Authors pickup truck in front of a winterized and frozen-in building in upstate New York (C) Daniel FriedmanWinterizing Guide: How to Freeze-Protect Building Drain Piping

  • DRAIN FREEZE PROTECTION - CONTENTS: How to avoid freezing drain piping, drains, traps, toilets, etc.How to winterize drain traps & drain pipes: frost protection for plumbing drainage systems. Use of heat tapes, heat sources, pipe routing, to prevent freezing drain pipes. What is the proper depth to bury sewer and septic system piping? What is the proper slope for drain line pipes?
  • POST a QUESTION or READ FAQs on Freezing Drain Lines: Cause, Prevention, Cure

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How to protect building drains from freezing:

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. You might think that because plumbing drains slope downwards (or should) that there is not freeze risk - that's not entirely true, as we explain here.

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. 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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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How to Avoid Freeze Damage to Building Drain Piping


Building drain clog (C) Daniel Friedman

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

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?

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

Reader 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?

Sewer piping being installed in NY (C) D FriedmanWhat 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

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:

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.

Reader Question: how much does it cost to bury a pipe below the frost line

Wonderful information. I am currently in the process of buying a home in Virginia, and a portion of the drainage pipe is exposed. Any idea how much (typically) it would cost to bury the pipe, or who I would contact to fix this? - Meaghan 5/31/12



if the drain pipe that is exposed is properly sloped and you don't have fixtures that run continuously into it (say a running toilet), the freeze risk is very low, especially in VA.

Speaking very generally, the 2013 cost to dig a 25-foot trench runs between $300. & $400. or about $13. to $16. per foot, with of course many variables such as depth, soil properties, and obstructions. There are online cost estimators that take your zip code and will give closer estimates, but even those can be way off as the cost estimator has no idea about local site conditions at your property. Simply hitting a big boulder, for example, can change the picture.

The cost to bury is something you'd best get from a local excavation contractor or landscaper who can see the actual site conditions, length of pipe to be covered, to what depth, thus estimating yards of soil, hours of machine and labor time, and finish landscaping requirements.

Without that data, an estimated cost to bury something arbitrary. Just be sure that your "burying" does not also bury building siding or you'll be asking for an insect problem.

Reader Question: what is the frost line depth in Nebraska's Platte Valley

Any idea what the specs might be in Nebraska's Platte River Valley? We are not far to ground water. - Gary 8/16/2012

Reply: easy to find frost line data for Colorado, but for Nebraska?

Gary, The spec is the frost line, right? Any local plumber will tell you. From a quick historical search about the frost line in Nebraska I learned from the Colorado Climate Center that for Colorado the frost line data is:

Typically the frost line in Nebraska in your area is around 10-14-inches from the ground surface

The deepest frost penetration in Nebraska in the last 20 years was at 20-inches. But watch out: more extreme variations are occurring in weather patterns everywhere so deeper frost penetrations may be in store.

In the 1930's in a cold winter with little snow cover the frost line extended down to 30"

So to be safe, and if you plan to move to Colorado, assume the frost level there is at least 20-inches and 30-inches would be safer.[8] OK, let's look for a better answer.

Take a look at the Air Freezing Index to Guess at Frost Line Depth for Your Area

Can we translate that data to Nebraska? Not directly. When you cannot find local frost line depth data, building codes adopted by your community still will have the answer. For example, the International Residential Code (IRC) for 2012 gives Air Freezing Index data for each U.S. state.

Figuring the Frost Line Depth for Nebraska

Most of Nebraska has an index of 1500 or less, except for Boyd, Burt, Cedar, Cuming, Dakota, Dixon, Dodge, Knox, Thurston counties where the air freezing index is 2500. Now we can do a little extrapolation and arm-waving:

The same IRC explains in Table R 403.3(1) that we can translate an air freezing index into a required footing depth. That data is in our table quotation just below. You can see that for an air freezing index of 1500 or less the minimum footing depth is 12-inches, and for an air freezing index of 2500 you'd have to make the footings at least 16-inches deep. Not very deep, right? Since the intent of footing depth is to protect a footing from frost heaves, I figure this is a good approximation of the frost line depth for your area: 12-16-inches.

1,500 or less 12
2,000 14
2,500 16
3,000 16
3,500 16
4,000 16

Watch out: local frost line data and local codes prevail; rules about minimum building footing depth, which you can use to guess at a safe frost line, may be different in your area from those assumed by and in the area addressed by teh IRC. Best bet is to ask your local building department.

Use of Heating Tapes on Sewer Pipes & Building Drains

Reader Question: what about using heat tape on sewer pipes - our mobile home sewer line keeps freezing

You don't say anything regarding using heat tape on sewer pipes. We have an annual problem with a mobile home sewer line freezing. The line connects a toilet, shower and tub so when it freezes it is a mess. - Anon. 9/19/11


Anon, you can add a heat tape on a sewer line provided the tape you choose is connected and secured to the sewer line the way the manufacturer instructs. Make sure that the bottom of the waste pipe is in contact with the heat tape, and provided the heat tape manufacturer permits, insulate the line as well. The combination of a proper heat tape and sewer line insulation is usually enough to stop freeze-up but here are some additional tips for mobile home waste line freeze problems:

Watch out: some heating tapes cannot safely be used on drain pipes, possibly because of a concern for heat damage to the piping. Details are in the article above.

Because the waste line typically has a vertical segment between the floor of the mobile home and the ground and because that segment is exposed to cold air in the crawl space beneath the home you will want to insulate it. But in termite prone areas the risk is that your insulation provides an insect path from the soil up into the floor of the home.

Ask your local pest company for advice about protecting this area from insects or perhaps construct a surrounding (openable for inspection and repair) partition wall that uses termite shields.

Watch out for dripping faucets and running toilets which can flood a septic soakaway bed or drainfield in any weather and can freeze, clog, and even burst drains in freezing weather.

A drain line does not normally contain standing water but when routed through freezing spaces and when there is a small rivulet of water in the drain due to a slow leak indoors such as a drip or running toilet, that water can freeze and accumulate until the line becomes blocked, frozen solid, and burst.

Reader Question: what kind of heating tapes can be used on PVC plumbing drains?

I have a PVC drain line for my water softener that goes outside. When it got very cold two winters ago, the drain line froze and it backed up into house. The drain line lays on the ground. I went to a local store to look for heat tape and they specifically stated not to use on drain lines. Is there some type of tape I could use that would be safe, and how should it be installed? - Don Corbett 9/30/11



A concern with heat tapes on plastic piping is that it may damage the pipes. And if your water softener drain is simply draining onto the ground surface, that is an improper and illegal disposition of wastewater in just about every jurisdiction. Unfortunately the proper fix is costly: reroute drainage to a drywell or other approved drainage destination.

If you use drain piping of sufficient diameter and proper slope, even exposed to cold temperatures the drainage will generally not freeze up in normal use. More help on avoiding freezing piping is at WINTERIZE A BUILDING.

Heating tapes and cables can be installed on PVC supply and drain piping provided you choose the right kind of heating tape or cable and that you follow the manufacturer's instructions. Details are in the article above.

Septic Drain Line Depth to Prevent Freezing

Reader Question: drain line depth for frost protection?

(June 4, 2014) Anonymous said:
how deep do the lines have to be

12/27/2014 Larry Slate said:
I want to extend the home sanitary line to the exterior of the house exposed under an exterior deck to a point where I would be able to dump the RV waste tanks via a macerator pump into the elevated pipe. The RV is parked approximately 10 ft below the interior home sanitary line. Normally the new exterior line would be empty.

Do I need to be concerned with freezing air entering the exterior part of the pipe and freezing the interior drain line. The 2 1/2 inch new drain line will connect 3ft inside the crawl space. If I need to be concerned, would a backflow preventer or even a valve prevent the cold air from possible freezing?


I am not sure I have quite a full picture, but if you are describing a waste line properly sloped and with no traps in a freezing area, the line is normally dry and so should not be a freeze risk

Reader Question: drain line depth for protection against breakage when more backfill is added

(Sept 7, 2014) said:
I have been searching for basic info on how deep you can bury 4" concrete sewer pipe under "normal" soil. The question arose because of a broken pipe buried below 9 feet of soil. When originally installed it was about 3' below grade, but the homeowner added 6 feet of soil above the area later in order to level the lot. What kind of depths can these pipes withstand?



Generally adding soil alone ought not to crush a pipe. Driving over it, or adding backfill that contains large stones or sharp objects can certainly result in broken pipe as can burying the pipe without proper bedding in sand or rock-free soil.

Improper trench grading, compaction or similar errors might also break a pipe as settling soils push down on the installation.


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