Sizing the Gutters Multiply the drainage area by the roof-pitch factor and rainfall intensity to find out the adjusted square footage. Then use the chart below to see what size gutter you need. (If a roof's various drainage areas call for different gutter sizing , go for the biggest one.)
Calculate the adjusted square footage by multiplying the square footage of the gutter's drainage area by the roof pitch factor and the rainfall intensity. This number will tell you what size gutter you'll need.
A K-style five-inch gutter has a capacity of 5,520 square feet. This is far less than our required drainage capacity. Instead, the six-inch K-style gutter with a capacity of 7,960 square feet should be used.
A common guideline is that you need one downspout per 40' span of gutter for proper drainage, but this is usually not enough for a 2”x3” downspout. A homeowner should also consider the size of the roof which will factor into how much water is going into the gutter.
Measure the length and width of each roof section. Multiply the length by the width to get the square footage of each section. Add the square footage of all sections to get the total roof area. This information will help you figure out the gutter capacity needed.
Your gutters should have a downspout around every 30 to 40 feet. This typically means installing downspouts at both ends of a gutter section.
As a general rule, contractors recommend having a downspout per every 20 feet of gutter, but depending on the gutter's capacity, it's not rare to see that distance go as far as 30-50 feet. Large gutters will offer a wider structure to channel more water at a time.
Remove any leaves, moss and other debris from the gutter and the rainwater pipe, this can significantly reduce the flow of water and in heavy rain that water will spill over the edge. Don't forget that mud and leaves etc does often get caught in the down pipe.
Anything more than a quarter of an inch of standing water in small spots can lead to algae growth and water damage. Standing water in gutters is a common symptom that can have multiple causes. Essentially it breaks down to two main possibilities with different variations between them.
6-inch gutters can handle almost 50% more water than 5-inch gutters. They are installed with bigger 3”x4” downspouts for better water management.
Gutter hangers should always be placed at least every three feet for proper support (2 feet in northern climates where snow and ice can add additional weight on the gutters).
5” gutters typically get the 2×3” downspout, so be aware that if you add 3×4” downspouts to 5” gutters, it will look very odd. Since work must be done to add your additional, larger downspout, now would be the best time to switch out to 6” gutters, as well.
As a general rule of thumb, choose 6-inch gutters for larger and steeper roofs and 5-inch gutters for smaller, low-slope roofs. Larger gutters typically hold more water, making them ideal for areas receiving heavier rains. However, different conditions may call for an adjustment in gutter size.
On average, for 6” aluminum gutters, the cost per foot is about 50 cents to 1 dollar higher than the cost of 5”.
Common downspout sizes include 2”x3” and 3”x4”. A good rule of thumb is that a 6-inch K-style gutter needs a 3”x4” downspout, whereas a 5-inch K-style gutter requires a 2”x3” downspout.
Calculating your effective roof area x rainfall intensity gives you a gutter flow capacity that can be crucial in finding the right size guttering, as you will see in the next section.
Gutters should be designed and installed to deal with any rainfall that flows off your roof, even when it gets heavy. If they can't cope with that, then something is wrong – and it needs to be put right. When your gutters overflow, they send rainwater down onto your walls and around the bottom of your building.
In general, it is recommended that the maximum length of a gutter with a single downspout should not exceed 30 to 40 feet (9 to 12 meters) to ensure proper water drainage. However, this is a general guideline, and it's always best to consult with a professional gutter installer or engineer for your specific situation.
A typical 20 foot run of gutters filled to capacity can hold approximately 200 lbs. To illustrate, consider a person weighing 200 lbs holding onto the gutters. The weight of the gutters can cause them to sag and pull away from the house.
Gutter Capacity
One foot of 5″ K-Style gutters holds 1.2 gallons of water. One foot of 6″ K-Style gutters holds 2.0 gallons of water.
Downspouts and gutters are sized in relation to rainfall on this basis. Plumbing codes typically use the vertically projected roof area for drainage design and they often use a square foot allowance per square inch of downspout for 1 in./hr. (25 mm/hr) rainfall that varies with diameter, for example, 3 in.
Burying downspouts improves the aesthetics of your home by hiding the pipes, enhances safety by reducing tripping hazards, and provides effective drainage that prevents water from pooling around your foundation, thus reducing the risk of basement flooding and soil erosion.
To get the best draining results from your gutters, we recommend having at least one downspout for 50 feet, and two 2 downspouts for over 50 feet.