A whole generation of leaky pipes underneath Toronto's suburbs is partly to blame for a planned decade of water-rate hikes, city officials say.

In the older downtown, water is still delivered through cast-iron pipes, some of which date back more than 100 years.

But many of these Victorian artifacts are outlasting the newer, thinner pipes used in the postwar boom that created much of Scarborough, North York and Etobicoke.

"The cast iron was made quite thick, and might last another 50 years," said Michael Price, general manager of Toronto Water, "whereas some of the very thin ductile iron up in North York is breaking on a regular basis."

In North York, city crews scramble to fix more than 1,000 water- main breaks a year, many of which flood homes or snarl traffic. In the old City of Toronto, only about 80 water mains pop each year, Mr. Price said.

To deal with the problem, the city plans to keep water rates on a steep upward slope.

After three years of significant increases, city council's works committee approved a proposal yesterday to raise 2005 water rates by another 6 per cent, as part of a plan that would see annual increases of 7 and 8 per cent until at least 2010, with smaller increases after that. The council as a whole still has to approve.

The city says next year's proposed increase would add just $21 to the average annual household water bill, bringing it to $373 from $352. By 2007, the typical yearly bill would hit $427 -- still cheaper than the current rates for Vancouver ($525), Winnipeg ($614) and New York ($610 Canadian).

The increases will finance what will eventually become a more than $500-million-a-year repair budget to speed the rehabilitation of the city's water and sewer infrastructure and replace those thin pipes with ones made of plastic or coated metal.

Councillor Jane Pitfield, chair of the works committee, said Toronto could charge even more to get its watery house in order.

She would prefer a 9-per-cent increase this year, but fears that heavy industrial water users -- such as mega-brewer Molson, with whom she met this week -- might leave the city if the increase were high. (Her committee backed off a 9-per-cent proposal last year, opting instead for 6 per cent.)

Most residential users won't even notice the increases, she said, especially compared to their gas or electricity bills: "Everyone should be able to afford this."

The problem of newer pipes dying before their 19th-century predecessors is not unique to Toronto; it is happening across Canada and North America, and replacement costs are rocketing into the billions of dollars. University of Toronto civil engineering professor Bryan Karney, a water infrastructure expert, said finding faults decades after the fact is nothing new in the water business. Predicting how long materials left underground will last is difficult.

"We never wanted to do things that were just pure shortsightedness," he said of the postwar suburban pipes. "Unfortunately . . . if you wait around 20 years to know whether something's going to last, you've already made your decisions by then."

Manufacturers made the problem pipes by spinning the metal instead of pouring hot iron into moulds. The pipes were thinner and cheaper. But it also meant they rusted more quickly.

Prof. Karney said the plastic or coated-metal pipes that are now replacing them should last much longer. One study in Germany found that plastic PVC (polyvinyl chloride) pipe, dug up after 50 years in the ground, still met all of its original technical specifications. "It looked like it was going to last another 100 years."

In addition to replacing its leaky pipes, the city's water system is struggling to meet heavy new demands. The city is expected to grow rapidly, and summers, thanks to climate change, are getting hotter, Prof. Karney said. Making people pay the full cost of their water service should encourage much-needed conservation, he added.

The city says it can treat and pump 1.2 million litres of water a day, or about enough to fill the SkyDome.

But on peak days in the summer, the city comes close to running down its reserves.

"We're expecting more of our systems," Prof. Karney said. "It's sort of like taking an old-style automobile and putting it on [Highway] 401 today. Everybody's doing 130, but cars in the fifties didn't regularly do that."