A 5 inch gutter takes a 2×3 inch downspout. A 6 inch gutter takes a 3×4 inch downspout.
Those two pairings cover almost every house in Chicago. The 3×4 moves about twice the water of the 2×3, so the size of your gutter decides the size of the pipe under it.
What size downspout fits a 5 inch gutter?
A 5 inch K style gutter pairs with a 2×3 inch downspout. That gives you 6 square inches of opening at the outlet.
Trade sizing tables allow roughly 100 square feet of roof for every square inch of downspout opening. One 2×3 drains about 600 square feet of roof.
Most Chicago bungalows put more than that on a single side. A 5 inch system usually needs two or three downspouts, and one is almost never enough.
What size downspout fits a 6 inch gutter?
A 6 inch gutter pairs with a 3×4 inch downspout. The opening measures 12 square inches, which drains about 1,200 square feet of roof.
We fit 6 inch oversized aluminum as our standard, so 3×4 goes on most of our jobs.
A 6 inch trough feeding a 2×3 pipe is the mismatch we find most often on other people’s work. The wide trough fills at the rate the roof sends it, and the narrow outlet cannot empty it that fast.
How many downspouts does a house need?
Plan on one downspout for every 30 to 40 feet of gutter. Add one at each inside corner, because two roof slopes drain into that single spot.
A long front elevation with one downspout at the far end is the most common sizing error we see. Water travels the whole length before it finds an exit, so the opposite end overflows first.
Corners, valleys and dormers all send extra volume to one place. Those spots earn their own outlet.
What goes wrong when the downspout is undersized
Water climbs over the front lip of the gutter during the heaviest ten minutes of a storm. From the driveway it looks like a sheet coming off the edge of the roof.
That water lands against the foundation instead of six feet out. A few seasons of soaking beside the house is how a dry basement turns into a damp one.
Small outlets also clog sooner. A 2×3 opening jams on one clump of maple seeds, and a 3×4 passes the same clump without stopping.
What Chicago weather does to the math
The National Weather Service puts Chicago’s yearly precipitation near 38 inches of rain and melted snow. A single summer storm can drop an inch of that in under an hour.
A downspout sized for an average week will overflow in that hour.
Winter adds a second problem. A 2×3 elbow packs with ice sooner than a 3×4 does. Once that elbow blocks, meltwater backs into the gutter and freezes at the roof edge, which is one of the ways an ice dam begins.
How to check your own house
Stand in the driveway during the next hard rain and watch the gutter edges. If water comes over the front lip, the system cannot move what the roof sends it.
Count the downspouts against the length of gutter on that side. A 60 foot front with one downspout is short by at least one.
Measure the outlet at the top of the pipe, not the bottom. Some houses have a 3×4 outlet stepped down to a 2×3 at the ground, which chokes the whole line.
What we fit, and why
Our standard is 6 inch oversized aluminum at .032 inch thickness, with 3×4 downspouts and QuickScrew hangers every 24 inches. We seal the corners with GeoCel and set gutter apron flashing behind every trough.
Every install comes with a lifetime warranty on the gutters and on the workmanship.
Read the full spec on materials and components. Or call us at (866) 488-7776 and we will size your downspouts off your own roofline, for free.
