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What is Negative Slope Drain & Its impact on drain flow ?

A Negative slope drain occurs when the angle of the drain pipe is not steep enough to allow water to flow freely. This can lead to water pooling, flooding, and other drainage issues.
This issue is mainly seen in underground sewer drains.

The main cause of the negative drain slope is improper rough-in and installation at the beginning.

Negative Drainage is one of those problems that you cannot see on the surface level. You need to know all your home’s plumbing fixtures and how they work to locate the problem firsthand. 


Everest Plumbing Jun 04, 2024
Negative Slope Drain - a white pipe with blue handle
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Common Causes of Negative Drain Slope

  • Improper Rough-in :
    This is one of the major and common causes of negative drainage.  if the drain is not laid out with the proper angle and with appropriate support of gravel underneath,  it can lead to slope issues. Drain pipe must have at least 1/4 inch per running foot slope according to National plumbing codes.
  • The poor foundation the pipe is laid on:
    Since the drain pipes are underground, you need to ensure that the pipes have a steady base because fixing them later can be a long and tedious job.
    The pipes need to be laid on gravel and granular materials that are at least 10 mm deep. Some of the pipes are laid on a trench. Consult with your plumber to see what is the best option for you.
  • Settling or shifting soil:
    The land is very unpredictable. For many different reasons, over time the ground and soil that the pipes are laid on can slide or lead to changes in landscaping’s natural slope. When this happens, very slowly but eventually, the drain pipe will start to slope negatively.
  • Tree Roots :
    Another cause of negative slope drain is Tree roots. Roots can easily penetrate the underground drainage pipe over time mainly when clay drain pipe is used. The rapid growth of tree roots over time can shift the pipe or misalign which ultimately contributes to a negative slope. It’s important to keep note of major trees and drain lines around the property to overcome future possible negative slope issue.
  • Debris Buildup: 
    Over time, debris such as grease, leaves, twigs, and any other organic matter can accumulate in your drainage system. This can increase the weight of the pipe leading to Drain collapse at a weak point. When debris clogs the drain pipe, water cannot flow freely, causing it to back up and pool in unwanted areas.

Understanding the common causes of negative drain slope is the first step in fixing the problem. By identifying which factors are contributing to the issue on your property, you can then implement the appropriate solutions to restore proper drainage.

What are the effects of a negative slope on the drainage system?

  • Slowing down the water drainage.

    The drain pipes are elevated ¼ inches per foot of the pipe. This is for the optimal flow of the waste into the sewage. But when the pipe starts slanting, it affects the speed of the flow. So if you notice that your fixture is taking a while to drain, this could be a sign of a negative slope of the pipe.

  • Possibility of the Multiple time Drains getting clogged.

    The negative slope can worsen over time. It tends cause also, cause backflow. This backflow can build up and solidify over time. This could lead to clogs in your drain pipes. This could become a health hazard and disturb your everyday routine. 

  • Your drainage pipe could potentially collapse.

    If the buildup and clogs are not taken care of in time, the pipe could be loaded. There is only so much negative pressure your drain pipe can take. At some point, your pipe will give away and collapse. This will create a big mess and cause major inconvenience for you and the people around you.

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Identifying Negative Slope Drain on your property.

Identifying the negative drain slope on your property is crucial in addressing the issue and implementing the right solutions. Here are a few signs that can help you identify whether you have a negative drain slope:

  • Water pooling: if you observe water pooling or backing up time and again in the same area or different then there is a chance of negative slope drain.
  • Erosion or soil shifting:  Another sign to look out for is soil shifting and landscaping changes. it’s important to keep an eye on the drain line if you are performing any landscaping work or renovation work. If you observe places where the soil seems to be eroding or shifting, it could be a sign that water is not flowing properly due to a negative drain slope.
  • Wet or Damp Basements: Wet or Damp basement is another crucial sign to look for a negative slope. consistent moisture or water seepage in the basement area is another sign of a negative slope drain.
  • Unpleasant odor: Water pooling and improper drainage can lead to stagnant water and the growth of bacteria, resulting in unpleasant odors. If you notice foul smells in certain areas of your property, it may be due to a negative drain slope.

Solution To Fix Negative Slope Drainage.

As difficult as the adverse effects of a negative slope are, you can always take care of it. Usually, there is no DIY solution to fix a negative slope drain. It required advanced equipment and years of expertise to realign the drain and catch the missed slope.  we highly recommend hiring a professional. Here are the steps that professionals follow for underground negative slope drains:

  • Video camera to inspect the negative slope.

    A plumbing professional will use a video camera to inspect the drain pipe. The camera is attached to a flexible cable. This makes the camera mobile enough to go into the drain pipes to figure out what the problem is. These cameras are so advanced they can even locate where exactly the drain starts getting a negative slope and by how many degrees. In this case, it helps you check how slanted your drain pipe is.

  • Find the spot where the slope starts.

    It would be expensive to completely dig out the entire drainage system just to see what part has the negative slope. For this reason, the professional will locate the area where the slope begins using the video camera. Then, they will start working on it. This will save both the parties time and money.

  • Digging out & Replacement of the pipe.

    There is no other way than digging out the place to fix the negative unless running a new drain line using Trenchless Technology. With the help of proper leveling equipment, the new pipe can be replaced in the section of the negative slope. Your appointed plumber will let you know if you need to work on the foundation the pipe is on or if just replacing the pipe is enough.

  • Fixing the pipe leveled as per the code.

    The ideal downward slope of the drain pipes is ¼ inch per foot of pipe. It’s important to use sand gravel underneath the pipe to have a proper slope and smooth seating of the pipe. You can consult a city official just to be sure about the slope requirement for the drain or you could hire a skilled and licensed professional to help you with the information and the installation.

Underground Drain Line Replacement

Conclusion and Final Thoughts

A negative drain slope can be a frustrating and inconvenient issue, causing water pooling, flooding, and other drainage problems. However, by understanding the common causes of negative drain slopes and implementing the appropriate solutions, you can restore your drainage system’s functionality and prevent future complications.

Preventive measures such as proper installation, regular maintenance, careful tree placement, appropriate landscape grading, and monitoring soil conditions can help minimize the risk of a negative drain slope in the future.

You must be curious, how much does it cost to hire a professional to fix negative slope drainage? Don’t worry we will explain you in detail.

While the cost of fixing a negative drain slope may vary depending on the severity of the issue and the chosen solution, investing in proper drainage maintenance and repair is essential for the long-term health and functionality of your property. Negative slope repair falls under the same category of underground drain line replacement like changing clay pipe to PVC drain. In a city like Toronto, it costs Approx. $550.00 – $2200.00 per ft. includes excavation, digging, trenching, backfilling & grading with material included. price range varies depending upon areas like inside the outside or outside the house and, a depth that needs to be dug out. This is the reason we recommend using professionals like Everest Plumbing a local plumber in Toronto or your local pros. to help you out with these Underground drain replacements.

Experiencing a negative slope,  Call us now for a Free Consultation and Quote at 416-383-7378  or Email our pros will respond to you back in no time.

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