One of the most common plumbing defects that I find in old Minneapolis and Saint Paul houses is a missing cleanout plug in the floor drain - especially during Truth In Sale of Housing Evaluations. A missing cleanout plug can allow hazardous sewer gas in to the home, and often indicates a clogged floor drain.

Every plumbing fixture has a trap. The purpose of a trap is to prevent foul-smelling sewer gas from coming back in to the house. The diagram at right shows a P-trap, which can be found at sinks, showers, and bath tubs. The left side of the trap connects to the plumbing fixture, and the right side connects to the sewer. The 'sewer' side will have sewer gases present, but the water sitting in the bottom of the trap prevents the sewer gases from entering in to the house.

Floor drains are no exception. The photo at right shows a floor drain, as viewed from the side. The shaded portion shows the trap where water will always sit, which prevents sewer gas from coming in. When you look at an installed floor drain, all that you typically see is the grill on top; the rest of the drain is always buried in the basement floor.

When the drain line gets clogged If the drain line for the floor drain gets clogged, it needs to be cleaned out with a drain cleaning tool. The floor drain has an area which bypasses the trap, which will allow a tool to be inserted in to the drain. I've highlighted this bypass in the photo below, left. Normally a plug is installed here, because this is an area where sewer gases dwell. When the plug is removed, sewer gases come in to the house. After the drain gets cleaned out, the cleanout plug needs to be replaced. The plug is circled in blue in the photo below, right.

What if the threads are destroyed? On some older floor drains, the threads that used to accept the cleanout plug are damaged or badly rusted, to the point that it's impossible to screw in the cleanout plug. The only acceptable repair for this situation is to install a rubber plug. One type of plug is a piece of rubber that is sandwiched between two pieces of metal that expand the rubber when tightened together. The two photos below show a rubber plug before it's tightened and after it's tightened. Another type of rubber plug can be found at Real-Tite Plugs - a company located in Golden Valley.

A missing cleanout plug usually means one of two things:

  1. The drain was clogged, someone removed the cleanout plug to clean the drain, and they forgot to put the plug back in.
  2. The bottom of the trap is clogged, and someone removed the cleanout plug to allow water to drain directly in to the sewer, instead of going through the trap.

When the cleanout plug is missing it needs to be replaced, and the floor drain may need to be cleaned out or replaced. If the drain cannot be cleaned, the entire floor drain needs to be replaced. When I perform re-inspections on homes in Minneapolis that have had missing cleanout plugs, about one in five floor drains need to be replaced because the drain couldn't be cleaned out. This is an expensive repair, as it involves breaking up the concrete in the basement floor, replacing the drain, then pouring new concrete.

Reuben Saltzman, Structure Tech Home Inspections - Email - Minneapolis Home Inspector