Downspout quantity is calculated based on roof area and local rainfall.
Standard Calculation
Rule: One 2x3" downspout per 600-900 sq ft of roof area
Rule: One 3x4" downspout per 1,200-1,500 sq ft of roof area
Example Calculations
Example 1: 1,000 sq ft single-story home
With 2x3" downspouts: 1,000 ÷ 750 = 1.3 → Need 2 downspouts
Typical placement: Front and back corners
Example 2: 1,500 sq ft ranch home
With 2x3" downspouts: 1,500 ÷ 750 = 2 → Need 2 downspouts
Typical placement: Left and right sides
Example 3: 2,500 sq ft two-story home
With 2x3" downspouts: 2,500 ÷ 750 = 3.3 → Need 4 downspouts
Typical placement: Each corner
Example 4: 3,500 sq ft two-story home
With 3x4" downspouts: 3,500 ÷ 1,300 = 2.7 → Need 3 downspouts
OR with 2x3": 3,500 ÷ 750 = 4.7 → Need 5 downspouts
Pacific Northwest Adjustment
Damascus receives 37" rainfall annually (vs US average 30"):
Standard calculation + 1 extra downspout recommended
Heavy rainfall = need more drainage capacity
Revised for Portland area:
- Less than 1,000 sq ft: 2 downspouts (vs 1 standard)
- 1,000-2,000 sq ft: 3 downspouts (vs 2 standard)
- 2,000-3,000 sq ft: 4-5 downspouts (vs 3 standard)
Placement Strategy
Best practice: Place downspouts at:
- Each house corner (corners = natural water division points)
- Midpoint of long runs (prevents overflow between corners)
- Low points where water naturally collects
Special Situations
Situation 1: Long run (80+ ft gutter)
- Add midpoint downspout to prevent overflow
- Example: 100 ft front gutter + 2 corners = add 1 extra in middle
Situation 2: Multiple roof levels
- Each level gets own downspout calculation
- Upper roof drains to upper downspouts
- Lower roof area calculated separately
Situation 3: Valley issues
- Valleys collect water from two roof planes
- Should always have downspout at valley base
Situation 4: Historic/steep roofs
- May need more downspouts for steeper pitch areas
- Gravity helps with downspout flow
Shop Downspouts →