Thaw Frozen Septic Line

Thaw a Frozen Septic Line

Late last Friday night we get to the cabin and find the toilet does not flush. Weird… it can’t be clogged, the flush was just water. Hmmm… I try running the sink and hear and see bubbles coming up through the toilet water.

Sh_t! The septic line is blocked — probably frozen! Ouch, over the years living in central Minnesota, I have heard of the nightmare that is a frozen septic system, but lucky had never yet experienced one. Until now.

Although we have owned the cabin for a few years now and never had a problem with the septic in the winter. But this year it was really cold. And, in addition to the cold, this year we have very little snow.

It was the perfect setup for frozen failure — a very cold winter, little snow cover or natural ground insulation and a relatively shallow system that we parked cars over. We made all the mistakes, and mother nature supplied the right ingredients.

Learn from our mistakes and avoid or fix the common risk factors for a frozen septic system:

Risks factors for a frozen septic system

  • Septic line too shallow – line installed above or too close to frost level
  • Septic line below compacted soil (driveways, paths) – tend to freeze deeper
  • Snow cover inadequate or compacted – loss of insulation effect of snow
  • Lack of vegetation or grass cover – vegetation acts as insulation for soil
  • Infrequent use of system – use of system flushes pipes and adds heat to system
  • Constant low volume water source (furnace condensation drainage, leaking faucet) – provides water to build ice without benefit of flushing system
Frozen septic line.

Frozen septic system line, cover removed ready to thaw.

Now what? Thaw it out!

OK, before you jump into this, seriously consider calling a professional who specializes in septic systems. The first time my system froze, I did just that. For $250 the problem was fixed in 15 minutes. So, like anything it’s risk vs. reward. If you like the idea of staying clean and warm in your home stop reading now and find the phone. If you still want to try this yourself, find some really old clothes and read on …


How to Thaw a Frozen Septic Line – Preparation

Preparation and Materials

Project Overview

  • Level:Basic
  • Time: Hours
  • Cost: $50

Project Big Picture

  1. Locate septic holding tank.
  2. Open septic tank cover.
  3. Pass hose into septic line.
  4. Run warm water to thaw line.
Materials – Water Heater and Plumbing
Item What I used Cost
Garden Hose Ace 50 ft Heavy Duty Garden Hose $45
Hose Nozzle Ace 6 in. Brass Water Jet Hose Nozzle $10
Anti Back Flow Valve Back Flow Preventer varies
  • Pry bar – lift cover
  • Shovel – clear snow, dig out cover
  • Old gloves – no explanation needed

How to Thaw a Frozen Septic Line – Step by Step

  1. Locate the first access cover of the septic system holding tank.

    Many systems have two access covers (one for the primary or “solid” compartment and the one for the secondary or “liquid” compartment). We are looking for the cover where the septic line from the house enters the holding tank (typically the cover closest to the house).

    Frozen septic system tank cover after digging out with cover lifted.

  2. Open access cover.

    This often requires a pry bar or crow bar to lift the concrete lid from the frozen ground. If the ground is frozen, take some time to trench out the dirt next to the lid.

    Sepic tank cover removed on frozen septic system.

  3. Gather your garden hose and nozzle.

    The nozzle should be longer than the diameter of the pipe (the pipe is typically 4″) to prevent the nozzle from turning in the pipe and therefore keep the stream pointing toward the blockage (Thanks to Nancy for the great tip).

    Non-kink garden hose to thaw frozen septic system.

  4. Find a water source.

    Ideally, you would use a source isolated from your domestic water supply, and therefore be sure the nothing from the septic systems mixes with your domestic water supply. Unfortunately, this may not be a realistic option. If you do use a source of water from your home — a hose faucet or utility faucet — use a back flow prevention valve to keep any water back flow from entering your domestic water supply.

    Most professionals use one of three water sources to clear ice from a frozen septic line: steam, pressurized hot water or high pressure cold water.

    I used a hose fitting connected to my hot water line from my utility room. It has the benefit of being fairly close to the septic tank and offered heated water.  Hot water is not necessary, but will speed up the process of clearing the ice blockage.

    Water source for thawing of the septic line -- use back flow prevention valve.

  5. Next, locate the septic pipe coming from the house.

    Many will have a “T” baffle and enter the tank on the side closest to the house (supply). Here is what mine looks like:

    Flushing frozen septic line - garden hose passed into septic line with water source turned on.

    Once you locate the line, you need to pass the hose, nozzle first, into the septic line so it is facing the blockage (heading back to the house). You may have to bend the hose slightly to get the nozzle into the septic pipe (I used a 6″ nozzle and needed to bend the hose a bit to get it in past the baffle).

  6. Turn on your water source and advance hose.

    Feed the hose into the pipe until you encounter resistance (this should be the ice blockage). Now the nozzle will be spraying water directly at the ice. As the ice melts, you will be able to continue advancing the hose until the ice has melted and you are through the blockage. Depending on the volume of waste water in the septic line behind the blockage, it should be fairly obvious when the ice has cleared. In my case a the volume of water returning into the septic tank increased dramatically and the water was soapy with white suds. If you are but not sure if the line has cleared, you can remove the hose (leave the water on until you are out of the septic line to prevent backflow) and have someone in the house run some water (with soap if it will help you identify the water) while you monitor for the waste water to flow into the septic tank.

  7. Remove the hose before turning off the water supply.

    Removing the hose prior to turning off the water supply will prevent back flow into the hose. Replace the septic cover and clean your tools and hose. To clean the outside of the garden hose, I pulled the entire length of hose through a handful of Chlorox disinfecting wipes several times and finished by pulling it through damp paper towels.

  8. Fix the underlying problems causing line freeze.

    After you have successfully thawed the frozen line, you should try to identify the underlying cause of your system freeze and make appropriate repairs. There are many excellent resources available online and I have listed a few below

Preventing A Frozen Septic Line

  • Ensure adequate natural insulation over pipe run.
    • Do not remove or compress snow over septic area (do not drive over or plow over septic system), snow has an r-value of 1 or more per inch of snow (12″ of snow = R12+).
    • Add layer of straw (R1.5 per inch) or wood mulch (R1 per inch) over pipe run and other areas of septic system or plant grass, vegetation in bare dirt areas over septic system
    • Avoid compressing dirt over septic line (cars, atvs, etc) as compressed ground freezes deeper.
  • Add insulation over and around septic system / line.
    • Add 2 – 4″ of rigid foam insulation around septic lines, and over holding tank with over lapping edges.
  • Fix or Avoid continuous, low-flow water sources emptying into septic line.
    • Fix leaking faucets, fixtures
    • Avoid using system to drain furnace condensation water.
  • Use the system regularly during cold months.
    • Regular use flushes the system and add heat to the system
    • Normal bacterial activity generates heat in holding tank.[/box]

Frozen Septic System Thaw Image Gallery

Additional Resources


  • Bruce December 7, 2013   Reply →

    Here is one more possible root cause of a frozen septic line… the ground has settled, and the line has a sag in it. The water that does not immediately drain can freeze in place if it sits in the line. You need to dig up the line and re-establish a continuous slope.

    • Cabin DIY December 8, 2013   Reply →

      Hi Bruce.

      Good point. In the past we would drive over this area with cars. I am certain the ground over the septic line has been compressed and likely there is some sag. I plan to insulate the pipe and will check the slope at the same time. Thanks!

  • George Ross February 16, 2014   Reply →

    Your advice saved me a hefty plumbing bill! At zero degrees F. There was n way i could dig up the septic without hiring a backhoe. The plugged line goes from the house into a crawl space. So i disconnected the line where it goes out of the crawl space. I put the hose in and gradually fed it about 10 feet or more. That did the trick. The excess water just pouring into the sand n the crawl space and disappeared. This was pure plumbing magic. Thanks. Cause i had no idea what to do!! Took me all of ten minutes once the tools were assembled and ready.

    • Cabin DIY February 16, 2014   Reply →

      Hey George,

      Awesome! Glad it worked. It sounds like you reversed the process and entered the soil line in the crawl space. The obvious down side to doing it this way is the emptying of the water / frozen pipe contents into your crawl space, but it sounds like you know what you were getting into doing it this way.

  • David February 19, 2014   Reply →

    Thank you for your writeup. I modified it a little bit. I was concerned about running hot water through a garden hose, but the hot water (red) hoses from the hardware store didn’t seem stiff enough to push into the pipe.

    I got a red hose and a 20′ length of PEX, as well as a PEX crimper, and garden hose to PEX adapters. The PEX worked very well for feeding into the pipe. Thanks again!

    • Cabin DIY February 20, 2014   Reply →


      Nice modification! You are right about the hot water and garden hose issue — the hose I used got fairly soft when running hot water through it. Glad it worked.

  • Maria April 26, 2014   Reply →

    Hi there, does anyone know why a line would break in the same area, two winters in a row? The area that was repaired was fine, it broke again the following winter down the line, just a foot ahead. Any ideas?

    • Cabin DIY April 27, 2014   Reply →

      Hi Maria,

      Sorry to hear about the septic line breakage.

      For the pipe to break, some force is being applied to the line in that region of the pipe run. It could be caused by compression from above (vehicles or other force on the ground over the area), movement (home settling relative to the line run, unequal ground freeze with shifting) or expansion within the pipe (pipe freeze).

      If the pipe is iron or clay, it could be just deterioration in this area — I assume its plastic and this is not the cause.

      If the pipe is plastic and breaking, some force is acting on the pipe. Consider the soil conditions, the support under and over the pipe and any force from above (cars, etc). If this is an area of traffic (driveway, etc), you probably will need a way to protect the pipe from the loads over the ground in that area. If the area over the pipe is undisturbed, think of reasons the pipe could be moving.

      Let us know what you find. Upload a photo or two if you have some.


  • Kirk May 13, 2014   Reply →

    Septic in Grand Rapids Minnesota area froze up. Has happened three times in last 10 years. Fairly easily fixed with conventional method (tank heater). Last fall added 18 inches of dirt plus foam plus straw. Bitterly cold winter so didn’t visit cabin much. Septic froze around the end of February Almost mid-May and line still frozen! Cover moved first of May. Super-insulating the pipe insured that once frozen it will stay frozen! May try your approach. Where is global warming when we need it?

    • Cabin DIY May 13, 2014   Reply →

      Wow, still frozen, that’s amazing. I assume your system is infrequently used during the winter and therefore you are not routinely adding heat to the pipe and septic system, making the insulation less of a factor.

      You could investigate wrapping the line with a heating tape or system designed to be safe with the pipe you are using (PVC, etc). I’m not familiar enough with these products to make a recommendation, but some of the newer tapes offer thermostatic temperature control and are low enough temperature to be used with plastic pipe. I’m not sure of there are products specifically designed for septic lines. From my reading, I would not recommend adding a heating tape to the inside of the line because of the risk of plugging within the line.

      Good luck. Let us know if you find a good solution.


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