Stop reading and shut off your main water valve. A pipe has already failed, and every minute it runs is more damage. Skip to what to do in the first five minutes. If you don't know where your main shutoff is, it's usually in the basement on the wall facing the street, near where the water line enters.
It's a cold morning in the Twin Cities. Someone turns on a tap and gets nothing — or a thin dribble that quits after a second. Everything else in the house works fine.
That's a frozen line, and here's the part worth understanding before you do anything: a frozen pipe and a burst pipe are two completely different situations. The first is an inconvenience you can often handle yourself in an afternoon. The second is one of the most destructive things that can happen to a house. And what you do in the next few minutes has a lot to do with which one you end up with.
Step one: figure out which problem you have
Before you start hunting for the frozen section, rule out the worse scenario.
- No water, and no water anywhere it shouldn't be. The pipe is frozen but almost certainly still intact. You have time to work.
- You can hear water running with every fixture in the house turned off. Something is open that shouldn't be. Shut off the main.
- A stain spreading on a ceiling or wall, a damp baseboard, water on a basement floor. The line has already split. Shut off the main.
- Water is gone and your water heater or boiler is behaving strangely. Treat this as urgent — a frozen line feeding an appliance that's still trying to fire is a different problem than a frozen line feeding a sink.
One more thing worth knowing: a pipe can freeze, split, and show you absolutely nothing until it thaws. The ice is plugging its own hole. That's why a house that's been empty through a cold snap sometimes floods twenty minutes after the heat comes back on — not during the freeze. If you're thawing a line that's been frozen for a while, stay in the room and watch it.
Why frozen pipes actually burst
Nearly everyone believes ice expands against the wall of the pipe until the metal gives out. That's not what happens, and the real mechanism is worth thirty seconds of your time because it points directly at the fix.
Research from the Building Research Council at the University of Illinois — the source of most of the standard guidance on this — found that once ice forms a complete plug, continued freezing drives pressure up in the water trapped between that plug and a closed faucet downstream. That pressure is what splits the pipe. And the split usually happens in a stretch with little or no ice in it at all.
On the other side of the plug, toward the water main, there's nowhere for pressure to build — the water can simply retreat back toward its source. So the damage concentrates on the dead-end side.
Which means: open the faucet. Right now.
If pressure can escape, the pipe doesn't burst — even if the water inside it is frozen solid. Go open the faucet that's giving you nothing, and leave it open.
This is the highest-value move in this entire article and it takes four seconds. It also gives you a free progress indicator: the moment water starts trickling out, the plug has opened up.
Finding the frozen section
Pipes don't freeze at random. They freeze where the heat isn't. In Minnesota houses the same handful of spots come up over and over:
- The rim joist — where the floor framing meets the top of the basement wall. Supply lines often run right through this band, and it's one of the leakiest parts of a house.
- Crawl spaces and unheated basements, especially with a vent or window left open.
- Attached garages — and particularly a garage door left up for twenty minutes in a cold snap.
- Exterior walls behind cabinets. A kitchen sink on an outside wall with the cabinet doors closed is a classic. The cabinet blocks the room's heat from reaching the pipe.
- Cantilevers and bump-outs — a bay window floor, an overhanging room section. Cold air gets underneath from three sides.
- The line feeding an outdoor spigot, particularly if a hose was left connected in the fall.
In older St. Paul housing stock there's one more: plumbing added during a remodel. A bathroom put in forty years after the house was built often has supply lines threaded through a wall that was never designed to carry water, with insulation on the wrong side of the pipe. Those runs freeze first, and they freeze repeatedly, every bad winter, until somebody reroutes them.
To narrow it down, trace the line back from the dead fixture and feel along it with your hand. A frozen section runs noticeably colder than the pipe on either side, and sometimes you'll find frost or condensation on it. A slight bulge in a copper line is a serious sign — that pipe is already deformed and may fail the moment pressure returns.
How to thaw it without making things worse
The principle is simple: gentle, steady heat, starting at the faucet end and working back toward the ice.
Direction matters. If you heat the middle of the plug first, you create meltwater with ice on both sides of it and nowhere to drain. Starting at the open faucet and moving backward means every bit of melt has a clear path out.
- Confirm the faucet is open.
- Apply heat with a hair dryer, a heating pad wrapped around the pipe, hot towels you re-wet as they cool, or a space heater aimed at the area from a safe distance.
- Work slowly along the pipe toward the frozen section. This can take a while — an hour is normal, longer isn't unusual.
- When flow returns, leave the faucet running at a trickle for several minutes and check the whole run for drips or spray before you walk away.
- Keep heat in that space for the rest of the cold snap, or it will simply freeze again tonight.
Never use an open flame. No blowtorch, no propane torch, no heat gun held against framing. It's a genuine fire risk inside a wall or joist cavity — that's how a frozen pipe turns into a house fire — and on copper, a torch can soften a soldered joint enough that it lets go the second pressure comes back. This is the single most common way a manageable frozen pipe becomes a catastrophe.
If the frozen section is inside a finished wall, behind tile, or somewhere you'd have to cut to reach, that's the point to stop. So is a pipe you can't locate at all. Neither of those is a DIY situation, and forcing it usually costs more than the call would have.
If it already burst: the first five minutes
A burst supply line under household pressure moves a startling amount of water very quickly, and the damage curve is steep. The order here matters more than the speed.
A few notes on those. Opening low faucets after shutting the main lets gravity pull the remaining water out of the system instead of out of the break. Don't wade into standing water near outlets, a furnace, or an electrical panel until the power to that area is off. And take the photos before you move anything — most homeowner policies cover sudden pipe failure, and adjusters ask for documentation of the condition as found.
Water damage also doesn't end when the water stops. Wet drywall and insulation start growing mold within a couple of days, so drying the space out properly is part of the repair, not an optional extra.
What actually decides whether this happens to you
The standard guidance you'll find everywhere is that pipes start freezing around 20°F. That figure traces back to the same University of Illinois field research, and it's worth knowing what it actually refers to: uninsulated pipe in an unconditioned attic, in southern-climate houses that were never built for cold.
That's not a Minnesota house. Homes here are built for this, which is why nobody in St. Paul loses a pipe every time it hits 20°F. What it means instead is that the outdoor temperature is a poor predictor on its own. What matters is whether a specific run of pipe is sitting in moving cold air without insulation between it and the outside. A house at 70°F on the thermostat can still lose a line in the rim joist, because the furnace was never heating that cavity in the first place.
So the useful question isn't "how cold does it have to get." It's "which pipes in my house are in a cold pocket, and what's between them and the outdoors."
| Situation | What it usually takes | Urgency |
|---|---|---|
| Frozen line, accessible, no damage | Gentle heat, patience, insulation afterward | Handle it today |
| Frozen line inside a finished wall | Locating it, possibly opening drywall | Call a plumber |
| Same pipe freezes every winter | Rerouting or insulating the run | Fix it before next winter |
| Visible bulge or split in the pipe | Section replacement | Don't restore pressure first |
| Active leak, water running | Shutoff, repair, then drying out the space | Emergency |
What a repair costs depends on access, pipe material, and how much has to be opened up to reach it — which is why it's worth getting more than one quote rather than accepting the first number over the phone.
The prevention that's actually worth doing
Most of this is cheap and takes an afternoon in October, which is a far better time to read this than during a January cold snap:
- Disconnect garden hoses and shut off the supply to outdoor spigots. A hose left attached is one of the most reliable ways to split the line behind a sillcock — the fall plumbing checklist walks through this in order.
- Insulate any pipe running through an unheated space. Foam sleeves cost very little and take minutes.
- Seal the air gaps, not just the pipe. Cold air pouring through a rim joist gap or a cable penetration in an exterior wall does more damage than the ambient temperature does.
- In a deep cold snap, open the cabinet doors under sinks on exterior walls so room heat can reach the pipes.
- Let a faucet on a vulnerable run drip. It's not that moving water can't freeze — it's that an open faucet means pressure never builds to the point of bursting.
- Leaving town in winter? Don't drop the thermostat below about 55°F, and have someone check the house. An empty house with a frozen line is how a small failure becomes a very large one.
If you're going into your first Minnesota winter in an older home, it's also worth reading through the problems that show up every time the season turns — frozen pipes get all the attention, but they're not the most common cold-weather call.
When to stop and call someone
Thawing an accessible pipe with a hair dryer is genuinely a homeowner job. Call a licensed plumber when:
- You can't find the frozen section, or it's behind a finished surface
- The pipe is visibly bulged, split, or dripping
- More than one fixture lost water at once — that points to a frozen main or a shared branch
- You've been heating it for a couple of hours with no change
- It's a frozen line feeding a water heater or boiler
- The same run freezes every year and you'd like it to stop
And if you're not sure whether what you're looking at is urgent, our guide on what counts as a plumbing emergency lays out the line between "call now" and "Monday is fine."
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