Engineers are warning that when the mercury drops, Toronto's century-old system of water mains and pipes are at risk of snapping and flooding streets, as the city faces a repair backlog of $1.7 billion to update its infrastructure.

The city repairs up to 80 kilometres of old cast-iron and metal pipes each year with PVC pipes, costing millions of dollars. The city is also constantly evaluating the total amount that needs to be fixed or replaced.

"You invest in something, you tend to think the problem is over. But pipes don't last forever, nothing humans build lasts forever," engineer Bryan Karney told CTV Toronto.

Digging below the frost line to fix the rusting and deteriorating pipes costs the city more than $100 million a year.

Lou Di Gironimo, general manager of Toronto Water, said that the city has found problems even in some of the newer parts of the city, such as in Etobicoke and North York, where the water mains were installed in the 1950s.

"It's a thinner wall of material," he said. "It's placed in clay soils, so you get a lot of corrosion."

The average age of pipes is 55 years in Toronto, though some have been in place since the turn of the 20th Century.

"Age is only one component of the problem that we have with the water mains in this city. The biggest problem is the type of materials, construction and the soils that the pipes are placed in," Di Gironimo said.

Because of the aging infrastructure, city taxpayers' water bills will increase nine per cent as of Jan. 1.

The increase, which is part of a multi-year nine per cent increase to keep the system in shape, will cost the average Toronto household an extra $62 a year.

"As residents and taxpayers know, that money is going directly to making sure we have clean and safe drinking water and they're OK with that," said Coun. Denzil Minnan-Wong.

Di Gironimo says that he deals with 1,400 water main breaks a year.

With files from CTV Toronto's Natalie Johnson. Follow her on Twitter at @NatalieCTV.