How many downspouts do I need?

Enough that no downspout drains more roof than it can carry, and at least one for every separate run. The installers' rule of thumb is about 600 sq ft of roof, measured in plan, for each 2×3" downspout and about 1,200 for a 3×4", with no more than about 40 ft of gutter feeding one outlet. A 28' x 44' house with a foot of overhang has 1,380 sq ft of roof over two 46' eaves, which comes to 4 downspouts, 2 a side. In heavy-rain country, SMACNA's method sizes them from local rainfall intensity and asks for more.

How far apart should gutter hangers be?

No more than 24" apart is what gutter makers commonly ask for, and closer — 16" to 18" — where snow and ice sit on the roof. Put one within a few inches of every end and every seam, and drive hidden hangers into the rafter tails or the fascia backing, not just the fascia board.

How much slope should a gutter have?

About 1/4" of fall for every 10 ft, toward the outlet. A 40 ft run with its downspout in the middle falls 1/2" from each end to the outlet; with the downspout at one end it would fall twice that, which starts to show against the fascia — one reason long runs get an outlet at each end or in the middle.

Why does each downspout need three elbows?

Two at the top bring it from the gutter outlet, which hangs out past the fascia, back across the soffit to the wall. One at the bottom turns it away from the foundation onto a splash block or into an extension. A downspout that runs past a deep overhang may want a short length of downspout between the top two, which is the same two elbows with a piece in the middle.

Gutters and downspouts

Enter the eaves, the roof area and the eave height, and get the order: feet of gutter in sections, hangers, end caps and corners, and how many downspouts, outlets and elbows the roof above needs.

Eave length

Every eave that gets gutter

Separate runs

Each has two ends

Roof area, in plan

sq ft

Footprint plus overhang

Eave height

Ground to gutter

Advanced settings600 sq ft a downspout · 40 ft max run · hangers 24" · 10 ft sections · 0 corners · 3 elbows each

Roof per downspout

sq ft

600 for a 2×3, about 1,200 for a 3×4 — a rule of thumb

Most gutter per outlet

About 40 ft is usual

Hanger spacing

24, closer under snow

Gutter section

Length it is sold in

Corners

Inside or outside mitres

Elbows per downspout

Two at the top, one at the foot

Downspout section

Length it is sold in

92 ft of gutter

4downspouts

Gutter sections

Hangers

Elbows

Order this

Gutter92 ft in 2 runs of 46 ft — or 92 ft of seamless, bought by the foot: 10 × 10 ft
HangersOne at each end of a run and every 24" between: 48
End capsA left and a right for every run: 4
OutletsOne drop outlet for each downspout: 4
Downspout4 downspouts at 10 ft, 40 ft in all: 4 × 10 ft
Elbows3 for each downspout: 12

Why this many downspouts

Each run is assumed to drain an equal share of the roof — 690 sq ft and 46 ft of gutter each — and gets 2 downspouts, set by the roof area at 600 sq ft a downspout. That leaves each one draining about 345 sq ft.

Both are installers' rules of thumb — about 600 sq ft for a 2×3" downspout, 1,200 for a 3×4", and no more than about 40 ft of gutter to an outlet. The proper method, in SMACNA's Architectural Sheet Metal Manual, sizes them from your area's design rainfall, and a heavy-rain region wants more downspouts than this. Valleys that pour onto one stretch of gutter want an outlet near them, whatever the arithmetic says.

Slope the gutter about 1/4" per 10 ft toward each outlet — about 5/16" of fall from the high point to the outlet here.

Where to buy this

10 gutter sections, 10 ftHome Depot Lowe's
48 gutter hangersHome Depot Lowe's
4 end capsHome Depot Lowe's
4 drop outletsHome Depot Lowe's
4 downspout sectionsHome Depot Lowe's
12 downspout elbowsHome Depot Lowe's

Gutter profile and size (5" or 6" K-style is usual) have to match across every piece, so buy one system from one shop.

Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.

Worked example: 92 ft of eave in 2 runs

92 ft of eave in 2 runs of 46 ft takes 10 sections of 10 ft gutter, 48 hangers at 24" and 4 end caps.

1,380 sq ft of roof at 600 sq ft a downspout, with no more than 40 ft of gutter to an outlet, needs 2 downspouts a run — 4 in all, each with its outlet and 3 elbows, 12 elbows altogether.

At 10 ft to the gutter that is 40 ft of downspout, 4 sections of 10 ft.

Gutter sections: 10
Hangers: 48
End caps: 4
Downspouts: 4
Elbows: 12

These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.

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About gutters and downspouts

How many downspouts do I need?: Enough that no downspout drains more roof than it can carry, and at least one for every separate run. The installers' rule of thumb is about 600 sq ft of roof, measured in plan, for each 2×3" downspout and about 1,200 for a 3×4", with no more than about 40 ft of gutter feeding one outlet. A 28' x 44' house with a foot of overhang has 1,380 sq ft of roof over two 46' eaves, which comes to 4 downspouts, 2 a side. In heavy-rain country, SMACNA's method sizes them from local rainfall intensity and asks for more.
How far apart should gutter hangers be?: No more than 24" apart is what gutter makers commonly ask for, and closer — 16" to 18" — where snow and ice sit on the roof. Put one within a few inches of every end and every seam, and drive hidden hangers into the rafter tails or the fascia backing, not just the fascia board.
How much slope should a gutter have?: About 1/4" of fall for every 10 ft, toward the outlet. A 40 ft run with its downspout in the middle falls 1/2" from each end to the outlet; with the downspout at one end it would fall twice that, which starts to show against the fascia — one reason long runs get an outlet at each end or in the middle.
Why does each downspout need three elbows?: Two at the top bring it from the gutter outlet, which hangs out past the fascia, back across the soffit to the wall. One at the bottom turns it away from the foundation onto a splash block or into an extension. A downspout that runs past a deep overhang may want a short length of downspout between the top two, which is the same two elbows with a piece in the middle.