How to Thaw Frozen Pipes Safely

When temperatures in Ontario drop well below -10°C, water pipes that run along exterior walls, through unheated crawl spaces or garages, or near the rim joist in the basement can freeze. Water expands by about 9% as it turns to ice, which puts enough pressure on the pipe to split copper and crack plastic fittings. Many burst pipes only start leaking once the ice melts, so a frozen pipe needs attention right away, not when the weather warms up.

The clearest sign is little or no water from one tap while others in the house still work. You may also see frost on an exposed pipe, a bulge in the pipe, or hear gurgling from the tap. If no fixture in the house has water, the freeze may be in the main service line or the problem may be on the municipal side. Check with a neighbour and call your city's water department before you start heating pipes.

A hair dryer, towels and a bucket are enough for most exposed pipes. A portable space heater helps for pipes inside a cold cabinet or wall, as long as it is kept at least 1 m from anything that can burn. For prevention afterwards, foam pipe insulation costs a few dollars per length at Home Depot, Canadian Tire or Rona, and a CSA-certified electric pipe heating cable (CAD $30–$80) can protect a pipe that freezes every winter.

1. Find your main water shut-off valve before you start (usually in the basement where the water line enters, next to the meter), so you can close it quickly if the pipe has split.
2. Open the affected tap, both hot and cold. Moving water helps the ice melt and relieves pressure in the pipe.
3. Trace the pipe back from the tap to find the frozen section. Look where it runs along an outside wall or through an unheated space, and feel for the coldest spot or look for frost.
4. Warm the pipe with a hair dryer, starting at the end closest to the tap and working back toward the frozen section so melted water can drain out.
5. For pipes under a sink on an exterior wall, open the cabinet doors and point a space heater into the cabinet from a safe distance. Wrapping the pipe in towels soaked in hot water also works.
6. Never use a propane torch, candle, barbecue lighter or any open flame. It is a serious fire risk and can damage plastic pipe and soldered joints.
7. Keep heating until full water pressure returns, then check the whole length of the pipe and the surrounding area for drips or wet spots.

Close the main water shut-off valve immediately. If water is near electrical outlets, fixtures or the panel, turn off power to that area only if you can reach the panel without standing in water. Open the lowest tap in the house to drain the lines, move valuables away from the leak, and take photos for your insurance claim. Call a plumber, and start drying the area within 24 to 48 hours to limit mould growth.

Insulate exposed pipes in unheated spaces, and seal air leaks around pipes where they pass through the rim joist or exterior walls. Shut off and drain outdoor faucets before the first hard frost (see our guide to winterizing outdoor faucets). Keep the thermostat at 15°C or higher at night and while you are away, and check your home insurance policy: many insurers require heat to be maintained, or the water shut off and drained, during longer absences. During a severe cold snap, open cabinet doors under sinks on exterior walls and let a vulnerable tap drip slowly overnight.

Call a licensed plumber if you cannot find or reach the frozen section, if the pipe is inside a finished wall or ceiling, if you see a crack or bulge, or if the same pipe freezes every winter. A plumber can reroute a vulnerable pipe into heated space, add insulation or heating cable, and repair or replace a burst section properly.