Finding the frozen section of pipe
Identify the location of the freeze. By checking the flow of water from all appliances, it should be possible to narrow down the position of the freeze. Once the section of pipe has been identified, you may be able to narrow down the exact point further by looking for clues. Draughts from air vents next to pipework, an unlagged section of pipe or pipework next to a vent in the roofspace are all possible causes.
Before attempting to thaw the blockage, inspect the pipe and any nearby fittings. It is possible for the pipe to have split, or the fittings to have been pushed apart by the expansion of the ice. In this case, melting the ice will result in a leaking joint. If you find such damage, the water supply to this section should be turned off first, and preparation made to contain the leaking water once the pipe is thawed.
Thawing with gentle heat
Open the tap on the affected pipe to allow water to flow as soon as the ice begins to thaw.
If you are unsure as to the location of the ice blockage on the length of pipe, start nearest to the tap and gradually work away from it.
Apply heat gently to the affected area using a hair dryer or hot water bottle.
A hair dryer, however, may only be used if you are certain that the pipe / joints are not damaged or split. Never use an electrical appliance in the vicinity of water.
If you use a blowtorch, guard against fire and apply only gentle heat. Excessive heating will cause soldered joints to fail.