What do I do if a pipe has frozen?

Turn the water off first, then thaw gently from the tap end back — and what to check once it flows again.

Difficulty: beginner
Time: 1 hr
Read: 2 min
Safety: warning

Short answer

Turn off the stopcock and open the affected tap first. Then thaw gently from the tap end backwards with warm cloths or a hairdryer on low. Never use a flame, and never start at the middle.

A frozen pipe is often already split — ice expands and cracks the pipe, and the leak only appears when it thaws. That is why the stopcock comes first, before any heat.

Safety

Never use a naked flame, a blowtorch or a heat gun on a pipe — it can cause the water to boil and burst the pipe, and it is a fire risk. Turn the water off at the stopcock before thawing, so that if the pipe has already split you are not flooding the house when it thaws. If you cannot reach the pipe, or it has burst, call a plumber.

What you'll need

Towels or cloths
Warm water
Hairdryer (optional)
Buckets

Step by step

Turn the water off at the stopcock.Before anything else. If the pipe has split, thawing it releases water into the house. This single step is the difference between a repair and a flood.
Open the affected tap.It gives the melting water somewhere to go and relieves pressure. You will also see the moment it starts flowing.
Find the frozen section.Usually where a pipe runs through an unheated space — a loft, a garage, an outside wall, under a suspended floor. It will be very cold to the touch, sometimes frosted.
Thaw from the tap end backwards.Start nearest the open tap and work towards the frozen section. Starting in the middle traps meltwater between two ice plugs, and the pressure can burst the pipe.
Use gentle heat only.Towels soaked in hot water and wrapped round, a hairdryer on a low setting kept moving, or a heat pad. Never a blowtorch or heat gun — the water can boil and burst the pipe.
Be patient.It can take an hour or more. Rushing with more heat is exactly how pipes split.
Check for leaks as it thaws.Watch the pipe and the area below it. Turn the stopcock back on slowly once flowing, and check every joint and the whole run.
Insulate it before next winter.Pipe lagging on any pipe in an unheated space. It costs very little and it prevents the whole problem.

Tips

If you cannot find or reach the frozen section, or the pipe has burst, turn the water off and call a plumber. Water damage escalates fast.
Leaving heating on a low background setting during a cold spell is far cheaper than one burst pipe.
Know where your stopcock is and check it turns before winter. A seized stopcock discovered during a burst is a serious problem.

Common mistakes

Thawing before turning off the water — If the pipe has split — which is common, since ice cracks pipes — thawing releases water into the house.
Using a blowtorch or heat gun — It can boil the water inside and burst the pipe, and it is a fire risk in a loft full of insulation.

Questions people ask

How do I thaw a frozen pipe?

Turn the water off at the stopcock, open the tap, then apply gentle heat starting from the tap end and working back — hot cloths or a hairdryer on low. Never a flame.

Will a frozen pipe burst?

Often it already has — ice expands and splits the pipe, and the leak only shows when it thaws. That is why turning the water off first matters so much.