Determining the correct number of downspouts for a roof is an important part of proper drainage and gutter system design. With too few downspouts, gutters can overflow and cause water damage. With too many, the system becomes overly complicated and expensive. There are several factors to consider when calculating downspout quantity, but the process can be summarized into three main steps:
- Calculate the roof area
- Determine the maximum spacing between downspouts
- Divide the roof area by the downspout spacing
Following these steps will help ensure your roof has an adequate drainage system to handle rainwater flows. We’ll go through each step in detail in this article.
How to Calculate Total Roof Area
The first step in determining downspout quantity is calculating the total square footage of the roof area to be drained. This includes all surfaces that will shed water into the gutter system.
There are a few ways to calculate roof area:
- Use the footprint dimensions – For simple roof styles like gable or hip designs, you can find the area by multiplying length x width of the structure’s footprint.
- Break up into rectangles – For more complex roofs, divide into smaller rectangles and triangles and calculate each individually, then add them up.
- Scale on plan – For existing structures, you can use an architectural plan scaled to the real dimensions and measure the roof sections.
Once you have the measurements, convert dimensions into square feet. This total roof area will be used to determine how many downspouts are needed.
For example:
- House with 35 ft x 45 ft footprint = 1,575 sq ft
- Dormer roof section = 12 ft x 15 ft = 180 sq ft
- Total roof area = 1,575 sq ft + 180 sq ft = 1,755 sq ft
Tips for Getting Accurate Roof Measurements
When calculating roof area, accuracy is important to ensure the drainage system is designed properly. Here are some tips:
– Use a measuring tape for dimensions rather than estimating or scaling from a photo.
– For sloped roofs, measure horizontal distance (not slanted roof surface).
– Include overhangs, dormers and any other sections that will contribute runoff.
– Double check math to avoid miscalculations.
– For existing roofs, verify measurements onsite if possible.
Taking the time to correctly calculate total roof area will pay off through properly spaced downspouts and adequate drainage capacity.
How to Determine Downspout Spacing
Once the total roof area is known, the next step is to determine the maximum spacing between downspouts. There are a few factors to consider:
- Roof pitch – Steeper roofs need closer downspout spacing.
- Climate and rainfall intensity – Areas with heavy rain need closer spacing.
- Gutter size – Larger gutters can handle more sq ft between downspouts.
- Roof surface – Smooth surfaces like metal can space downspouts further apart.
Most building codes require downspouts every 40 feet or less. A good rule of thumb based on common roof pitches is:
| Roof Pitch | Maximum Spacing |
| Flat (1/12 or less) | 50 ft |
| Low slope (2/12 – 4/12) | 40 ft |
| Steep slope (5/12 or more) | 30 ft |
For a standard gable roof with 4/12 pitch, start with a 40 ft downspout spacing. Adjust as needed based on climate or other factors.
How to Adjust Spacing for Heavy Rainfall
In regions with heavy rainfall, the downspout spacing should be decreased:
- For areas with 65 in/hr and above rainfall, use 30 ft spacing.
- Between 50-65 in/hr rainfall, use 35 ft spacing.
- Between 30-50 in/hr rainfall, use 40 ft spacing.
Check average rainfall intensity for your location to determine if a tighter downspout spread is needed.
Calculating Number of Downspouts
Once you know the total roof area and the desired downspout spacing, you can calculate the required number of downspouts using this formula:
Total Roof Area (sq ft) / Downspout Spacing (ft) = Minimum Number of Downspouts
Let’s see this in an example for a 2,000 sq ft roof with standard 4/12 pitch in a region with 35 in/hr average rainfall:
- Roof area = 2,000 sq ft
- Spacing (adjusted for rainfall) = 35 ft
- 2,000 sq ft / 35 ft = 57 downspouts
Round up to the nearest whole number, so this roof would need a minimum of 58 downspouts.
You can then space the downspouts evenly around the roof perimeter based on the desired maximum spacing.
Tips for Adjusting Downspout Quantity
The calculated downspout quantity should be considered a minimum. Here are some tips for adjusting:
- Round up to the next whole downspout – Avoid fractions.
- Account for existing obstructions on the roof that may block drainage flow.
- For very large roofs, you may want to decrease spacing by 5-10 ft and add more downspouts for better drainage.
- Follow manufacturer’s instructions for specific gutter types and local building codes.
It’s better to have slightly more downspouts than required than too few. The goal is to prevent overflowing gutters and safely direct roof runoff.
Locating and Sizing Downspouts
Once you know the number of downspouts needed, the next steps are determining their locations around the roof perimeter and sizing each one properly.
Where to Locate Downspouts
Ideally downspouts should be positioned:
- At the ends of gutter runs
- Near valleys or low points where water collects
- Near roof intersections or changes in roof direction
- So rainwater can drain fully from all roof surfaces
Avoid locating downspouts:
- Where they will be blocked by landscaping, decking, or other obstructions
- Too close to electrical service panels or outlets (minimum 10 ft clearance)
- Against surfaces that could be damaged by drainage
Proper placement is crucial for directing runoff safely away from the structure once it reaches the bottom of the downspout.
Sizing Downspout Width and Quantity
Each downspout must be sized correctly based on the roof area draining to it. Manufacturers provide size charts based on square footage and rainfall rate.
For example, a 3×4 inch rectangular downspout can typically handle 500-600 sq ft of roof in moderate rainfall areas. Upsize to 4×5 inch for roof areas over 1,000 sq ft.
In heavy rainfall climates, a secondary overflow downspout is often required to handle peak storm loads.
Use multiple smaller downspouts rather than one large one when possible for better drainage across all roof surfaces.
Consult manufacturer sizing guides to choose the proper downspout width and quantity of downspouts based on your specific roof dimensions and rainfall.
Special Considerations for Large Roofs
For homes or buildings with very large roof expanses, calculating downspouts takes some additional considerations:
- Span across two or more exterior walls with gutters and downspouts.
- Step down the roof height with internal drainage valleys to break up roof sections.
- Pitch the roof area back toward the interior of the structure so the perimeter has shorter slope lengths.
- For flat roofs, create roof sumps where downspouts can be located.
- Use overflow scuppers spaced between downspouts to prevent pooling.
Work with an experienced gutter contractor or engineer to ensure proper drainage on large roof designs.
Installing Downspouts
Once you’ve determined the correct quantity and locations, proper installation of the downspouts is essential for directing roof runoff safely away from the structure. Here are some best practices:
- Use hangers or straps spaced every 3 to 6 feet to securely attach downspouts.
- Allow a 1/4 to 1/2 inch gap between gutter bottom and top of downspout.
- Use elbows and bends to direct water at least 5 feet away from the home’s foundation.
- Connect downspouts to drainage pipes that carry water towards storm sewers or away from the property.
- Seal all connections to prevent leakage and detachment.
- Ensure outlet is free of obstructions and not draining onto paved surfaces.
Proper downspout installation is just as important as correctly calculating quantity and placement. Directing the roof runoff well away from the structure prevents costly water damage.
Downspout Maintenance Tips
Once installed, downspouts require periodic inspection and maintenance:
- Check for leaks at all connections and fittings.
- Clear any clogs or debris blocking the downspout.
- Repair any loose attachments or hangers.
- Extend downspout outlet away from home if necessary.
- Paint to maintain appearance and prevent corrosion.
- Ensure water flows freely from gutters without overflowing.
Well-maintained downspouts are critical for moving roof water safely away from your home’s exterior and foundation.
Hiring a Professional
While it is possible for a knowledgeable DIYer to calculate, install, and maintain downspouts, hiring a professional gutter contractor is recommended for most homes. Benefits of professional downspout installation include:
- Expertise in properly calculating downspout quantity and placement.
- Access to special equipment like lifts for tall roofs.
- Efficiency of completing the job quickly and safely.
- Higher quality materials and installation work.
- Ability to handle existing roof drainage issues.
- Knowledge of local building codes.
For complex roofs or drainage problems, professional help can ensure your downspouts are correctly installed to protect your home.
Conclusion
Determining the proper quantity and placement of downspouts for a building involves calculating roof area, choosing maximum spacing, dividing area by spacing, adjusting for rainfall, and correct installation. Following the steps outlined, even DIYers can ensure their roof has adequate drainage capacity to handle rainwater without overflowing. For homes with large or complex roofs, hiring a professional contractor is recommended to get the downspout system designed and installed correctly. Properly calculated and installed downspouts will prevent costly storm damage for years to come.
