Gutter and downpipe calculator
Enter the dimensions of one section of roof that drains into one gutter. The calculator matches a downpipe diameter and gutter girth using a Slovak manufacturer’s nomogram, and checks the result against the effective area, which allows for driving rain.
- Sheet
- Gutter calculator
- Revision
- Sources
- 4 items
- Type
- calculation aid
How the calculator works
1. Plan area A = LR · BR, divided by the number of downpipes. For a downpipe in the middle of the gutter we divide it by 1.6 before matching it to a band, because according to KJG a central downpipe increases capacity by 60%.
2. Band from the KJG nomogram (based on DIN 1986):
| Plan area per downpipe | Downpipe Ø | Gutter |
|---|---|---|
| up to 37 m² | 60 mm | girth 200 |
| 37–83 m² | 80 mm | girth 250 or 280 |
| 83–150 m² | 100 mm | girth 280 or 333 |
| 150–243 m² | 120 mm | girth 333 or 400 |
| 243–443 m² | 150 mm | girth 400 or 500 |
3. Check with the effective area to EN 12056-3: for driving rain (26° from the vertical) half the height of the roof is added to the plan depth, A = LR · (BR + HR/2), where HR = BR · tan α. If the effective area falls into a higher band, the calculator flags a borderline case.
4. Flow Q = r · A · C (C = 1) is shown only if you enter a rainfall intensity. EN 12056-3 leaves this to national and local regulations; we do not choose it for you.
Next: Downpipes · Eaves gutters · Build an enquiry
Technical sources
The sources this sheet draws on. We checked prices and values from standards on the date shown; for prices, whatever the source currently states always applies. Sources marked “in Slovak” or “in Czech” are only available in that language.
| Item | Source | Published by | Version / date | Checked |
|---|---|---|---|---|
| 01 | Nomogram – sizing a rainwater system from the plan area of the roof (based on DIN 1986) (in Slovak) | KJG a.s., Čachtice | current file on the manufacturer’s website | 14 October 2026 |
| 02 | EN 12056-3:2000 Gravity drainage systems inside buildings – Part 3: Roof drainage, layout and calculation (relationship Q = r · A · C, effective roof area allowing for wind) | CEN; explained in: HR Wallingford, Manual SR 620 | 2000; manual 2003 | 14 October 2026 |
| 03 | Installation guide – KJG half-round rainwater system 2025 (bracket fall, expansion joints, thermal expansion) (in Slovak) | KJG a.s. | 2025 | 14 October 2026 |
| 04 | Lindab a.s. warranty certificate – conditions: min. gutter fall 5 mm/m, no contact with copper (in Slovak) | Lindab a.s. (Slovakia) | valid for products bought after 15 April 2019 | 14 October 2026 |
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