for this roof area
RecommendedGutter Length Is the Eave Run
Gutter runs along the horizontal edges of the roof, the eaves, and nowhere else. Rake edges, the sloped edges at a gable end, do not get gutter. So the length needed is simply the total of every eave run on the house.
Gutter lengthLinear feet of gutter = the sum of every eave run on the house
Measure at the fascia, not off a plan, and take each run separately so the downspout positions can be worked out per run rather than as one total.
A simple 30 ft by 40 ft gable roof house has gutter on the two 40 ft eaves and nothing on the 30 ft gable ends, so it needs 80 linear feet. The same footprint with a hip roof has eaves on all four sides, which is 140 linear feet. A house with dormers, a porch roof, a bay and an attached garage can easily have eight or nine separate runs, and each one needs its own drainage plan.
Add about 5 percent to the material order for cuts and waste on sectional gutter. Seamless gutter is rolled on site to the exact run length, so there is very little waste, but the end caps, inside corners, outside corners, outlets and elbows are all separate pieces that have to be counted individually.
Sizing the Gutter
Gutter capacity is a question of how much water arrives, how fast. Five inch K style is the standard on the great majority of US homes and handles a normal roof in normal rainfall without trouble. Six inch K style is the step up, and it carries roughly 40 percent more water while its 3 in by 4 in downspouts move about double what a 2 in by 3 in downspout does.
Go to 6 inch when any of these apply:
- A large roof area draining to a single run, generally more than about 1,800 square feet.
- A steep roof. Water runs off a 12 in 12 pitch far faster than off a 4 in 12, and the same rainfall arrives at the gutter in a shorter burst.
- A metal roof, which sheds water quickly and can overshoot a 5 inch gutter entirely.
- A region with high peak rainfall intensity, which is the number that matters rather than annual total.
- A valley that dumps a large area onto a short section of eave. A diverter or a larger gutter local to the valley is often the answer.
Half round gutter is the traditional profile and holds less water than K style at the same nominal size, so it usually needs to be sized one step up. Half round in 6 inch is roughly comparable to K style in 5 inch.
Downspouts
Downspouts, not gutter size, are usually what limits a system. Water backs up and overflows at the corner furthest from the outlet long before the trough itself is full.
Downspout countOne downspout per 30 to 40 linear feet of gutter, and enough total capacity for the roof area draining to that run
A 2 in by 3 in downspout drains about 600 square feet of roof. A 3 in by 4 in downspout drains about 1,200 square feet. Use whichever of the two rules gives the larger number.
A 40 ft eave draining 900 square feet of roof needs one downspout by the spacing rule, but a 2 in by 3 in outlet only handles 600 square feet, so either the downspout goes to 3 in by 4 in or a second 2 in by 3 in is added at the other end. Two downspouts on a long run also halve the distance water has to travel, which reduces the total fall the gutter needs.
Roof area draining to the runRecommended gutterDownspout sizeDownspouts on the run
Under 600 sq ft5 in K style2 in x 3 in1
600 to 1,200 sq ft5 in K style3 in x 4 in1
1,200 to 1,800 sq ft5 in K style3 in x 4 in2
1,800 to 2,400 sq ft6 in K style3 in x 4 in2
2,400 to 3,600 sq ft6 in K style3 in x 4 in3
Over 3,600 sq ft6 in K style or larger3 in x 4 in4 or more
Roof area here means the actual sloped area draining to that run, not the building footprint. On a steep roof the sloped area is significantly larger than the plan area, and steep roofs also deliver water faster, so both effects push toward larger downspouts.
Where downspouts go
Put downspouts at corners where they can be tucked against a wall, away from doors and walkways, and away from windows. Discharge has to move water well clear of the foundation. An extension carrying water at least 4 to 6 feet from the wall, or a buried drain line running to daylight or a dry well, is what keeps a basement dry. A downspout dumping at the foundation is a common cause of wet basements and settling.
Hangers
Hangers hold the gutter to the fascia. Space them every 24 to 36 inches, and tighten that to 18 to 24 inches in any area that gets snow and ice, because a gutter full of ice weighs many times what a gutter full of water weighs. Add a hanger within 6 inches of every end cap, corner and outlet.
Hidden hangers that clip into the gutter lip and screw into the fascia are the current standard and hold far better than the old spike and ferrule. Every hanger screw should hit solid wood, ideally a rafter tail rather than just the fascia board. On a house with rotten fascia, the fascia has to be replaced before the gutter goes up, since no hanger holds in soft wood.
Slope
Gutter is not installed level. It needs a slight fall toward each downspout so water keeps moving and debris flushes through instead of settling.
Gutter slopeAbout 1/4 inch of fall per 10 feet of run, toward the downspout
A 40 ft run to a single end downspout drops 1 inch total. The same run with a downspout at each end is sloped from a high point in the middle, so each half falls half an inch.
More slope is not better. Too much fall looks wrong against a level fascia line and can let fast water overshoot the gutter at the low end. Too little and water stands in the trough, which breeds mosquitoes, holds debris and adds weight. Run a string line and mark the fall before hanging anything.
Sectional Versus Seamless
Sectional gutter comes in 10 ft lengths from a home center and joins with slip connectors sealed with gutter sealant. It is the practical choice for a homeowner installing their own, it can be carried in a car, and a damaged section can be replaced on its own. Every joint is a potential leak, and the sealant needs redoing periodically.
Seamless gutter is rolled on site from a coil by a machine on a truck, so a 60 ft run arrives as one unbroken piece with joints only at corners and outlets. It leaks less and looks cleaner, and it is what most professional installers use. It cannot be done without the machine, so it is not a DIY option, and a damaged length means replacing the whole run.
Aluminum in 0.027 in thickness is standard and 0.032 in is heavier and worth the small extra on long runs or in snow country. Copper and steel are available, cost considerably more, and both have their own maintenance profile.
Gutter Guards
Guards keep leaves out and reduce cleaning, and they range from cheap foam inserts and plastic screens to micro mesh panels and reverse curve systems. None of them make a gutter maintenance free. Micro mesh performs best against pine needles and shingle grit but the mesh itself needs brushing off. Reverse curve systems can shed water past the gutter entirely in heavy rain.
Guards also add weight and can complicate hanger spacing, and some products void a roofing warranty when they are fastened under the shingles. Check the roofing manufacturer's position before installing anything that slides under the first course.
Sizing depends on local rainfallGutter and downspout sizing is properly based on the peak rainfall intensity for the area, the roof pitch and the drainage area, and local code or a manufacturer's chart may call for larger sizes than the general figures here. Confirm requirements locally, and get quotes from installers who measure the roof rather than the footprint.