Residential Downspout Sizes A 2x3 inch downspout can handle roughly 600 square feet of roof area for every inch of rainfall within an hour. On the other hand, a 3x4 inch downspout can handle twice that - 1200 square feet of roof area per inch of rainfall in an hour.
Most homeowners install 5-inch and 6-inch gutters. Larger 7-inch and 8-inch gutters are more suitable for commercial properties. 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 Downspouts Based on Gutter Length
The general rule of thumb is one downspout for every 20-30 feet of gutter. This may vary depending on several factors such as the pitch of your roof, the volume of rainfall in your area, and the capacity of your gutters.
Larger downspouts are also less likely to clog. Leaves, twigs, and other debris that might get caught in a smaller downspout can easily pass through a larger one. This not only reduces the frequency of clogs but also diminishes the need for regular cleaning.
A 2x3 inch downspout can handle roughly 600 square feet of roof area for every inch of rainfall within an hour. On the other hand, a 3x4 inch downspout can handle twice that - 1200 square feet of roof area per inch of rainfall in an hour.
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.
Even though 68mm round and 65mm square downpipes are among the most common, most rainwater systems are available in multiple sizes. For example, this might be 75mm, 87mm, 100mm and 120mm for a certain range, with corresponding gutter sizes to match.
Instead, the six-inch K-style gutter with a capacity of 7,960 square feet should be used. Downspouts must also be sized to handle this level of rainfall. Rectangular 2 x 3-inch downspouts have a capacity of 600 square feet, while 3 x 4-inch downspouts can handle 1,200 square feet.
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.
To keep it easy, we recommend that you install one downspout every 20-40 feet of gutter. This will ensure that you are taking the proper precautions to defend against potential water damage to your house. The average home has 2-3 downspouts.
How many downspout installations should your gutters have? If we have to sum it up in a single rule, gutters need a downspout around every 30 to 40 feet. For most homes, this means installing spouts at either end of a gutter section.
The right way to do it is to have the downspout end at least 12 inches from the ground level, leaving enough room to pitch the extension at the base without disrupting the flow of water. If the building is located at the top of a slope, you don't have much to worry about.
If your roof's square footage requires 5-inch gutters, you'll need a 2-inch by 3-inch downspout. That creeps up to a 3-inch by 4-inch downspout for 6-inch gutters. It's necessary to create custom downspout solutions for gutters wider than 6-inches that are being installed to address excessive pitch or rainfall levels.
This downspout has a drainage capacity of 186 gallons per minute.
5 inches and 6 inches gutters are the most common sizes installed for residential use. However, 7, and 8 inches are available although the larger sizes are more suitable for larger buildings or business properties. As far as downspouts go, they run at 2″x3″ and 3″x4″ in size or 3 or 4 inches in diameter.
The downspout size may be 2”x3” or 3”x4” as suggested in Table 4–5. Use of the larger size downspout, if architecturally acceptable, may help reduce the potential for clogging from debris.
No bigger isn't better unless you plan on replacing stock exhaust. Think about it - having a larger taper than transitioned back to the smaller stock exhaust pipe makes no sense from a performance perspective because you are still limited to the smaller opening.
A three-inch pipe is what's used in homes to pipe toilets. The four-inch pipe is used as the building drain under floors or in crawlspaces to transport all the wastewater from a home out to the septic tank or sewer. The four-inch pipe may also be used in a home if it's capturing two or more bathrooms.
Should Downpipes go into the Ground? No! The aim of a downpipe is to keep water away from the building, especially its foundations. As such a downpipe should direct water into a surface water drain or soakaway, which will allow the rainwater to gradually soak into the ground safely.
Generally, in a residential context, 5" gutters with 2x3 is the typical "normal" size and 6" gutters with 3x4 downspouts is the typical "large" size. If you currently have normal/small gutters, then the next size up might help.
Six-Inch Gutters Can Handle More Water
Compared to five-inch gutters, six-inch ones can hold twice as much water even though they're just an inch bigger. Upsizing your gutters allows them to efficiently divert a more significant amount of water from your roof and home.
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.