What Size Downspouts for 6-Inch Eavestroughs in Ottawa?

If you have upgraded to 6-inch K-style eavestroughs, you need 3-inch by 4-inch rectangular downspouts to handle the increased water volume those larger gutters collect. Pairing 6-inch eavestroughs with undersized 2-by-3-inch downspouts is one of the most common drainage mistakes in Ottawa, and it creates a bottleneck that defeats the purpose of the larger gutter.

A 6-inch K-style eavestrough holds roughly 40 percent more water per linear foot than a standard 5-inch profile, which is exactly why Ottawa homeowners with steep roofs, large roof areas, or valley-style rooflines choose the bigger size. But all that extra capacity is meaningless if the downspout cannot drain the trough fast enough during a heavy storm. Ottawa regularly experiences summer thunderstorms dumping 25 to 50 millimetres per hour, and a 2-by-3-inch downspout on a 6-inch gutter simply cannot keep up. The eavestrough fills, overflows at the seams, and sends water cascading down your fascia and siding — exactly the damage you paid more to prevent.

Sizing by the Numbers

Standard 2-inch by 3-inch downspouts can drain approximately 600 square feet of roof area effectively. 3-inch by 4-inch downspouts handle roughly 1,200 square feet of roof area, doubling the drainage capacity. For a typical Ottawa two-storey home with a roof area of 1,500 to 2,000 square feet, you would need a minimum of two 3-by-4 downspouts, though three or four is the practical standard to ensure redundancy if one becomes partially blocked by debris.

The outlet hole where the eavestrough connects to the downspout also needs to match the larger size. If you are upgrading from 5-inch to 6-inch eavestroughs, the contractor should install 3-by-4-inch outlets cut into the new gutter trough. Retrofitting larger downspouts onto existing 5-inch eavestroughs rarely makes sense — the outlet hole in a 5-inch trough is not large enough to feed a 3-by-4 downspout efficiently.

Round downspouts are another option, with 3-inch and 4-inch round being the common residential sizes. A 4-inch round downspout has slightly less capacity than a 3-by-4 rectangular but offers a cleaner look on certain home styles. Round downspouts are more common on half-round eavestrough systems used on heritage homes in Rockcliffe Park, New Edinburgh, and the Glebe. For 6-inch half-round gutters, a 4-inch round downspout is the appropriate match.

Cost-wise, upgrading to 3-by-4-inch downspouts adds $50 to $100 per downspout over standard 2-by-3 sizing when done as part of a new installation. Retrofitting larger downspouts onto an existing system runs $200 to $400 per downspout since the outlet openings need to be reworked. Given that proper drainage protects your foundation from thousands of dollars in potential water damage, this is one of the most cost-effective upgrades in the eavestrough system.

For professional sizing advice specific to your roof configuration, browse eavestrough contractors through the Ottawa Construction Network directory at justynrookcontracting.com.