With more rain bearing down on the region, a Toronto architect says the city must find new ways to soak up excess water.

"We have an incredibly hard surface downtown core," said Lisa Bate on CBC Radio's Metro Morning.

Bate is an architect with B+H Architects and Canada's representative to the World Green Building Council.

She says the city's inability to absorb record-setting rainfall or cope with high Lake Ontario water levels stresses the sewer system and forces more permeable land in Toronto's suburbs to take on an extra burden.

She suggested three ways that Toronto could steel itself against the increasingly unpredictable — and this spring, very wet — weather.

Waterproofed buildings

Bate says downtown buildings must be be built or renovated in anticipation of increasing rain and water levels.

Some buildings could be built on stilts, she said. Others might make better use of green roofs — roofs covered in absorbtive vegetation — to help soak up rain.

She said that work has already started at some of the city's largest downtown buildings, including the TD Centre, First Canadian Place and RBC Centre.

"These kinds of developers have continual improvement plans to improve permeability," Bate said.

Rainwater harvesting

Along with making structures more permeable and absorbent, Bate says buildings could find ways to re-purpose extra water.

The RBC Centre, for example, recently installed a rainwater harvesting system which is collecting water to flush toilets.

Separating the sewer system

The water that makes it past collection systems or soaks through green spaces ends up in Toronto's sewer system.

In cases of a heavy downpour, that can send a mix of storm and sanitary water into Lake Ontario, due to the city's combined sewer system.

While the city has dedicated reserves for storm water, it has no choice but to pump the mixed sewage and storm water into the lake during extreme rainfall.

Eastern Avenue flooding

Lake water has spilled out of sewers after heavy storms. (Shannon Johnson)

"There's nothing they can do, we cannot pump any more and we're seeing more discharge into the harbour," Bate said.

That system may be contributing to dangerously high bacteria levels in the Toronto harbour.

Toronto has started work to separate the system, but it will not be complete until at least "a decade," according to the city's deputy manager John Livey.

The city says it is committed to overhauling its water and wastewater infrastructure, which is expected to cost more than $12 billion over the next 10 years.