Why do freezing pipes burst?

Bursting water pipes are a major concern when winter temperatures drop below freezing. But why do frozen pipes burst?

The simple answer is that as water freezes into ice, it expands, resulting in solid ice filling more volume compared with the liquid that used to be running through the pipes. That ice creates pressure inside the pipes that can cause a rupture.

"If there's no relief, ice is just going to burst a pipe," John Galeotafiore, who has an engineering background and is an associate director of product testing at Consumer Reports, told Live Science. His organization produces home maintenance guides on matters like pipe safety.

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"I've seen copper pipes that burst," Galeotafiore added, but said bursting can be just as much due to a weak fitting as to a weak spot in the pipe. Also, plastic piping is not immune, especially as the material ages.

"I've heard that some people say, 'Well, I have PVC or PEX plastic piping.' But while that might offer a slight delay in causing issues, it's really not the solution," Galeotafiore said. 

When water freezes, its molecules crystalize into an open hexagonal arrangement, which takes up more space than when the molecules are in their liquid form — that is, the water molecules expand as they freeze.

As the ice expands, it pushes liquid water toward the closed faucet. This causes an immense amount of water pressure to build between the ice blockage and the faucet — eventually, the pipe ruptures under the pressure, usually at a spot where there's little or no ice.

The pipe segments upstream of that ice clog generally aren't in danger of bursting because the pressure isn't great enough. In this case, the water isn't blocked, and can always retreat back to its source.

Pipes that are the most at risk of bursting are those that are exposed: located outside of building insulation, or in unheated interior areas like basements, attics and crawl spaces. Also at risk are mobile homes and three-season cottages that have thin walls and little or no insulation; Galeotafiore said you'll need to flush out your piping systems before winter to reduce the risk of damage to your property.

The typical danger point is at temperatures of about 20 degrees Fahrenheit (minus 7 degrees Celsius). Danger signs to look for include water not flowing at all, or water flowing more slowly than usual, Galeotafiore said. "That's a big sign the pipes are starting to freeze."

Heating or insulating these vulnerable pipes with fiberglass or foam sleeves can help prevent them from bursting. Additionally, keeping the faucet open and letting the water run — even at just a trickle — will help prevent the water pressure from building to dangerous levels. Galeotafiore also urges people to go into their attic or crawlspace on a sunny day and plug any holes in the wall that let in daylight.

If more insulation is required, however, call a professional as there might be safety concerns or other problems associated with putting large amounts of modern insulation in a house at once. Be especially careful if your house has asbestos insulation, which will cause health issues if disturbed improperly and requires special care by licensed contractors.

If you're away from the house for more than a day errand, Galeotafiore said to make sure your interior temperature is more than 62 F (17 C) to reduce the risk of pipes freezing. Also shut off the water at the source if at all possible. While heating is expensive, he acknowledged, "don't worry about the cost savings, because it's going to be a lot less expensive than dealing with a broken pipe."

If the worst happens and you do have a clogged or burst pipe, here are a few steps you can take before calling a plumber. If the pipe is underneath a kitchen or bathroom sink in a cabinet, you can try opening the doors to expose the pipe to your residence's heat. Turn off the water main for a few minutes and then try running a hairdryer in the coldest area of the pipe to break up the ice. When you're ready, turn the main back on slowly, Galeotafiore urged, to make sure there are no leaks.

If the pipe has completely burst and flooded an area, Galeotafiore said to never wade into standing water, as you can't be assured of safe footing or of staying safe from possible electrical shock. That requires a professional to deal with. Only shut off the water if you can safely do so without going into the flooded area; otherwise, your plumber will take care of it.

Galeotafiore added that residence owners, whether you're renting or buying, should always be aware of the age of the pipes, the type of pipes and the health of the pipes as soon as possible before or after moving in — and to stay on top of maintenance. Plumbing is among the top things to know about, along with roofing, electrical systems, heating systems and windows, he said.

Repiping is a massively expensive procedure, typically costing several thousands of dollars and taking several days to do. Your walls will also need to be replastered and painted after the plumbing is replaced. But that investment will go a long way to preventing leaks and ice vulnerability in the future. If you're lucky, insurance may cover the procedure, but check first.

Originally published on Live Science on Feb. 14, 2014 and rewritten on June 06, 2022.