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What Causes a Slow Drain?


Slow Drains Can Be a Simple Fix… or a Sign of a More Serious Problem

It's a kind of plumbing problem that can sneak up on you. Not too bad at first, so maybe you ignore it. But each day it gets a little bit worse... until you find yourself brushing your teeth over a sink filling up with toothpaste foam or showering ankle-deep in dirty water. It's not a fully clogged drain, yet, but it's draining slowly and getting slower. If you ignore it too long, it could become a more serious problem.

What's slowing down our drains here in Denver? What can you do to prevent slow drains? And how can you get your drains flowing smoothly again?

If you’re trying to figure out what’s causing your slow drain, answer this important question first: Do you have only one slow drain or many?

One Slow Drain Could Be a Simple Fix

A single slow drain is likely caused by a partial clog somewhere along that drain. In some cases, you may be able to fix it yourself.

Bathroom Sinks, Showers, and Tubs

We send a lot down the drains of our bathroom sinks, showers, and tubs. Hair can get caught in the sink stopper or clog up the P-trap (the curved section of pipe below the drain). Shaving cream and beard trimmings can form a sticky cement. And soap scum can build up along the inside surfaces of your drain, constricting the flow.


There are only three things you should flush down your toilet: water, toilet paper, and human waste. Maybe the invitingly large size of the toilet drain is to blame, but people flush all kinds of materials that can clog their toilet drains. If any of the following have gone down your toilet, they may be the source of your slow drain problem:

  • Wet wipes
  • Cat litter
  • Feminine hygiene products
  • Dental floss, cotton swabs, and bandages
  • Cigarette butts
  • Excessive amounts of toilet paper

Kitchen Sinks and Dishwashers

The food debris we send down our kitchen sinks and dishwashers may be biodegradable, but it can hang around a long time in your drain.

Grease, oils, and fats cause the most common clogs. They stick together and to the insides of your drain, slowing and eventually fully stopping water flow.

Food particles may get bound up with grease, worsening the clog. And both coffee grounds and eggshells can accumulate as a sediment at the bottom of your P-trap.

Clothes Washers

It’s obvious in your dryer, where fibers from your clothes gather in the lint trap with every load. But fibers release in the wash cycle too and head down the clothes washer drain. Sometimes these fibers can cling together, just as in the dryer lint trap, and form clogs inside the drain.

Fixes for Single Slow Drains

Because the clogs and buildups that cause single slow drains are usually easier to access, you may be able to fix some problems yourself. You can remove hair from your bathroom sink stopper. And a good, old-fashioned plunger can often clear smaller clogs.

Avoid using harsh chemical drain cleaners, many of which can damage your plumbing.

Multiple Slow Drains Suggests a More Serious Problem

If you have several slow drains, there may be a problem deeper in your home’s system of drain and vent lines, in your main sewer line, or in your septic system.

Broken or backed up sewer or drain lines can badly damage your home with anything from flooding to foundation problems. It’s unlikely to be something you can fix yourself, and you definitely don’t want to delay. Call a plumber promptly to diagnose the problem.

Drain Pipes

Your home has a network of drain pipes leading from each drain to the main sewer line. All the materials that cause single slow drains can work their way into the drain pipes and cause similar constrictions and blockages that impact many drains.

In homes with hard water and galvanized steel drain pipes (used in home construction until the mid-1970s), limescale can build up inside drain pipes, gradually restricting flow.

Vent Pipes

Vent pipes maintain neutral air pressure in your drain pipe system. As water flows down drain pipes, it compresses air ahead of it and creates a partial vacuum behind it. Vent pipes allow fresh air to flow in from outside, filling the vacuum and equalizing air pressure. Without this free air flow, the unequal air pressure will slow down drainage and can cause more serious problems too.

During our Denver winters, heavy snowfall can obstruct vent pipes. Bird nests in the spring and leaves in the fall can also block airflow. Inside the house, vent pipes connect to drain pipes where they can be obstructed by the same materials blocking your drains.

Main Sewer Line

All of your home’s drain pipes transfer wastewater to your home’s main sewer line, which carries it to your public sewer system or septic tank. Everything that goes down any household drain passes through the main sewer line. So, all the potential problems mentioned above can also clog your main sewer line.

Because your main sewer line continues outside of your house and underground, a settling foundation can partially collapse the sewer line pipe. Tree roots can pierce it and obstruct flow. Cracks and leaks in the pipe can let in dirt and other debris.

Septic System

If your home has a septic system, a wide variety of problems can cause sludge to back up into your main sewer line. This will cause slow drains throughout your home and can cause more serious problems too.

Fixes for Multiple Slow Drains

Because the causes of multiple slow drains are usually located in less accessible areas of your plumbing system, you’ll need a qualified plumber to fix the problem.

If the drain pipes or main sewer line are clogged, they may be able to clear them with a professional-grade machine auger or water jet.

If vent pipes are blocked, they’ll clear the obstruction.

If the main sewer line is damaged, they may be able to repair it with one of several no-dig solutions, or, if necessary, with a traditional trench dig repair.

And if your septic system is backing up, they’ll get it flowing smoothly again.

Don’t Ignore Your Slow Drain

A slow drain is more than an annoyance. It’s an early warning sign that your drainage system is at risk of more serious problems.

Fix single slow drains promptly before the materials clogging them work their way deeper down your drainage system. And if you already have multiple slow drains, call in a plumber promptly before the problem gets worse. Addressing the problem now can prevent more costly problems later.

Call Denver's Drain Cleaning Experts

Plumbline has decades of experience and can unclog the drain in your Denver area home. If you're in the Denver metro area and are experiencing issues with any drains in your home, give us a call today. Our drain and plumbing experts are ready to help. We have have been clearing drains in Denver, Centennial, Arvada, AuroraLongmont, Littleton, Broomfield, Thornton, and the Denver metro area since 1998.

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