Floor Drain Basics

One of the most common defects that I find in old Minneapolis and Saint Paul houses is missing cleanout plugs in floor drains - especially during Truth In Sale of Housing Evaluations.  Missing cleanout plugs can allow hazardous sewer gas in to the home, and often indicate a clogged floor drain.   To learn why and how, read on.

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 Drain

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 always in place 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 that is sandwiched with 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.

 
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.

Any time I see a missing cleanout plug, I tell my client that the cleanout plug 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.

Just for fun, here are a few photos of missing cleanout plugs that I've taken within the past couple months.




For more information on common Truth In Sale of Housing defects, click on any of the links below.

• Missing Jumper Wire
• CO Alarms
• Vacuum Breakers (backflow preventers)
• Toilet Ballcocks
• Smoke Detectors

Reuben Saltzman, Structure Tech Home Inspections - Email - Minnesota Home Inspections