Close Up of Septic Tank Cover / BlakeDavidTaylor

As they age, septic tanks can sustain damage that isn’t immediately obvious, but that can still cause issues such as sewage backups or water supply contamination. A septic dye test makes it easy to spot major problems without using more invasive inspection methods. Although common, a dye test is far from exhaustive, so make sure you understand the limitations of this test before you schedule one.

What a Septic Dye Test Can Tell You

Question Marks / GOCMEN

A septic dye test is one of the simplest methods of pinpointing issues with the septic system. It’s most often used to check for leakage of septic effluent (liquid sewage) due to broken pipes or incorrect installation, but can also be used to verify that a new household appliance or drain is correctly connected to the septic tank system.

The inspector who performs the test only needs to put dye into the septic system, then wait to see if it shows up above ground. If it does, you know you have a problem somewhere. Many certified home inspectors offer the test as an add-on service, but in some jurisdictions, you might need to hire a certified septic inspector.

While you can have the test done for your own septic system if you suspect there’s a problem, it’s more often done as part of a home inspection to provide the prospective buyer with some basic assurance that the system is running correctly. Because incorrect installation can cause malfunctions, even brand new septic systems should be inspected.

As popular as the septic dye test method is, its usefulness is highly limited. Dye test failures are rare, and when a system does fail, it’s usually already showing signs of leaks or clogs, such as backed up toilets and drains, sewage odors, and standing water around the tank and drain field.

It’s also possible to get inaccurate results by using too little or too much water. Recent rain or snow, a layer of leaves, and tall grass can obscure any dyed effluent on the soil surface so much neither you nor the inspector notice it. What’s more, even if no dyed effluent appears, that doesn’t conclusively prove the system is working optimally.

Most septic systems need to be pumped every three to five years, and they’re usually inspected at the same time. If you’re looking to buy a house where the septic system was recently pumped and inspected, ask the owner to show you the invoice. This should list any issues found or repairs made. If no invoice or other report is available, consider holding off on the dye test until the system contains the amount of water needed to test the drain field effectively.

A septic dye test should satisfy your home loan lender, but if you want to know the system’s true condition, you’ll need a full septic tank inspection. If you’re serious about buying the house, ask the owner if they’d be willing to have the system pumped and inspected at a cost you can both agree on.

The Septic Dye Test Process

Blue Dye in Water / AlekZotoff

A septic dye test is done by adding dye, usually green or red, to the septic system, flushing water into the system, then waiting to see if the dye appears anywhere above ground. The dye makes any escaping effluent visible and traceable. If there’s a problem with the system, the dye might show up in the drain field, your yard, or a nearby waterway.

The home inspector starts by determining the capacity of the septic tank, then calculating the amount of dye required to color that amount of water. Next, they’ll determine the water flow rate in gallons per minute from the source they’ll be using to add water to the tank. With this information, they can calculate how long to let the water run to fill the septic tank.

If the goal is to test for escaping septic effluent from any source in the house, the inspector will either add the dye tablets directly to the septic tank and add water from a garden hose or flush the dye down a toilet and run a faucet. If you’re trying to verify that a household appliance or drain is connected to the septic system, the inspector will add dye directly to that appliance or drain.

They’ll then run the water for around 10 to 15 minutes to push the dye through the septic system, then into the drain field. Exactly how long the water should run depends on the size of the tank and the faucet’s water flow.  

If a pipe, inlet, outlet, or other part of the system is damaged, the dyed effluent might escape and travel to the soil surface at the point of damage. If the drain field is clogged, dye might show up there or nearby. Again, though, just because damage exists, that’s no guarantee the dye will show it.

In most cases where dye appears, it surfaces within 15 minutes to several hours. In rare cases, it can appear in nearby waterways as much as five days later. The inspector or health department regulations determine how long to wait, but this should never be less than three hours. Some inspectors prefer to check back several days later.

Dye appearing on the soil surface means the system has failed the test. If you’re considering buying the house, a failed dye test can affect your ability to secure a loan unless you have a plan for replacement. If it’s your house, you’ll need to consult a septic tank company for advice on what to do next.

A septic dye test alone isn’t enough to prove a septic system is working, but it can show you if any major issues are present and help you secure a home loan. If you’re considering having a system dye tested, work with a home or septic system inspector who’s trained in performing the test and interpreting the results.

Editorial Contributors
Henry Parker

Henry Parker

Henry Parker is a home improvement enthusiast who loves to share his passion and expertise with others. He writes on a variety of topics, such as painting, flooring, windows, and lawn care, to help homeowners make informed decisions and achieve their desired results. Henry strives to write high quality guides and reviews that are easy to understand and practical to follow. Whether you are looking for the best electric riding lawn mower, the easiest way to remove paint from flooring, or the signs of a bad tile job, Henry has you covered with his insightful and honest articles. Henry lives in Florida with his wife and two kids, and enjoys spending his free time on DIY projects around the house. You can find some of his work on Today’s Homeowner, where he is a regular contributor.

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