Where this starts
Ice occupies more space than the water it came from. In a closed pipe that expansion has nowhere to go, so pressure builds between the ice plug and the nearest closed tap or valve until something gives — and what gives is usually a fitting or a length of pipe some distance from the ice itself.
THE SPLIT DOES NOT LEAK WHILE IT IS FROZEN. The same ice that caused it is plugging it. So a house can be through the cold snap with no water and no visible problem, and then flood on the afternoon it warms up. That is why the sequence matters more than the technique: know where the stopcock is, and be present when things thaw.
WHERE PIPES FREEZE is predictable and short: anywhere outside the insulated envelope. Lofts above the insulation, unheated garages and outbuildings, under suspended floors, in external walls, and anywhere a pipe runs past an airbrick or an unsealed penetration where cold air blows directly onto it. A pipe that has frozen once will freeze again in the same place unless something changes.
THAWING: gently, and from the right end. Start at the tap or outlet nearest the blockage with it OPEN, and work back along the pipe toward the ice — that gives melting water somewhere to escape and lets you see progress. Warm cloths, a hairdryer on low, or a room heater near the run. Never a blowtorch or any open flame: it boils water inside the pipe, which can burst it far more violently than the ice did, and it sets fire to joists.
IF THERE IS NO WATER AT ALL and the weather has been cold, check the simple things before assuming ice: whether neighbours have water, whether the stopcock has been knocked closed, and whether it is only some outlets — because a loft-fed system with a frozen storage cistern behaves quite differently from a mains-fed one with a frozen service pipe.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
A pipe outside the insulated envelope
Most commonLoss of flow after a hard frost, affecting outlets fed by pipework in a loft, a garage, an outbuilding or an underfloor void.
How to check: Which outlets have lost flow, and where those pipes run. Cold pipework in an unheated space is where it happens.
Cold air blowing directly onto a pipe
Most commonOne pipe freezing repeatedly while others in the same space do not, often near an airbrick, a vent, a gap at the eaves or an unsealed penetration.
How to check: Whether there is a draught at the spot. Moving cold air freezes a pipe far faster than still cold air does.
Insulation absent, gapped, or on the wrong side
CommonLagging that stops short at a bend, a valve or a tank connection, or loft insulation laid OVER the pipes so they sit in the cold zone.
How to check: Whether the lagging is continuous, and whether loft insulation passes over or under the pipe. Under the insulation keeps the pipe in the cold.
A pipe that is rarely used
CommonAn outside tap, a garage sink or a pipe to an unused bathroom freezing while everything in daily use stays clear.
How to check: Whether the affected run gets used. Moving water is much harder to freeze than water standing still.
A frozen storage cistern or its feed
CommonOn a loft-fed system, some outlets losing flow while the kitchen cold tap still runs, because that one is fed direct from the main.
How to check: Whether the kitchen cold tap still works when others do not. That pattern points at the loft rather than the incoming supply.
The incoming service pipe frozen underground
Less commonNo water at any outlet including the kitchen, often with neighbours affected too, after a prolonged freeze, on a shallow-buried supply.
How to check: Whether anything in the house has water, and whether neighbours do. Nothing anywhere points outside the property.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
The split reveals itself on the thaw, not on the freeze. Being able to shut the water off in seconds is the difference between a repair and a flood.
- 2
Everything dry points at the incoming supply; some outlets dry with the kitchen cold tap working points at a loft cistern or its feed on an indirect system.
- 3
It gives melting water somewhere to go, relieves pressure, and lets you see progress as flow returns.
- 4
Working from the far side traps melt water behind the plug. Warm cloths or a hairdryer on low — never a flame, which can burst the pipe far more violently than the ice.
- 5
The split is often some distance from where the ice was, at a fitting, and it will only start leaking once the ice has cleared.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where a pipe has split, since the repair has to be done with the water off and a temporary fix on a mains-pressure pipe is not a fix.
- Where you cannot find or cannot close the stopcock, which should be sorted out before the next cold spell rather than during it.
- Where the incoming service pipe is frozen outside the property, because that is the supplier's pipe up to the boundary and their responsibility beyond it.
- Where the system is unvented, sealed or has a hot water cylinder involved, since freezing in those brings pressure and safety components into it.
- Where a heating system has frozen — typically the condensate pipe on a condensing boiler — which stops the boiler and is a different repair from a water pipe.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.