Q: We just bought a house with a 20-year-old septic system. It’s a basic gravity system with a leach field. We had the system inspected before buying and they told us that everything “looked good.” The sellers did not keep records, but said they had the tank pumped “a few times”.
How many years can we expect to get out this system before it needs to be repaired or replaced? How much should we budget for this project? — John. B
A: The average lifespan of a conventional septic system is 20 to 30 years. The 20- to 30-year life span, commonly cited in the industry, is for systems that were properly designed and built, well-maintained, and not overloaded. However, there are many factors that can reduce or increase a system’s useful life beyond that range.
I recently discussed the issue with a sanitary engineer who has designed septic systems for more than 40 years. He said that he typically starts to hear from his old customers after about 20 years when they first experience drain-field problems. He has also seen systems last 40 or more years, but these are the exception. He emphasized that it is impossible to predict the lifespan of an individual system. There are just too many variables.
Don’t Forget Maintenance
The first component to fail in a septic system is usually the leach field (drain field). The drain field is sized based on the number of bedrooms, assuming two people per bedroom. So a three-bedroom drain field is sized for six people.
All things being equal, a lightly used drain field will outlast one that gets heavy use. If only two people use a three-bedroom system, use low-flow fixtures and appliances, and pump the system regularly, it should have a long life span. A system that is only used seasonally should also have a long life. Dumping chemicals, grease, and food scraps down the drain will shorten a system’s life. A garbage disposal should not be used unless the system is designed for one.
The other major component is the septic tank. A well-built concrete tank should last at least 40 years. Steel tanks tend to fail in 20 to 30 years and good-quality plastic tanks may last from 30-40 years.
Many factors affect a system’s longevity. Some, such as the design and installation of the system, and the soil type, are out of your control. Others, like proper care and maintenance are fully under the homeowner’s control. Proper maintenance and care of your system can add years or decades to its service life.
The most important factors that the homeowner can control are regular pumping, household water conservation, and watching what they flush down the drain — no harsh chemicals, paints, grease, food scraps or other solids.
It’s also important to protect the drain field area from damage. Direct drainage of yard and roof water away from the drain field so the soil is not saturated. Do not drive or park over the field, or use it in any way that will compress the soil. And keep trees and large shrubs a safe distance away as the roots can clog the perforated drain pipes. Grass is the best ground cover.
In most cases, drain field failure happens gradually as the soil around the leaching trenches gets blocked by the naturally forming”biomat” and clogged with solids and grease that spilled out of the septic tank (due to high-volume water usage and inadequate pumping).
The first signs may be sluggish drainage, backups on the lower floors of the home, or soggy areas over the leach field with a whiff of sewage.
If the tank is in good condition and you have a designated area for a replacement drain field, as required in some jurisdictions, then the cost for a new drain field will typically range from $3,000-$10,000, depending on size and local costs for labor and gravel.
A new concrete tank will typically cost from $3,000 to $6,000 installed. If you need a completely new system, the cost can easily exceed $15,000 or twice that if you need to install an alternative septic system.
Once you start using your replacement drain field, the original field will have time to naturally recover, and should be ready to use when needed.
New Perc Test?
Many towns will make you conduct a new perc test and deep-hole test before issuing you a permit to replace your existing leach field or entire septic system. In most cases, if the site passed the perc test in the past, it will pass again. However, this is not always the case as site conditions may have changed (e.g., higher water table), or the town’s test procedures and standards may have changed. You could need to upgrade to a more expensive type of “alternative” septic system than you originally used.
Bottom line: It’s always cheaper to take good care of your current system and extend its life than to ignore it and let it fail.
— Steve Bliss, BuildingAdvisor.com
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