisements are often printed on the outside of the pipe. Again, your local code enforcement office also should be able to give specific direction. Use proprietary pipe straps to secure the vent to internal wall and loft timbers if running the vent up inside the building; use correctly sized PVC two-part pipe clips screwed to the wall if making an exterior installation.