Inputs
in/hr
Use local design storm. This scales downspout capacity.
ft
Used to estimate drainage area from each section length.
ft
Cold climates often require closer spacing.
ft per downspout
Custom capacity
sqft per downspout
Used only if capacity rule is set to Custom.
in/hr
If your rainfall is higher than this, effective capacity reduces proportionally.
Roof Sections (eave runs to be guttered)
Add each eave run length. Optionally override its drainage depth and corner count.
| # | Run Length | Depth (opt.) | Inside Corners | Outside Corners |
|---|
Results
Enter values and click Calculate to see outputs.
Example Data Table
| Section | Run | Depth | Area | Inside | Outside |
|---|---|---|---|---|---|
| A | 40 ft | 12 ft | 480 sqft | 1 | 1 |
| B | 24 ft | 10 ft | 240 sqft | 0 | 1 |
| C | 36 ft | 14 ft | 504 sqft | 2 | 0 |
These illustrate typical inputs. Use the "Load Example" button to preload similar values.
Formulas & Method
- Total Gutter Length: sum of all eave runs, minus intentionally un-guttered segments.
- Estimated Drainage Area per Section: Ai = Li × Di, where L is run length and D is average roof depth. Add pitch factor if desired.
- Hanger Count: Nh = ⌈Ltotal / sh⌉ + 1, where sh is hanger spacing.
- Downspouts by Spacing: Ns = ⌈Ltotal / sds⌉.
- Downspouts by Capacity: effective capacity scales with rainfall intensity: Ceff = Cref × (Iref / I). Then Nc = ⌈Atotal / Ceff⌉.
- Recommended Downspouts: N = max(Ns, Nc). Corner fittings and end caps are reported from inputs.
This tool provides planning estimates. Confirm with local codes and manufacturer data.
How to Use
- Select units and set your design rainfall intensity.
- Enter average roof depth from each eave to ridge line.
- Add each guttered eave run. Include optional section depth and corners.
- Set hanger spacing, downspout spacing, and pick a capacity rule.
- Click Calculate to see lengths, fittings, and downspout counts.
- Export results to CSV or PDF for quotes and takeoffs.
FAQs
Capacity values are typical baselines. They vary with outlet geometry, bends, leader length, and rainfall. Use manufacturer data and local codes for definitive sizing.
Steeper roofs shed water faster. As a conservative simplification, multiply drainage area by 1.10–1.25 for steep slopes if local guidance recommends such adjustments.
Common practice is 2 ft in temperate climates, closer in heavy snow zones. Always follow manufacturer recommendations and regional requirements.
Enter counts per section. The report totals both types to assist with fittings procurement and fabrication planning.
Yes. The tool scales the reference capacity by the ratio of your design storm to the reference intensity, yielding a conservative effective capacity.
Use the CSV or PDF buttons to download lengths, fittings, and counts. Attach the file to your estimate or material takeoff.