Why pipes freeze, and why they burst when they thaw
Water expands by about 9% when it turns to ice. In a pipe, that ice forms a plug. As more water freezes, pressure builds between the plug and the nearest closed tap, and the pipe gives way at its weakest point.
That weak point is often not where the ice is. It is also usually not noticed until the thaw, when the ice melts and water pours out of the split. A pipe that looked fine while frozen can start leaking the moment the temperature rises.
This is why the first sign of trouble matters. A tap that runs slowly or not at all in cold weather is a warning, not just an inconvenience.
Which pipes are most at risk
Pipes in unheated or exposed places freeze first. Check these:
- The loft: pipes and the cold water tank sit above the insulation, where it is coldest.
- Garages, outbuildings, cellars and porches: anywhere without heating, including a utility room on an outside wall.
- Outside taps and garden supply pipes: fully exposed to the weather.
- Pipes in external walls and under floors: common in older homes, including many stone cottages and farmhouses.
- The boiler condensate pipe: the plastic pipe that carries waste water from a condensing boiler, often running outside.
- Empty homes: second homes, holiday lets and anywhere nobody is turning the heating on.
Rural properties often have longer runs of pipe in barns, yards and outbuildings, so more of the system is exposed to the cold. If you live in the countryside, it is worth walking the whole property before winter, not just the house.
Before the freeze: a checklist
These jobs take an hour or two and cost very little compared with a flood.
- Find your stopcock and test it. It is usually under the kitchen sink, in a utility room or under the stairs. Turn it fully off and on again. If it is stiff or stuck, ask a plumber to look at it now, not during an emergency. Make sure everyone in the house knows where it is.
- Insulate exposed pipes. Foam pipe insulation is cheap and easy to fit. Focus on the loft, garage, outbuildings and anywhere a pipe runs along an outside wall.
- Insulate the cold water tank in the loft, but not underneath it. Leave the space beneath the tank clear so warmth from the house can rise and reach it.
- Deal with outside taps. If there is an isolation valve, turn it off and open the tap to drain it. Fit an insulated cover over the tap.
- Fix drips and leaks now. They are easier to repair in October than at 2am during a freeze.
- Keep the heating on. A low background temperature, or timed bursts in the morning and evening, is much safer than leaving it off. On very cold nights, open the cupboard doors under sinks on outside walls so warm air can circulate.
- Get the boiler serviced. Boilers are most likely to break down on the coldest days. Use a Gas Safe registered engineer.
- Save two numbers in your phone: a plumber you trust, and your insurer’s claims line.