Calculating Downspouts Based on Gutter Length The general rule of thumb is one downspout for every 20-30 feet of gutter.
Correct Number of Downspouts for Your Gutter System
Your gutters should have a downspout around every 30 to 40 feet. This typically means installing downspouts at both ends of a gutter section. Although there are some factors that can affect the total number of downspouts you need for your house.
This means more water needs to be diverted away from the building and therefore more downpipes may be required. A good rule of thumb is one downpipe for every 9m (30ft) of guttering.
Dividing the total "effective" roof catchment area by the catchment area for the gutter selected, will give the number of downpipes required. This is not normally a whole number, so it is rounded up to the nearest whole number.
Size of the Gutter Drainage Area: As a rule of thumb, one downspout should be placed every 25-35 linear feet of guttering. Depending on the local climate and rainfall intensity, some homeowners may need more downspouts. Many homeowners add extra downspouts to boost the drainage capacity of their gutters.
An insufficient number of downspouts can lead to water overflow, while too many can disrupt the aesthetic appeal of your home's exterior.
A maximum 12 m gutter length served by each downpipe is to ensure effective fall and adequate capacity to discharge all water anticipated during a storm having an average recurrence interval of 20 years.
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.
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.
The installation of downpipes, especially near valley gutters, is designed to ensure rainwater from areas on the roof that have concentrated water flows is adequately removed. Particular consideration needs to be given to box gutters, valley gutters etc. located above the internal areas of a building.
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.
When choosing a colour, think about how the downpipes will look against the wall colour, and also ensure it matches the gutters. Typically, you would choose the same colour of downpipes and guttering, but it really is up to you.
Cast iron and copper guttering repairs cost significantly more, at around £50 to £65 per metre. Replacing a downpipe is typically more expensive. uPVC downpipe replacement costs around £22 per metre, while cast iron downpipes cost between £70 to £80 per metre to replace.
Downspouts on large gutters
If your downspout measures 2” by 3” it can have a hard time handling the increased volume. Adding another downspout can evenly distribute the water to avoid risk of failure.
However, placing them up to every 40 feet apart is adequate when this is your only option. Each downspout can only handle so much water. It is essential to have enough downspouts in your gutter system. Without enough downspouts, the water will not drain from your horizontal gutters quickly enough.
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. Choosing the 3 x 4-inch rectangular downspouts, we'd need at least seven to handle the expected water flow.
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.
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.
Your new gutters should be installed at a certain angle to ensure rainwater is diverted to the drains. As a rule of thumb, gutters should have a slope of 1/4 inch for every 10 feet.
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 you only have one downspout, on a stretch of gutter, then you may want to add a downspout on the opposite side. It's important to note that the pitch of the gutter should be sloping toward your existing downspout so water will flow in that direction.
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.
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.
✔ Hold a downpipe clip centrally over the line and mark the fixing holes on the wall with a pencil. ✔ Repeat down the wall, spacing pipe clips no more than 1.8m apart. ✔ Drill the fixing holes. ✔ Leave a 10mm gap between the end of the pipe and the bottom of the pipe socket to allow for expansion.