Frozen copper water line shows freeze stresses (C) Daniel FriedmanMechanics & Forces of Freezing Water
Effects of ice and freezing water on building plumbing systems

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This article describes the expansive force of freezing water, or the force exerted by ice as it freezes and expands. The pressure exerted by freezing water depends on temperatures and other physical conditions, but it can be tremendous - enough to lift buildings, burst pipes & plumbing fixtures, and crush the hulls of ships trapped in ice.

What is the definition of the strength or force exerted by freezing water as it forms ice; how much expansion occurs as water freezes into ice & what are the effects on building plumbing systems; how can we predict where frozen pipes will burst?

Here we describe the typical effects of freezing water and ice on buildings and on or in building plumbing pipes & fixtures or in appliances such as water tanks or heating boilers.

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Water Supply or Drain Pipe Freezing Mechanics - The Forces of Freezing Water

Frozen pipe burst damage pattern by joint separation (C) Daniel Friedman Temperature at which pipes freeze vs when pipes will break

As we elaborate below, while water begins to crystallize into ice at 0°C (or 32°F), its expansive forces generally do not cause water pipes to burst until temperatures further fall to around 20°F or until temperatures remain below freezing for some protracted time period.

As water temperatures drop, water actually can become supercooled by a few degrees before it begins to actually crystallize into ice. That's because the expanding ice crystals give up latent heat, warming the surrounding water back up to 0°C.

When water freezes its volume, in the form of ice, increases by about 9% under atmospheric pressure.

If the melting point (or freezing point) is lowered by large increases in pressure, the increase in volume on freezing is even greater (for example 16.8% at -20°C (-4F).

Factors Affecting the Temperature at Which Water Pipes Will Freeze

The actual point at which water inside of water pipes freezes solid in a building is determined by a combination of factors including the following:

In general, 20°F (-6.67°C) is a practical number for the point at which water pipes are likely to freeze solid in a period during which temperatures dip below freezing overnight but don't remain there for days at a time.

Effect of Altitude, Atmospheric Pressure, or Water System Pressure on the Freezing Point of Water

Freezing point of water as affected by pressure - Wikipedia Commons 2018-01-20

[Click to enlarge any image]

The phase change chart for water shown above, from Wikipedia commons, adapted from Chaplin, plots changes in the point at which water freezes as a function of pressure. You can see that at atmospheric pressure the freezing point of water is 273.2°Kelvin or 0°C or 32°F. Pressure is mapped on the vertical axes of the chart.

The nearly-perfectly-vertical line between the blue area (solid or frozen water) and green area (liquid water) forms of water mapped against the vertical axes giving pressure show that freezing point of water changes only very slightly in response to very large changes in pressure.

Does Leaving a Faucet Open Prevent Freezing Pipes from Breaking?

We know that running water, by moving warmer water from some building locations to colder pipe locations that would otherwise freeze, we can defer or even prevent frozen water supply pipes.

Watch out: at DE-WINTERIZE a BUILDING we warn that while running water may prevent supply pipes from freezing you may cause the building drain to freeze, block and even burst. Some building experts advice that when faced with freezing pipes or already-frozen water pipes we open the faucets, reasoning that if pipes are frozen you might reduce the chances of freeze-burst piping or reduce its extent by opening faucets.

Allowing even a small amount of water (from the un-frozen pipe sections) to drain out of the building supply piping might reduce some of the in-pipe pressure even if no water is flowing from the faucet. - Building Research Council (1996)

Really? While there is no question about the tremendous force of freezing water it appears to be less clear the direction in which those forces work in plumbing systems. Let's look at the force exerted on building pipes along the length of the pipe vs. across the diameter of the pipe as ice freezes.

Horizontal freezing water-forces (ice pressure) along piping lengths

An exception to our horizontal split burst frozen pipe rule that describes what we have observed most often in buildings is the occasional separation of pipes at 90 degree elbow solder joint (photo at above left). In the photo above we show that clearly horizontal forces along the pipe pushed this pipe joint apart.

But look more closely at the solder joint and you will see that the original soldering job was poorly executed without proper cleaning and use of flux. The solder was not bonded to the copper and was not uniform in the joint - this was a weak point in the piping system so it's no surprise that the pipe failed here.

Lateral water freeze-forces (ice pressure) across building piping

Frozen pipe split pattern (C) Daniel FriedmanMore often, where I observed that pipe joint separation a plumbing elbow (a common freeze point in buildings), often the elbow of bronze rather than just thin-walled copper resisted splitting while nearby softer copper piping did not.

While expanding ice inside of water supply pipes may slightly increase pressure (as water is not compressible) our field experience and photos of frozen burst pipe such as the photographs at immediate left and again at below left suggest that most often frozen water pipes burst or split from the expanding ice pressure within a small section of pipe, creating first a bulge and then a split from forces across the diameter of the pipe rather than along its length.

It certainly appears from physical evidence that the water pipe shown in my photo bulged and then split by forces across the diameter of the pipe.

A rational view is that freezing water applies force in all directions rather uniformly. But the damage done by that force occurs where the force is confined.

But freezing water appears to be immediately confined more by the circumference of a pipe than along its length. Why might this be? Perhaps because the expansive force of ice increases as the temperature of the freeing water drops. A plug of ice forms in the pipe, the continues to expand as temperatures drop.

Why then might forces across the pipe be more confined than forces along the water pipe? We are not sure we not agree with one building expert who opines that the fill valve on toilets allows water, pushed along by freezing ice, to enter toilet tanks.

Really? Building water pressure varies over a range all the time for a variety of reasons (faucets open and shut, pumps starting and stopping) without ever pushing water through a closed toilet tank fill valve.

But there are building components that can absorb increasing water pressure:

What is the Expansive Force of Freezing Water?

Frozen copper water line shows freeze stresses (C) Daniel FriedmanHow much does water expand or contract as a function of temperature and how much does ice expand? Various sources give different expansion forces for freezing ice.

Depending on its state, freezing water (or ice as temperatures continue to drop) can expand by as much as nine percent at a maximum force between about 25,000 and 114,000 psi.

Actually water reaches its maximum density above freezing, at about 4°C. It is the expansion of ice as it continues to freeze that explains why icebergs float - the iceberg displaces a volume of water that weighs more than the (expanded) ice itself. The tip of an iceberg seen above water usually represents about 8% of the iceberg's total volume.

Stated another way we can explain why ice is lighter than water by explaining its expansion as water changes from liquid to frozen state.

As ice freezes forming hexagonal crystals (comprised of two H molecules join with an O molecule at an angle of 104°) the water in this form takes up more space than liquid water.

But the crystals formed by freezing water take on varying forms (and affecting the pressures exerted by confined ice) as temperatures continue to fall. - Debenedetti (2003)

Before modern physicists and engineers began calculating the expansive force of freezing water, Florentine academics had measured the strength of freezing water by enclosing water in a brass globe of known thickness and strength, then allowing it to freeze. Those academics observed that a one-inch globe of water, when freezing, could exert a 27,000 pound force. - Platts (1880)

Luckily copper plumbing has some bending or flexing ability and does not break immediately. Copper water supply pipe strength varies by pipe type and wall thickness (K, L, or M copper) but break at around 3000 psi.

The thermal expansion of water is detailed at THERMAL EXPANSION of HOT WATER.

Expanding ice forces are very strong when confined. Entire building movement caused by freezing and ice are discussed at FOUNDATION DAMAGE by ICE LENSING


Watch out: while some experts advise leaving faucets open or dripping to avoid freezing pipes, this advice is risky if the building drains are exposed to freezing. In many areas the building main drain exits above the frost line and can be exposed to very cold conditions.

During normal plumbing use the surge of wastewater down the drain makes it past this cold spot without freezing. But a dripping faucet or running toilet, sending a small but continuous trickle of water down the drain can accumulate as ice until it expands, blocks the drain (leading to a sewage backup in the building) or until the drain line freezes and breaks.

Frost-Pushed-Up Sewer Piping - Cause & Prevention of Frost Heave Damage

Reader Question:

4/16/14 Anonymous said:

How do I prevent the frost from pushing my toilet drain pipe up through the floor and dislodging the toilet from the flange on the floor. This is at a cottage.


Anon, you ask an important question, for which, with apology, I have to speculate a bit to try to answer.

Normally the toilet waste line doesn't push up through a floor.

To fix your frost-heaved waste piping we need to know why it's happening. I speculate that

A "band aid" approach that I don't like is chopping out a section of flooring and packing foam insulation around the drain line in the fantasy that we can protect it from frost heaves. We need to either install construction components below the frost line or keep water out from below the foundation and slab.

Instead, start outside looking at where and how water is saturating the ground under your foundation and slab. While you're at it, and since this is a cottage that perhaps is not occupied and heated all year, also take a look at WINTERIZE - HEAT OFF PROCEDURE


Thank you for your helpful information about freezing water pipes!

But I fear that your reference to 20°F will mislead many of your readers. It seems to be generally said on the Internet that when outdoor temps get down to 20°F or below that freeze/break of indoor pipes becomes more likely. This is quite different from the actual temp of the indoor pipe itself, that may freeze/break...

It takes over 1000 psi to lower the freezing point of water by one degree F! It seems inevitable that the vast majority of water pipes would freeze (and presumably break?) long before the real temp of the pipe and the water inside got down to 20°F, which would require a pressure of over 10,000 psi.

-k seeking facts and understanding in Boston

As we elaborate below, while water begins to crystallize into ice at 0°C (or 32°F), its expansive forces generally do not cause water pipes to burst until temperatures further fall to around 20°F.

The actual burst point for freezing pipes has more variables including the pipe material, thickness, and even possibly its overall diameter and shape. But 20°F is a good number for the freeze point of pipes.

pressure lowers freezing point of water a slight amount

* Explanation of the Density Anomalies of Water

"Pressure reduces ice's melting point (13.35 MPa gives a melting point of -1 °C)"

"In water, this line is backward sloping with slope 13.46 MPa ˣ K-1 at 0 °C, 101.325 kPa."

Many people want to know if the pressure in water pipes lowers the freezing point. Almost all Internet info including WP is not helpful: science phase diagrams in unfamiliar units that do not show appropriate detail, only that the melt temp does not vary much with pressure under common water pipe conditions.

It would be great to have a graph of temp in C and F (not K) vs pressure in PSI for 0-10,000 PSI for inclusion in WP articles. Until then, 13.35 MPa converts to 1936.25 psi (pounds per sq. inch). We could say something like:

* Pressure in water pipes reduces the freezing point slightly: a pressure of 1936 psi lowers the freezing point to -1 °C.

The graph in the good Martin Chaplin LSBU article appears to show that 100 MPa (14,500 psi) would lower the melt temp by about 10 °C.

Typical water pipe pressures are usually around 100 psi, which apparently would lower the freezing point by a small fraction of a degree. If the slope at zero is 13.46 MPa per degC, that works out to 0.051 °C (0.092 °F) for 100 psi.

Or maybe a more useful rule-of-thumb to remember would be 1000 psi lowers melt point by about 0.5 °C (0.9 °F).

It would be a relevant factoid to include in some articles, if we could be sure of the science/physics facts. - Anonymous by private email 2018/01/11


Thank you for the comments. In our article we do mention that the impact of water pressure on freeze-point of water in pipes is very small - perhaps I should say "not really a factor" because of the numbers involved.

About the freeze point, as we both know that water freezes at 32F (at sea level, for fresh water), [actually water super-cools a bit before it freezes] we can pretty much figure that water in pipes at that point, too, depending on a range of factors we list in the article above.

The 20 degree figure is, I agree, a subjective ballpark number based on field experience for occupied homes during a temporary power outage. Certainly a home that is unattended and /or that remains un-heated for a protracted period is likely to have frozen pipes at somewhat higher temperatures.

A separate question is the hard freeze temperature and the relationship between that and burst copper pipes. I pose that colder temperatures increase the risk of bursting - but we need some science to support that claim.

I'll review the article to add our discussion, for which I'm grateful, and we can both look for more data.


Research on the Freezing Point of Water


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FREEZING FORCE of ICE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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