How to tell if a pipe has burst
A burst pipe rarely announces itself. Most of them are inside a wall, under a floor, or in an attic, and the first thing you notice is a symptom two rooms away. This is how to work out whether that is what you are looking at, and what to do in the first ten minutes.
If water is running right now
- Shut off the main water valve. In most homes it is at the street meter box or on the wall where the supply line enters the house.
- Open the lowest faucet in the house. That drains the pressure still sitting in the lines instead of pushing it out through the break.
- If water is anywhere near outlets, a breaker panel, or a furnace, kill power to that circuit before you go near it.
- Take photographs before you move or dry anything. Your insurer will ask, and the pictures are worth more than your account of it.
- If you smell gas, do not touch a light switch. Leave the building and call from outside.
The signs, in the order you usually notice them
Almost nobody sees a burst pipe. What you see is the second or third consequence of one, and the useful skill is recognising which consequences point at a pipe rather than at something cheaper.
- Pressure drops everywhere at once. One weak fixture is that fixture. Every fixture in the house going weak at the same time is supply.
- The meter keeps moving with everything off. Shut every tap and appliance, then watch the meter's low-flow indicator. Movement means water is leaving the system somewhere you cannot see.
- A hot spot or a cold spot on a floor. A slab leak on the hot side warms the tile above it. It is often the first thing anyone notices, and it is usually mistaken for underfloor heating or sun.
- Sound with nothing running. A hiss, a trickle, or a rush behind a wall when the house is quiet.
- A stain that spreads, or paint that bubbles. Clean water tracks along framing before it comes through, so the wet patch is frequently nowhere near the break.
- The water bill jumps with no change in habits. A slow burst can run for weeks before it shows anywhere else.
- Discoloured water when you turn a tap back on, or a smell of damp that does not clear.
Why pipes burst, and why it is seasonal in both directions
Freezing is the cause everyone knows. Water expands as it freezes, and the pressure that builds between the ice plug and a closed tap is what splits the pipe. The split is often not at the ice: it is downstream of it, wherever the wall is weakest.
A burst that happens in a cold snap usually shows itself during the thaw rather than the freeze, because the ice that split the pipe is also plugging the hole. A house can look fine for two days and then flood the moment the temperature comes back up.
In hot, dry regions the causes are different and less seasonal. Ground movement in expansive soils shears supply lines under slabs. Water that runs consistently hard scales the inside of a pipe, narrows it, and drives up velocity and pressure. Older galvanized steel corrodes from the inside and fails without warning once the wall thins past a point. Polybutylene, installed widely between the late 1970s and the mid 1990s, degrades from contact with treated water and is a known failure class in its own right.
Plastic is not immune to freezing. PEX is far more forgiving than copper or CPVC because it can expand, and it frequently survives a freeze that would split a rigid pipe, but it is not freeze-proof and the fittings on it are less forgiving than the tube.
Narrowing down where it is before anybody arrives
You do not need to find the break. You need to tell whoever arrives enough that they are not starting from nothing, because the search is most of the labor on a concealed leak.
Start with the meter test. Shut the main, then watch the meter. If it stops moving once the main is closed, the leak is inside the house. If it keeps moving with the main closed, the break is between the meter and the house, which is a different repair and often a different trade.
Then isolate hot from cold. Close the cold inlet valve on the water heater. If the meter or the sound stops, the break is on the hot side, which on a slab usually means a shorter search because hot runs are more predictable.
Note the age of the house and what the supply pipe is, if you know. Copper, galvanized steel, PEX and polybutylene each fail differently and each points at a different likely location.
What it costs, and what actually drives the number
The repair itself is rarely the expensive part. Reaching the pipe is. A visible break in an accessible basement or attic run is a straightforward job. The same break under a slab, behind tile, or inside a finished wall costs multiples of it, because the work is demolition, access and reinstatement rather than plumbing.
That is why an honest quote over the phone is a range and not a number, and why anybody who gives you a firm figure before seeing it is guessing. Ask for the diagnostic or call-out fee, ask whether it is credited against the repair, and get both answers before a truck rolls.
Ask separately who handles the drying and the reinstatement. Many plumbers fix the pipe and leave the wall open, which is reasonable and is not always made clear up front.
When it can wait until morning, and when it cannot
It can wait if you have shut the main off, nothing is running, and the water that escaped is contained and on a hard surface. You have bought yourself the night and you will get better rates and a less rushed job in the morning.
It cannot wait if water is still moving and you cannot stop it, if it is anywhere near electrical equipment, if it is coming through a ceiling, or if it is running into a cavity you cannot see into. Every additional hour of water inside a wall is more drying, more mold risk, and more reinstatement.
It also cannot wait if the water is not clean. Sewage in the house is a health problem before it is a plumbing problem.
Common questions
Do frozen pipes always burst?
No. Freezing is necessary but not sufficient. The split happens when pressure builds between an ice plug and a closed outlet with nowhere to go, which is why leaving a tap barely running during a hard freeze helps: it gives the pressure an exit. Plenty of pipes freeze solid, thaw, and are fine.
Will PEX freeze and burst?
PEX can freeze, and it survives freezing far better than copper or CPVC because the tube can expand and return. It is not freeze-proof. The fittings and the transitions to rigid pipe are the usual failure points rather than the tube itself.
How do I know if the leak is under the slab?
The tells are a warm or damp patch on the floor, the sound of running water with everything shut off, and a meter that keeps moving. Confirm it by closing the cold inlet at the water heater: if the symptom stops, it is a hot-side slab leak, which is by far the more common of the two.
Should I turn the water heater off too?
If you have shut the main and drained the lines, yes, turn off the power or gas to the heater. An electric element that fires in an empty tank will destroy itself in minutes.
Will my insurance cover it?
Policies commonly cover sudden and accidental water damage and commonly exclude damage from a leak that has been going on a long time, and the repair of the failed pipe itself is often treated separately from the damage it caused. Photograph everything before you clean up and report it before you start repairs.
If it is happening now
Each city page below carries one number, answered day or night, that reaches a licensed plumber on call in that area.
Sources
- American Red Cross, preventing and thawing frozen pipes. The freeze mechanism and the pressure-between-the-ice-and-the-tap explanation
- United States Environmental Protection Agency, WaterSense. The water meter test for finding a concealed household leak