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.
Residential Downspout Sizes
Typically, residential downspouts come in two sizes: 2x3 inches and 3x4 inches. The former is often used for smaller roofs, while the latter is preferred for larger homes. A 2x3 inch downspout can handle roughly 600 square feet of roof area for every inch of rainfall within an hour.
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.
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.
To calculate for heavy rainfall conditions, however, sizes should be based on a figure of 10cm² of gutter and 8 cm² of downpipe for 7 m² of root area. Conversely, if dry conditions prevail, calculations should be based on 10 cm² of gutter and 8 cm² of downpipe being sufficient for 14 m² of roof area.
Gutter size is determined by measuring across the top of the gutter. To help you understand gutter measurement and dimensions, here is a relative comparison in size between 2×3 inch downspouts vs. 3×4 inch downspouts… likewise, a comparison between 3 inch round downspout and 4 inch round downspout.
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.
For most residential properties, the standard downspout size is typically 2×3 inches. However, this may not suffice for areas with frequent heavy rainfall or for larger roofs that channel a significant amount of water. In these cases, upgrading to a larger downspout is recommended to handle the increased water flow.
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.
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.
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.
While 5-inch gutters are the standard size, some homeowners opt for larger ones. 6-inch gutters are more expensive, but they can handle large amounts of water and are less likely to get clogged.
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.
Homeowners With Bigger Roof Systems
Because 6-inch gutters hold 50% more water than their 5-inch counterparts, rainwater has more space to run. In turn, there's less chance of it flowing over the sides of the gutters, helping protect your fascia and siding from water damage.
pipe, you also increase your pounds per square inch. You have a higher water pressure which prevents clogging. You can run up to two downspouts on a 3 in. line.
3×4 Downspout
The MPS 3X4 downspout is a heavy duty commercial grade downspout system. The profile is a corrugated downspout with a double lock seam. All types of construction including commercial, industrial, and residential. For both steep sloped or flat roof systems.
As far as downspouts go, they run at 2″x3″ and 3″x4″ in size or 3 or 4 inches in diameter. Gutter professionals have experience matching the gutters and the downspouts. For example, a 6-inch K-style gutter requires a 3″ x 4″ downspout while a 5-inch K-style gutter uses a 2″ x 3″ downspout.
To ensure proper coverage and protection, bury your downspout extension at least 12 inches deep. This depth helps protect the pipe from surface activities and ensures efficient water drainage.
While 4-inch corrugated is the industry standard, there are several reasons why a 3-inch downspout pipe could be the right choice for particular drainage application. 3-inch corrugated pipe moves roof runoff water at a higher velocity.
As mentioned above, the water should be diverted at least 4 to 6 feet away from the house. If a home has basement walls, it should be at least 6 feet away. The furthest away from the foundation the better. Putting gravel or rocks at the end of the downspout to avoid erosion is a great idea.
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.
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.
To calculate the minimum number of downpipes, divide the roof catchment area by the allowable maximum catchment per downpipe. To calculate the average catchment per downpipe, divide the roof catchment area by the number of downpipes.