Royal Bank of Canada economists are fretting over the condo construction boom.

They’re flagging other issues, as well, notably that of housing affordability in Vancouver and Toronto.

But condo construction gets a red flag across the board, as this RBC monitor below shows.

As this second graphic shows, the number of multiple units under construction was high in the first quarter of the year compared to longer-term trends, at 5.7 per 1,000 population.

That latest number is just below the 5.8 units reached in 2014, according to the report this week from RBC chief economist Craig Wright and senior economist Robert Hogue.

“This level is well into the ‘high risk zone’ (4.5 units or higher),” Mr. Wright and Mr. Hogue wrote in the report.

“Such high levels of condo construction are concentrated in the Toronto (33 per cent of total) and, to a lesser degree, Vancouver (14 per cent), Montreal (11 per cent) and Calgary (7 per cent) markets.”

The RBC economists said they see little sign of overbuilding of single detached homes.

And, for that matter, condo purchases have been “quite solid” over the past year.

It’s current condo building that RBC is flagging.

A “large part” of that is because of policy-driven changes, such as restrictions to contain urban sprawl, and the key issue affordability, given the lower cost of a condo, the economists wrote.

“Nonetheless, the prospects for high levels of condo completions in the period ahead in markets such as Toronto, Montreal and Calgary maintain above-average absorption risks.”