What is the cost to upgrade round ductwork to rectangular for a Toronto basement ceiling?

Converting round trunk ductwork to rectangular in a Toronto basement typically costs $2,000 to $6,000, depending on the length of the trunk run, the number of branches and fittings, and whether the goal is to gain ceiling height, accommodate a bulkhead, or improve aesthetics for a basement finishing project. For a straightforward swap of a single 20 to 30 foot trunk line, expect $2,000 to $3,500 including removal of the old round duct and installation of the new rectangular system.

The primary reason GTA homeowners make this change is ceiling clearance. A 10-inch round trunk duct hangs 10 inches below the floor joists, while a 20x8 rectangular duct carrying the same airflow hangs only 8 inches — gaining 2 inches of headroom. In Toronto basements where ceiling height is already tight (many older homes have 7-foot or even 6-foot-8 basement ceilings), those 2 inches can make the difference between meeting the Ontario Building Code minimum of 6 feet 5 inches for finished basement ceiling height and falling short. Some homeowners go further with low-profile rectangular ducts — a 24x6 or 28x6 configuration can carry equivalent airflow while hanging only 6 inches below the joists.

The cost breakdown for a typical basement trunk conversion includes: custom rectangular duct fabrication ($15 to $30 per linear foot for material), installation labour ($30 to $60 per linear foot), transition fittings where the new rectangular trunk meets existing round branches ($40 to $80 each, typically 6 to 12 needed), a new supply plenum if the furnace connection needs modification ($200 to $600), sealing all joints with mastic and UL 181 foil tape (included in quality installation), and removal and disposal of the old ductwork ($200 to $500). If the project is part of a basement renovation, the ductwork conversion should happen before framing and drywall to keep costs down.

One important technical consideration: rectangular duct has more surface area than round duct of equivalent airflow capacity, which means slightly higher friction loss. A qualified contractor will calculate the equivalent rectangular dimensions using duct sizing tables or Manual D calculations to ensure your HVAC system maintains proper airflow after the conversion. Simply guessing at dimensions or choosing the flattest profile possible can result in undersized ducts that restrict airflow, increase static pressure, create noise, and reduce your furnace and air conditioner efficiency.

For GTA homes with post-war galvanized steel round trunk ducts that are 50 to 70 years old, this conversion is an opportunity to replace aging ductwork with new sealed and insulated rectangular duct — effectively killing two birds with one stone. Old round trunk ducts often have leaky joints, minimal sealing, and surface rust. New rectangular duct with mastic-sealed joints can reduce duct leakage by 20 to 30 percent, improving both comfort and energy efficiency.

Get at least three quotes and make sure each contractor provides the specific rectangular dimensions they plan to use and confirms that airflow will be maintained. Toronto Ductwork can match you with experienced sheet metal contractors through the Toronto Construction Network.