Trained Eye


Slow Draining Sink


I am seeking your advise on a slow draining bathroom sink.

I installed a new bathroom sink with pop-up. The basin is a one piece 37 X 22 vanity top, made by General Marble purchased at Home Depot. The hole for the water overflow is about the size of a quarter. As part of the installation I cleaned out the drain, therefore I know there is no blockage caused by material in the pipes.

My problem is that the basin is very slow draining and the problem seems to be caused by air in the drain. When the pop-up is first opened, water is forced back up the overflow. I seems like it can't go down so it first comes up. If I lift the rubber seal of the pop-up with my finger the water bubbles with air as it slowly drains. Once the air is released it seems to drain somewhat better but still slower than it should.

 There is no leakage of water from the drainpipes inside the vanity. It all appears normal.    


Thank you for any advice you can offer.


Plumbing fixtures within homes are designed with drains, which can often be seen beneath the fixtures. This is only one half of the system, the other half often hidden in walls of the home. The second part of the system is the venting, which terminates in an open-ended pipe above the roof of the home. We will explore both parts of the system to try and determine the cause of your slow draining sink.

When drains are slow or sluggish, the first place normally checked is the drain or trap, for debris, which may partially clog the drain. This debris is often hair and soap that stick to the drainpipes and reduce the effective size of the drain. This is more of a concern when there is a mechanical drain, such as in your sink. The mechanical drain components often get coated with soap and hair often wraps around the working parts within the drain. The drain stopper should be removed and the area cleared of debris and some regular drain cleaner poured in to help clear the pipes. Once this is done, the mechanical drain should be reinstalled and checked for proper operation.

One of the most common causes of slow draining sinks is improperly adjusted or poor working mechanical drains. When opened, the pop-up section of the drain stopper should protrude above the sink drain high enough to allow water and debris to drain properly. This can be tested, to see if this is the cause of the problem, by filling the sink and manually removing the stopper. If the sink drains properly, and the bubbling you describe is not present, then a simple adjustment is all that may be required. Many homeowners are not aware that mechanical drains can be easily adjusted to allow better operation. This is normally a simple operation accomplished by moving the rod from the mechanical sink drain to a different hole in the bracket beneath the sink or adjusting a set screw on a clamp on the rod. Many mechanical drain stoppers also have an adjustment bolt underneath that allows fine adjustment of the height of the plug. This can be adjusted, simply, by removing the stopper and changing the depth of the bolt with a small wrench.

If these drain adjustments do not correct your situation, then inadequate or missing drain venting is the likely culprit. As previously mentioned, the venting is the unseen part of the drain system, but is critical to proper operation. The vents have a dual function.

Firstly, the vents allow sewer gas to escape the plumbing system through the top of the pipe, above the roof. Secondly, they allow air to enter the top of the drain piping, which helps to force the water and waste down the drain. This can be easily understood by anyone who has tried to pour liquid from a sealed can or container. If the container has a small opening at one end only, the liquid will come out in uneven spurts with much bubbling and effort. If a small hole is punched in the other end of the container, the pouring becomes smooth and regular. This is the same principal in use. The overflow hole in your sink may be bubbling and partially filling because it is trying to act like the vent for the sink. This may help the flow, but is not a substitute for a properly vented drain. Look under your sink and you should see the sink tailpiece dropping from the bottom of the sink, running into a P-shaped trap and disappearing into the wall behind the vanity. If the drain trap is an S-shape, with the drain exiting through the floor of the vanity, then proper venting is not present.

This is a commonly found problem during the course of a normal home inspection and is the result of amateurish sink installation. Many home handymen and handywomen don’t understand the need for proper venting of plumbing fixtures and will omit the proper connection to the vent when a fixture is replaced or removed. This may be easily rectified, if the old vent is still installed and capped behind the vanity. If the vent is not present, due to relocation of the sink, installing a proper vent may be difficult.

One solution, if traditional vent installation is not practical, is to install a mechanical vent under the sink. The older style of these small devices are commonly called “cheater vents” and do not meet current building codes. Today, there are approved mechanical vents that may be easily installed under sinks where connection to the main venting is not possible. Contact a licensed plumber for information and prices on installation.




P.O. Box 69021
#110-2025 Corydon Ave
Winnipeg, MB
R3P 2G9