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.
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.
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.
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.
Most residential homes will use a gutter size between 110mm and 125mm. Areas with high rainfall and larger roofs need gutters with more capacity. Different types of gutters offer different capacities. For example, ogee gutters handle more water than half-rounds of the same size.
For homes requiring enhanced water flow capacity, 3×4 inches downspouts are often recommended. These oversized downspouts can handle approximately twice as much water as their standard-sized counterparts.
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.
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.
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.
Install downspouts every 20 to 50 feet along the gutter. The more closely downspouts are spaced along the gutter, the more water can be conducted away during peak rainstorms.
The good news is: There's no shortage of options. 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.
A 5 inch gutter is typically recommended for your standard shingle and slate roofs with little to no overhang. Roofs with an overhang, commonly seen with tile or metal roofing, should have a larger 6 inch gutter system.
To ensure proper coverage and protection, bury your downspout extension at least 12 inches deep.
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.
P2912. 5 Roof Gutters and Downspouts.
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.
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.
The downspout requirements for five and six inch gutters are about the same even though gutter downspout sizes vary. The general guideline for downspouts is one for every 25 feet of gutter, with 40 feet between downspouts being the maximum limit.
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.
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.
The best type of gutter depends on factors such as the building's architecture, local climate, and style preferences. Half round gutters offer easy flow and minimal debris accumulation, square line gutters maximise space efficiency, while ogee gutters combine functionality with decorative appeal.