How to Rough Plumb a Bathroom Lavatory

Consult the building plans to learn the precise location in the wall behind the lavatory to be built. By far the most common distance from the back wall to the center of the waste-water vent from a regular domestic lavatory is 12 inches from the face of the drywall, although 10- and 14-inch models are made. The client should provide a manufacturer’s specifications sheet if the toilet to be used is anything other than the typical size. A space of 18 inches between each side wall and the center of the toilet is considered adequate for most wheelchair users to access the facilities.

Design the wastewater line so it joins the main sewer line out of the property at the nearest convenient point; use the most direct route available to you. The run of pipe does not need to have a P-trap -- often called a U-bend in industry-speak -- because lavatories have integral traps. The fall for a toilet drain is typically mandated to be no less than 1/4 inch per linear foot, but again this should be confirmed with local code enforcement. There are also regulations mandating the maximum linear runs of both vertical and horizontal pipe between the lavatory and the nearest vent.

Install the toilet flange so that its underside is flush with the floor. Make the run from the flange to the sewer using unplasticized polyvinyl chloride pipe designed for lavatory wastewater; it is entirely resistant to the kinds of household products, including bleach, that are used to clean toilets. Use only proprietary fittings designed for lavatory wastewater drainage, and ensure both the cleaning primer and the solvent -- the “glue” -- are made for the type of pipe you are using.

Locate a 1/2-inch water-supply point close to the location to be occupied by the lavatory cistern. Floor-mounting is entirely acceptable, but wall-mounting is likely to require a less obtrusive final connection, and therefore to result in a more esthetically-pleasing finish. Floor-mounted supplies should be left 6 inches above the anticipated finished floor height, and 6 inches to the left of the flange as seen from the front. Cap off the supply with an isolation valve, and ensure it is oriented to the “off” position even if the water main to the property is not yet live.