Aluminum Gutter Downspouts: Sizes, Spacing and Where to Put Them

An aluminum gutter downspout is the vertical pipe that carries water from the gutter to the ground. Most houses use 2x3-inch or 3x4-inch rectangular aluminum downspouts. The 3x4 carries about twice the water of a 2x3 and clogs far less, so it's the better choice for most new installs unless the gutter is a small 5-inch run on a short roof.

Sizes and how much roof they drain

A rule of thumb many installers use: one square inch of downspout opening for every 100 square feet of roof it drains. It's a rough guide that assumes average rainfall. Use more downspouts in heavy-rain regions.

Downspout	Opening	Roof area per downspout (rule of thumb)	Pairs with
2x3 rectangular	6 sq in	about 600 sq ft	5-inch K-style
3x4 rectangular	12 sq in	about 1,200 sq ft	5- or 6-inch K-style
3-inch round	about 7 sq in	about 700 sq ft	5- or 6-inch half round
4-inch round	about 12.5 sq in	about 1,250 sq ft	6-inch half round and up

Measure roof area as the footprint of the roof plane draining to that gutter, then add some for steep roofs, which catch more wind-driven rain.

How many you need

• At least one for every 30 to 40 feet of gutter.
• One at each end of long runs, with the gutter sloping from the middle to both ends.
• One near each inside corner where two roof planes dump into the same gutter.

Two smaller downspouts often beat one big one, because a single clog doesn't take out the whole run.

Parts

• Outlet (drop): the short tube that fastens in the gutter bottom. Cut the hole cleanly and seal around the flange.
• Elbows: "A" elbows turn front to back (toward or away from the wall); "B" elbows turn side to side. Most runs use two A elbows to step from the gutter back to the wall.
• Straps: fasten the downspout to the wall every 6 to 8 feet and near each elbow.
• Extension or splash block: at the bottom, to move water away from the foundation.

Get the water away from the house

The downspout's job isn't done at the ground. Dump roof water right at the foundation and it ends up in the basement or crawlspace. Discharge it at least 4 to 6 feet from the house, farther on clay soil or if the yard slopes toward the house. Options are hinged extensions that flip up for mowing, buried solid pipe to a pop-up emitter, or a rain garden. Check local rules before tying into a storm sewer, and don't send water onto a neighbor's lot.

Aluminum gauge and finish

Downspouts come in thinner and thicker gauges. Thicker resists dents from mowers and kids. Match the color to the gutter or the wall. Some homeowners paint downspouts to disappear against the siding rather than match the trim.

Common problems

• Clogs at the top elbow: add a downspout strainer or outlet screen, or upsize to 3x4.
• Leaks at joints: each upper piece should slip inside the one below it, so water can't leak out at the joint.
• Washed-out soil at the bottom: add a splash block or extension; erosion at the corner is how basement leaks start.