What causes pipes to burst when they freeze?

Presidents Grant, Nixon and Carter slept here. So did President George H. W. Bush, a string of governors and other celebrities [source: Thayers Inn]. But the excitement surrounding their visits to Littleton, N.H.'s historic Thayers Inn probably paled in comparison to one of its more recent attention-getters.

On a Friday in January 2014, a waterfall erupted at the top of the inn's grand staircase, rivers appeared in hallways lining the 1843 structure's 39 guest rooms and lakes formed in its first floor parlors and restaurant.

A string of bitterly cold temperatures had caused a water pipe to burst in the building's attic, sending an estimated 500 gallons (1,900 liters) gushing for every minute the break remained unchecked. It took 10 to 15 minutes to turn the water off, resulting in enough leaked liquid to fill a residential swimming pool. Ceilings collapsed, wood floors buckled and antique furniture warped [source: Sullivan].

Burst pipe disasters like this are compounded by ongoing (and potentially expensive) problems, including deep structural damage and mold growth. Even a hairline crack in a water pipe can release up to 250 gallons (950 liters) of water a day.

So how do you know if your home's pipes are frozen and creating a disaster just waiting to happen? The answer to this is surprisingly simple: If you turn on the water faucet and water does not come out -- or if you flush a toilet and it does not refill -- your water pipes are probably frozen. Alternatively, if you turn on a faucet full strength and only a trickle is released, this could also be a sign the water pipes are blocked by ice. If you suspect a water pipe is frozen, it's time to call a plumber and turn off the water where the main line enters your home [source: House Logic].

Of course, this advice usually only applies to areas in which temperatures have dropped near or below freezing (32 F or 0 C). As a general rule, temperatures below 20 degrees Fahrenheit (minus 7 degrees Celsius) put water in pipes at risk for freezing. However, once the wind chill is factored in, water pipes could potentially freeze above the 20 degree Fahrenheit mark, too [source: The Weather Channel].

Bursting at the Seams

As the hydrogen and oxygen molecules that make up water begin to transfer their heat to the freezing air around them, they bind more tightly. If the pattern of the hydrogen and oxygen molecules remained unchanged, all this huddling would result in solid, frozen matter that would actually take up less space than the liquid form. But water just has to be different. Instead, the pattern of its molecules changes as it freezes, taking up more space and expanding as it turns into ice [source: The Physics Van].

And when it comes to the water pipes in your home, this expansion of water as it freezes can put a lot of stress on stiff metal and plastic pipes, as well as pipe joints. Take note, inventors: If water pipes were made out of an expandable material, freezing water wouldn't affect their performance and would eliminate damage to thousands of homes each year.

So this means that when water expands into ice it causes pipes to burst, right? Not directly. Instead, the problem occurs when a pipe becomes blocked by ice, often in multiple locations within the pipe. These blockages -- located between the water source and the closed faucet -- cause a buildup in water pressure. Upstream from the ice blockage, the water can retreat back to the source without pressure buildup. Downstream from the ice blockage, the water is trapped between the blockage and the closed faucet. If more ice forms, it has nowhere to go and the pipe will burst. Pipe joints are particularly susceptible to leaks, but cracks can occur along other sections of a pipe, too [sources: The Weather Channel, The Physics Van].

This is one reason why it's a good idea to leave a faucet slightly open if you're worried your pipes may freeze. Some people think the constant drip keeps the water from freezing but actually water can freeze while even flowing. The real benefit is relieving the pressure [source: The Weather Channel].

Although pipes will burst or begin to leak as they thaw, leading some to believe the melting process causes problems, that's not the case. The actual damage will have occurred days earlier when the pipes became frozen; you simply won't know how much damage was done until the thaw begins because the ice blockage had been acting as a plug.

Prevent a 'Pipe'tastrophe

In colder regions of the U.S., most homes are built so the water pipes are protected from the elements. Insulated water pipes are placed in heated, interior areas of a home, steering clear of unheated attics, crawl spaces and exterior walls. In warmer regions of the U.S., homeowners aren't always as fortunate. Freezing is rare, which leads to less attention to water pipe insulation and placement during construction. During a cold snap, this could translate into a frozen -- and then watery -- mess [source: The Weather Channel].

What's the best way to prevent frozen water pipes, wherever you live?

Insulate water pipes exposed to cold. These water pipes could be located in an attic or crawl space or along an exterior wall. Wherever they are, add a layer of insulation and to help protect pipes from freezing temperatures. For an extra measure, wrap the insulated pipes in heat tape.
Seal leaks that let cold air into your home, especially those near your water pipes. Common culprits include spots around electrical, telephone or cable wiring that enters your home through an exterior wall. Also caulk around dryer vents and pipes.

If you're right in the middle of a cold snap, there are few additional precautions you can take, too.

Allow a steady drip or slight stream of water to flow from a faucet all day and night. If you have hot and cold taps, be sure to keep both open.
Keep the thermostat turned up, even at night. A warm interior can help some heat reach your home's water pipes.
Open cabinet doors under sinks and along exterior walls so warm air can enter the space near water pipes.
If you suspect a pipe has frozen, or is freezing, warm it with a hair dryer. Apply heat near the faucet and then work your way backward [source: State Farm].

Lots More Information

Author's Note: What causes pipes to burst when they freeze?

We have some family friends who just spent five weeks without the use of their kitchen because of a burst water pipe. They waited and watched as new floors, cabinets and appliances were installed -- and begrudgingly took their baby and toddler to dine out for nearly every meal. The expense, she said, was astounding. And the food choices were more limited than my friend expected, especially for someone who leans toward an organic, whole food diet. I can't imagine ushering young children to a restaurant every day. It's challenging enough while on vacation. Fitting into a daily routine would be exhausting, don't you think?

Sources

State Farm. "Preventing Frozen Pipes."(Jan. 16, 2014) http://learningcenter.statefarm.com/residence/maintenance/preventing-frozen-pipes/
Sullivan, Adam. "Historic NH Inn Cleans Up From Burst Pipes." WCAX.com. Jan. 14, 2014. (Jan. 17, 2014) http://www.wcax.com/story/24449827/historic-nh-inn-cleans-up-from-burst-pipes
The Physics Van. "Q&A: Freezing Water." (Jan. 15, 2014) http://van.physics.illinois.edu/qa/listing.php?id=1744
The Weather Channel. "Preventing Frozen Pipes." (Jan. 15, 2014) http://www.weather.com/activities/homeandgarden/home/hometips/severeweather/pipefreeze_prevent.html