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This article explains how to prevent frozen drain piping and freeze-damage or burst drain pipes in buildings and in the building connection to a septic system or to a municipal sewer line.

The articles at this website will answer most questions about freeze protection for piping and other building plumbing and heating system components: how to winterize a building to avoid frozen pipes, and how to thaw frozen water supply & drain piping, wells, & water tanks.

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How to Avoid Freeze Damage to Building Drain Piping

Our page top photo shows what might happen if a building drain is clogged, leading to standing wastewater in the pipes which then froze and burst.

Is the building heat going to be left "on" or "off" - the answer determines the extent of freeze-proofing needed.

• If heat is to be left on in a partially winterized building see the procedure at: Winterize - Heat On (continuing below)
• If heat is to be turned off and the building completely winterized, see the procedure at: Winterize- Heat Off

What? My Building Drains Can Freeze Too?

Freeze damage risk in buildings is not confined to water supply piping or hot water heating piping. Building drains can also freeze and break, including fixture traps and building drains. Building drains can freeze inside the building and even outside a drain line that is not below the frost line can easily freeze in some conditions which we outline below. Even if the building water supply piping has not frozen a drain line can freeze. You may first notice the problem when a toilet, sink, or tub simply stops draining. In a building where water is left on at some plumbing fixtures, be sure that there are no leaky supply valves or running toilets. In freezing weather a slow drip or water running slowly into a drain pipe can lead to ice build-up, a frozen or even a frozen and burst drain pipe, or a water backup into the building.

What Causes Building Drain Pipes to Freeze?

• A building main drain or other building drain pipe is exposed to freezing temperatures outdoors
• A building drain is routed through an un-heated crawl space or basement, or an attic or attic knee-wall that is unheated indoors
• A plumbing fixture drain trap extends down into an un-heated crawl space or basement or is in the path of a cold draft even if the crawl space or basement is heated.
• A toilet is running or a faucet dribbles continuously into the drain system
• A water softener or other water treatment equipment is left on and running improperly, perhaps sending a continuing stream of water into the drain system
• A roof gutter drains into the sewer piping (bad), perhaps through a drain line that enters the building (still worse) en route to the sewer connection; clogged gutters that dribble water slowly into the drain system increases the freeze risk.

Handling Roof Gutters that Drain into the Sewer Piping

Some older homes include roof drainage downspouts that are carried to drain pipes which connect to the building sewer drain piping. In some installations the gutter downspouts connect to a drain line inside the building, perhaps running out of the building under the basement floor onwards to a public sewer line.

Such drains can easily freeze in an un-heated home, leading to broken drain lines, frost-heaved basement floors, and severe basement flooding.

We recommend that drain connections between roof gutters and downspouts and the building sewer line be abandoned.

If necessary, install temporary above-ground downspout extensions to conduct roof runoff well away from the building rather than into the sewer line via the basement floor. Incidentally, because the volume of roof runoff sent into a sewer line can overwhelm municipal sewage treatment plants this connection is prohibited in some communities.

What Else to Do to Avoid Frozen Building Drains

• Make sure there are no "running" toilets or faucets or other equipment
• Make sure there are no blocked or clogged drains that will hold standing wastewater (that freezes and breaks the pipes)
• If heat is left off in a building, use a non-toxic antifreeze in plumbing traps
• Find & Fix Water Pipe Freeze-Up Points, including adding insulation and heat in problem areas if necessary

Questions & Answers on Freezing Drain Lines: Cause, Prevention, Cure

Question: What is the required or recommended depth for connecting house drain pipes(such as a bathroom waste pipe) to a septic tank to avoid freezing?

What is the required or recommended depth for connecting house drain pipes(such as a bathroom waste pipe) to a septic tank in New York State in order to avoid freezing?

If a house drain pipe is connected toward the top of a septic tank, is it vulnerable to freezing and, if so, how much soil should cover that pipe to protect it from freezing?

- Jane

Reply: House drains may be above the frost line and depend on proper slope and drainage to avoid freeze-damage.

Plastic drain pipes are usually buried at least 18" and bedded in sand to avoid mechanical damage to the pipe. But building main drains (upper right circle in our photo at left) are often not below the frost line outdoors and in a freezing climate such as New York, the lines depend on proper slope to drain into a septic tank or into a municipal sewer main to avoid freezing.

So the waste piping depth is more determined by using the required pipe slope (1/8" to 1/4" per linear foot) and working backwards from the septic tank or municipal sewer line connection point and the property's own elevation and slope.

Our sewer piping photo (above left, courtesy Galow Homes - click to enlarge) shows a new sewer line (at left and foreground) being connected to an existing house drain (circled areas) in a New York home. You will notice that where the existing main sewer line exits the building the plumber included a cleanout (upper circle in photo) and that the depth of the waste piping is just about 12" below the ground surface. At this particular installation the drain piping slope exceeds 1/4" per foot in the area of the new connection because the ground slope in this area falls rapidly away from the home.

From the point of entry into the septic tank or into a municipal sewer main, drain piping lines need to slope 1/8" to 1/4" per foot to drain properly. Some piping sections may slope more than this due to terrain properties (our photo above) but slope should not be less.

We don't find that in-use septic tanks freeze up - most likely due to the combination of the warmth generated bacterial action, the entry of warm water from the building, and the fact that a good portion of the tank will be below the frost line in most installations. Also see SEPTIC TANK DEPTH.

Watch Out though: the whole theory behind allowing drain lines to be above the frost line in your state, to avoid a freeze-up of the drain line, depends on:

• Proper drain line slope
• Drain lines that are not clogged or blocked
• The absence of any ongoing plumbing supply leaks such as a running toilet

If the outside (or inside for that matter) building drain piping is not properly sloped it will clog.

If the drain line is blocked, the holding of wastewater in a drain line that is above the frost line means that in prolonged cold weather the line will freeze and often will burst

If there is a running toilet or other small plumbing supply leak, that rivulet of slow but continuous wastewater can freeze in the bottom of shallow outdoor drain lines in a freezing climate; over time and without sufficient wastewater passage to thaw out and flush out that freezing of water on the bottom surface of the drainline the whole line can slowly fill with ice, leading to a frozen waste line, blocked line, sewer backup in the building, or a burst sewer line.

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FREEZE-PROOF A BUILDING
Key Winterizing Topics
How to Winterize a Building

Winterize - Heat On Procedure
  Thermostat Settings
  Turn Water Off ?
  Winterize Water Softener & Treatment Equip.
  Find & Fix Water Pipe Freeze-Up Points
  Heat tapes, Heat, Insulation prevent Freeze-Up
  Freeze Protect Drains

Winterize- Heat Off Procedure

De-Winterized a Building
  Thawing Frozen Pipes
  Repairing Burst Leaky Pipes
  Turning on Heating
  Restoring Drains
  Turning on Water
  Water Supply/Drain Pipe Leak Types
  Restoring Water Softener & Treatment Equip

• "New Electric Heat Tapes Help Prevent Fires," US Consumer Product Safety Commission (CPSC) #00936
• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• "Freezeproof Your House," Mike McClintock, Rodale's New Shelter, p. 30, October 1985 (approximate date)
• Eric Galow, Galow Homes, Lagrangeville, NY. Mr. Galow can be reached by email: ericgalow@gmail.com or by telephone: 914-474-6613. Mr. Galow specializes in residential construction including both new homes and repairs, renovations, and additions.
• "How to Winterize Your Pipes," Mike McClintock, Homeowners How-To Magazine, p. 59-62, Nov-Dec 1979.

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