Gutter Footage & Downspout Sizing
Gutter footage to buy, hangers, guards, and downspout count sized against your roof area and local rainfall intensity, so the water actually leaves.
Capacity tables are per downspout: a 2x2 handles about 600 sq ft per inch of rain per hour, 3x2 about 1,350, 4x3 about 2,780, 5x3 about 4,230. Over that, water leaves at the scupper.
How the math works
- Gutter footage: linear run at eaves and rakes + 3% splice loss seamless, 8% sectional. Gutters run at the fascia line, so measure the run, not the roof area.
- Downspout capacity: the standard table gives 600 sq ft per inch/hour for 2x2, 1,350 for 3x2, 2,780 for 4x3, 4,230 for 5x3. Divide by your design rainfall to get sq ft of roof per downspout.
- Downspout count: roof area ÷ capacity per downspout, minimum two per continuous run, placed away from corners where the water migrates and toward the lowest point of the run.
- Slope: gutter runs 1/16 in per 10 ft toward the outlet, minimum 1/4 in over a 20 ft run; a 50 ft run needs 1¼ in of fall, which is why the outlet goes at the middle of a long span, not the corner.
- Hangers & guards: hangers every 24 in (spaced spikes on the fascia at 24 in), leaf guards every 5 ft of run, 2 elbows and an outlet per downspout, extensions 4 to 6 ft from the foundation.
How many downspouts for a 1,200 sq ft roof?
At 2 in/hr design rain, a 3x2 downspout carries 675 sq ft, so a 1,200 sq ft roof needs two, minimum, and three if the run is longer than 40 ft with water travelling both directions.
Are 5 inch gutters worth it over 5?
5 in K-style gutters carry about 40% more volume than 5 in on a like-for-like radius, cost about 20% more, and resist overflow in ice and leaf conditions. For a 40 ft run with a 1,400 sq ft catchment, 5 in is the answer.
How far apart are gutter hangers?
Every 24 in in the run, 12 in where ice and snow load are factors, and a hanger within 6 in of every splice and outlet. Spike-and-fascia at 24 in with a 1/16 in per 10 ft slope is the standard install.