Why do pipes burst in winter?
Water expands as it freezes. Once ice plugs a pipe, more freezing pushes water toward the closed faucet and the pressure in that trapped stretch climbs until the pipe splits, usually where there is little or no ice. Pipes in crawl spaces, attics, garages and outside walls, especially where cold air leaks in, are the ones that freeze. The leak often shows only when the ice thaws.
Free to ask. Local shops send quotes, and you pay only when you book one.
By Kasra Taheri, Founder, LoclisUpdated
Researched and drafted with AI, then checked claim by claim against the sources listed below before it was published. Prices and rules change; the dates beside each source say when it was last checked.
Step by step
Read when to stop and call a pro, further down this page, before you start.
Step 2: Look for cold air reaching them
On a cold, windy day, feel around those pipes for drafts from vents, gaps, sill plates and holes where cable or phone lines come in.[1]
Step 3: Check the outside faucets
Disconnect the hoses, then shut off and drain the pipes leading to your outside faucets so no water is left to freeze.[3]
Step 4: Check the thermostat habits
Keep the heat steady day and night in a cold snap, and no lower than 55 degrees when the house is empty.[3]
Step 5: Watch the faucets in a freeze
If a faucet served by an exposed pipe slows to a trickle or stops, treat the pipe as frozen and leave that faucet open.[1]
Step 6: Inspect after the thaw
When the weather warms, look and listen along those same pipes for drips, wet insulation or stains, since a split often leaks only once the ice melts.[2]
The short answer
Ice does not split a pipe the way most people picture it. As Loudoun Water puts it, water expands as it freezes and this can put tremendous stress on whatever is containing it, including metal or plastic pipes.[3] When a short section of pipe freezes solid, it becomes a plug. As more water freezes behind that plug, the water trapped between the ice and the closed faucet has nowhere to go, and its pressure climbs until the pipe gives way.
How the pipe actually breaks
In fact, ice forming in a pipe does not typically cause a break where the ice blockage occurs.[1] Instead, following a complete ice blockage in a pipe, continued freezing and expansion inside the pipe causes water pressure to increase downstream,[1] in the stretch between the ice blockage and a closed faucet at the end.[1]
Two things follow from that:
- The split is often not where the ice is. Usually the pipe bursts where little or no ice has formed.[1] A plumber may find the crack a few feet from the frozen section.
- Only one side is at risk. Upstream from the ice blockage the water can always retreat back towards its source, so there is no pressure build-up to cause a break.[1] The danger is on the faucet side.
It also explains why the damage shows up late. In most cases, the actual break doesn't appear until the water begins to thaw.[2] While the pipe is frozen, the ice holds the crack shut. When the weather warms, water starts running through the split, often in the middle of the night or while the house is empty.
Which pipes freeze
Loudoun Water says that, usually, pipes that freeze are exposed to severe cold weather, like outdoor hose bibs, water supply pipes in unheated interior areas like basements, garages, or kitchen cabinets.[3] Virginia American Water's list of areas that might be prone to freezing is similar:
- crawl spaces, unheated rooms, basements, garages, and exterior walls;[4]
- sprinkler and irrigation systems, for which its advice is: "Make sure everything is turned off and fully drained."[4]
In a house on a crawl space, the culprit is often the line that comes up through the floor to a sink on an outside wall, or the pipe feeding a hose bib through a cold rim joist.
Why cold air matters more than cold weather
Moving air freezes pipes faster than still air. The University of Illinois research found that wind chill can play a major role in accelerating ice blockage, and thus bursting, in water pipes.[1] Drafts reach pipes through foundation vents, gaps at the sill, and less obvious places: "Holes in an outside wall where television, cable or telephone lines enter can provide access for cold air to reach pipes."[1]
What the pipe is made of matters less. Pipe size and material are relatively minor factors in pipe bursting compared with the absence of heat, pipe insulation and exposure to a flow of subfreezing air.[1]
How cold is too cold
There is no single temperature. In field tests, for un-insulated pipes installed in an unconditioned attic, the onset of freezing occurred when the outside temperature fell to 20 degrees F or below.[1] That threshold is meant primarily for homes in the south and other areas where freezing may occur only once or twice a season,[1] and the same research warns that freezing incidents can occur when the temperature remains above 20 degrees F[1] when a pipe sits in a draft.
Underground lines have their own limit. In Montgomery County, the Public Service Authority says: "The frost line in Montgomery County is 25 to 30 inches. The PSA recommends 3 feet of cover."[5]
What makes a burst more likely
Turning the heat down. Loudoun Water's advice is to keep your thermostat set no lower than 55F[3] whether you are home or away, and it warns that travelers will often make the mistake of turning their thermostats off thinking it will save on their heating bill.[3] For an empty house, Virginia American Water suggests a freeze alarm, which will call a user-selected phone number if the inside temperature drops below 45 degrees.[4]
Closed faucets on exposed pipes. Because the damage comes from pressure, an open faucet protects the pipe even if it freezes: opening a faucet will provide relief from the excessive pressure that builds between the faucet and the ice blockage when freezing occurs.[1] The drip can be tiny. "Even the slowest drip at normal pressure will provide pressure relief when needed."[1] And: "If the dripping stops, leave the faucet(s) open, since a pipe may have frozen and will still need pressure relief."[1]
Outdoor faucets left charged. A hose left connected can hold water in the spigot, and the pipe to an outside faucet often runs through a cold wall. Loudoun Water's advice: "Shut off and drain the pipes leading to your outside faucets and/or hose bibs so no water is left to freeze, expand and cause a leak in these lines."[3] Fairfax County Fire and Rescue says the same for the rest of the house: "Shut off water to unheated areas of your home and to outdoor faucets. Don't forget to drain these pipes."[2]
Stop and call a pro if
- A pipe has already split, or water is coming through a ceiling or wall. Shut off the main valve first.
- A faucet has stopped and you cannot find or reach the frozen section.
- The same pipe freezes every winter. A plumber can insulate it properly or reroute it inside the heated part of the house.
- The underground service line or the meter has frozen. Call your water utility.
- Water is near outlets, the panel, a water heater or a furnace.
Next steps
If a pipe has already burst, start with what to do when a pipe bursts. For a pipe that is frozen but intact, see how to thaw a frozen pipe safely, and for prevention, how to protect pipes in an unheated crawl space and how to keep pipes from freezing. You can also find plumbers in Blacksburg or browse all plumbing repair answers.
If a pipe keeps freezing or has split, describe the burst pipe once and licensed local plumbers quote it.
Common questions
- Why did my pipe start leaking after the cold weather ended?
- Because the split happened while it was frozen, and the ice was plugging it. Fairfax County Fire and Rescue notes that in most cases the actual break does not appear until the water begins to thaw. Check pipes in cold spots for drips as soon as the weather warms.
- How cold does it have to be for pipes to freeze?
- There is no single number. University of Illinois field tests found uninsulated pipes in an unconditioned attic started freezing when the outside temperature fell to 20 degrees F or below, but that threshold is primarily for homes in the south and other areas where freezing may occur only once or twice a season, and pipes in a drafty spot can freeze at warmer temperatures.
- Does a dripping faucet stop pipes from freezing?
- Not reliably. Its real job is pressure relief: an open faucet lets the pressure that builds between the ice and the faucet escape, so the pipe does not split even if water freezes inside it. Fairfax County Fire and Rescue cautions that a trickle may or may not keep the pipe from freezing.
- Are PEX or plastic pipes safe from bursting?
- Any pipe can burst. Loudoun Water notes that freezing water can put tremendous stress on whatever is containing it, including metal or plastic pipes. Separately, University of Illinois research found pipe size and material are relatively minor factors in bursting compared with the absence of heat, pipe insulation and exposure to a flow of subfreezing air.
- Can an underground water line freeze?
- Yes, if it is buried too shallow. Montgomery County's Public Service Authority puts the local frost line at 25 to 30 inches and recommends 3 feet of cover for a water service line.
Sources
- [1] Freezing and Bursting Pipes: How pipes freeze and burst, and what to do, City of Paola, Kansas. Checked October 26, 2026.
- [2] Frozen Water Pipes, Fairfax County Fire and Rescue Department. Checked October 26, 2026.
- [3] Frozen Pipes, Loudoun Water. Checked October 26, 2026.
- [4] Virginia American Water Urges Customers to Prepare for Cold Weather (December 3, 2025), Virginia American Water. Checked October 26, 2026.
- [5] Water/Sewer: PSA frequently asked questions, Montgomery County, Virginia, Public Service Authority. Checked October 26, 2026.
Want a pro to handle it?
Describe the job once. Licensed local businesses send quotes, and you pay only when you book one.
Get a quote for Plumbing Repair