Every cold snap brings the same calls: a pipe that split overnight in a cellar, a ceiling that came down under an exterior wall, a hose bib that let go in the side yard. Most of them have something in common. They happen in older houses with galvanized pipes. Freezing NYC nights hit that pipe harder than anything else in the house. Here’s why, and what you can do before the next freeze.
Ice doesn’t burst the pipe. Pressure does
It seems obvious that ice expands and splits the pipe where it forms. That isn’t quite how it works. Once ice fully blocks a pipe, continued freezing pushes on the water trapped between the ice and the closed faucet downstream. That water has nowhere to go, so its pressure climbs until something gives. Insurance-claims guidance drawing on research from the Insurance Institute for Business & Home Safety describes exactly this (CLM, on frozen-pipe claims), and it’s why the pipe often splits somewhere with little or no ice at all (AAA).
Two things follow. The burst happens at the weakest point in the trapped section, not necessarily at the cold spot. And a slightly open faucet relieves that pressure, which is the whole reason for letting taps drip on the coldest nights.
Why galvanized is the weakest link
Galvanized steel was the standard supply pipe in NYC houses through the first half of the last century, and it typically lasts 40 to 60 years. In Brooklyn, 74% of one- to four-family lots were built before 1940; in Queens the median build year is 1935 (MapPLUTO). Plenty of that original pipe is still in service.
Old galvanized fails under that pressure for two reasons:
- Thin threads. Galvanized is joined with threaded fittings, and cutting the threads removes metal. Decades of corrosion take the rest, so the joints are the thinnest part of the system.
- Internal rust. NYC water is soft and corrosive. DEP’s annual water quality report puts the average Langelier index at −2.21, and a negative value means the water dissolves metal rather than coating it. Galvanized rusts from the inside, and the scale narrows the pipe while the wall gets thinner.
When pressure builds in a frozen section, it finds a thinned threaded joint long before it finds sound pipe.
The runs that freeze first
DEP notes that most pipes and water meters sit in basements, crawl spaces and other unheated areas, and that protecting them is the homeowner’s job (DEP: protect your pipes from freezing). In NYC houses, the usual suspects are:
- Supply lines on outside walls, especially a kitchen sink or bathroom on the back wall of a rowhouse, where the pipe sits in an uninsulated cavity against brick.
- The cellar near the front wall, where the service enters and cold air leaks around old windows and the meter.
- Unheated vestibules, porches and extensions, common in Queens and Brooklyn attached houses.
- Hose bibs and their supply lines, which stay full of water unless someone shuts and drains them.
Before a cold snap
DEP’s own checklist covers most of it, and every item is cheap compared with a burst:
- Repair broken or cracked windows, doors and walls; add storm windows to basement windows; seal crawl-space drafts.
- Insulate exterior walls where pipes run, and ask a plumbing supplier about pipe and meter insulation.
- Shut off outdoor faucets, disconnect hoses and drain those lines.
- Open cabinet doors on outside walls so warm air reaches the pipes.
- If you’ll be away, have someone run a faucet periodically.
- Know where your main shutoff valve is, and make sure it actually turns.
If a pipe does freeze, DEP advises opening a faucet near it and warming it with a hair dryer, heat lamp or small space heater, and never an open flame. If you can’t thaw it, call a Licensed Master Plumber; if the cold damages your meter, call 311. DEP may bill you for a meter that froze because reasonable precautions weren’t taken.
After a burst
Shut the main, kill power to any wet circuits, and start removing water. What to do when a pipe bursts walks through the first hour. Published NYC prices for an emergency burst-pipe repair run $500–$2,000 (Handyman NYC price guide), before any ceiling or floor repair. Homeowners insurance generally covers sudden, accidental water damage, not the replacement of pipe that wore out (is repiping covered by insurance?).
A burst in galvanized is often the first of several, because the freeze found the weakest joint, not the only weak one. If the house also has brown water or weak upstairs pressure, read should I replace galvanized pipes? and look at galvanized pipe replacement before next winter.
Get help fast
If a pipe has already split, burst pipe repair is available across the five boroughs, with calls answered 24/7 and a same-day callback. The repair is made in copper, and you get a written, itemized price before the work is booked. Call (929) 659-6658.