Septic systems in winter: preventing and dealing with freeze-ups

In cold climates, a frozen septic system is one of the most frustrating winter problems a homeowner can face. Drains stop working, and fixing it in frozen ground is hard. The good news is that most freeze-ups are preventable. This guide draws on the Minnesota Pollution Control Agency (MPCA) and the University of Minnesota's Onsite Sewage Treatment Program, which deal with some of the harshest septic winters in the country.

Why septic systems freeze

Wastewater leaving a house is warm, and deep soil holds geothermal heat. Normally that's enough to keep a septic system working all winter. The University of Minnesota program identifies the common reasons it isn't:

• Little snow cover. Snow is an insulating blanket. Without it, frost goes deeper and can reach pipes, the tank or the drainfield.
• Compacted snow. Driving cars, ATVs or snowmobiles over the system, or even repeated foot traffic, compacts snow so it insulates less and drives frost deeper.
• Low or no use. In cabins left empty or homes with only one or two people, not enough warm water flows to keep the system from freezing.
• Pipes without proper slope. Pipes installed without enough fall, or that settled or sagged, let sewage collect in low spots and freeze. Pump lines can develop a dip near the tank where backfill settled.
• Leaky fixtures. A dripping faucet or running toilet sends a trickle of water that can freeze in the line, instead of an occasional warm surge.
• Compacted soil. Driveways, paths and livestock areas freeze deeper, so any pipe or tank under them is at higher risk.
• Small, steady drips from appliances. High-efficiency furnaces and humidifiers can send a thin trickle down the drain that freezes the same way a leaky faucet does.
• Open or broken risers and lids. Uncapped inspection pipes and cracked manhole covers let cold air straight into the system.
• A system that's already failing. If effluent is surfacing or seeping out the side of a mound, the University of Minnesota calls it a prime candidate to freeze, because that wet effluent freezes and blocks everything behind it.
• New systems with bare soil. Systems installed late in the year without established vegetation are especially vulnerable in their first winter.

Preparing in the fall

The MPCA and University of Minnesota recommend:

1. Mulch the system. Place 8 to 12 inches of loose material, such as straw, hay or leaves, over the pipes, tank and drainfield. It must stay in place and not compact. Remove it in spring so grass can grow. Don't add mulch if the system is already frozen, because it will delay thawing in spring.
2. Let the grass grow longer over the tank and drainfield in late summer and fall for extra insulation.
3. Cover and insulate risers, inspection pipes and manholes. Make sure every one has a tight cover so cold air can't get in. Insulating the lids helps.
4. Fix leaky fixtures before winter.
5. Keep traffic off the system. A snow fence around the drainfield discourages vehicles and traps snow for extra insulation.
6. Have the tank pumped before winter if it's due, not during freezing months, unless it's an emergency.

During cold snaps

If you're worried the system is starting to freeze, the MPCA says to use water, the warmer the better. Run one warm or hot laundry load a day, use the dishwasher and take hot baths. This is a rare case where septic experts encourage normal water use instead of conservation. Don't leave water running continuously, though. That overloads the system and can make things worse.

If you'll be away

Unused systems are at higher risk. Before leaving for an extended period:

• ask someone to run warm water through the house periodically, if practical;
• make sure insulation and snow cover are in place;
• consider having the tank pumped before you leave if your provider recommends it. In high water table areas, ask first, because an empty tank can be pushed up by groundwater.

If your system freezes

The MPCA's advice:

1. Call a septic professional. They can send cameras down pipes to find where freezing occurred, and use heat tape, tank heaters, steamers or high-pressure jetters to thaw lines.
2. If it can't be thawed, you may need to use the septic tank as a holding tank until spring. Have a licensed maintainer pump it when it starts to fill.
3. Cut water use sharply in the meantime: fewer toilet flushes, short showers, and only full dishwasher loads.

After spring thaw, find out why and where the system froze so you can fix the cause, whether that's a sagging pipe, missing insulation or a leaky fixture.

Never do these

The MPCA is clear about what not to try:

• Don't add antifreeze, salt or septic additives to the system.
• Don't pump sewage onto the ground surface.
• Don't build a fire over the system to thaw it.
• Don't run water continuously to try to unfreeze it.

Long-term fixes for repeat freeze-ups

The University of Minnesota suggests adding more insulation: insulated pipe, foam board over septic tanks, or more soil cover. Where traffic over a sewer pipe, tank or drainfield is unavoidable, use insulated pipe. A septic professional can also correct pipe slope problems. Don't place insulation sheets permanently over a drainfield, though. They limit oxygen transfer and evaporation, which the soil needs to treat wastewater.

Frequently asked questions

My drains are slow in winter. Is it frozen? Possibly, but slow drains can also mean a full tank or a failing system. Have a professional check. See signs your septic system is failing.

Do septic systems in mild climates freeze? Rarely, but an unusual cold snap with no snow cover can affect shallow pipes anywhere.

I live alone. Is my system more likely to freeze? It can be. The University of Minnesota points out that one- or two-person homes may use only a small share of the flow a system was designed for, and that low, slow flow may not keep it warm enough. Spacing out warm loads of laundry and dishes helps.

Is it safe to plow snow over the drainfield? Avoid it. Plow traffic compacts snow, and piling plowed snow can add weight. Keep vehicles off the system all year.

Key takeaways

• Snow and loose mulch are your best insulation, so keep both uncompacted.
• Fix leaks, cover risers and keep traffic off the system.
• Use warm water normally during cold snaps, but never leave it running.
• If the system freezes, call a professional. Never use antifreeze, salt or fire.