# Roof Flashing Calculator

Flashing is measured on developed width; fixings every 300 mm.

- Page: https://jianzhujisuan.com/en/roof-flashing
- Category: Construction (https://jianzhujisuan.com/en/c/construction)
- Chinese version: https://jianzhujisuan.com/zh/roof-flashing.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Parapet / gable runs 40 m, Gutter / valley runs 20 m, Pipes through roof 4 pcs, Perimeter per pipe 0.6 m, Developed width 0.3 m, Sheet thickness 0.6 mm, Lap allowance 10%): **Total flashing run 62.4 m**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Parapet / gable runs | number | 40 | m |  |
| Gutter / valley runs | number | 20 | m |  |
| Pipes through roof | number | 4 | pcs |  |
| Perimeter per pipe | number | 0.6 | m |  |
| Developed width | number | 0.3 | m |  |
| Sheet thickness | select | 0.6 mm | — |  |
| Lap allowance | number | 10 | % |  |

## Formula

- **Total run**: L = parapet + gutter + pipes × perimeter
- **Developed area**: A = L × width × (1 + lap)
- **Unit weight**: q = thickness(mm) × 7.85 (kg/m²)
- **Total weight**: W = A × q
- **Fixings**: n = (L / 0.30 + 1) × 2

## Worked example

40 m parapet, 20 m gutter, 4 pipes at 0.6 m perimeter, 300 mm developed width, 0.6 mm sheet, 10% laps: 62.4 m total run, 20.6 m² developed, 4.71 kg/m², ~97 kg of sheet and ~418 fixings.

## Questions

### Why does flashing leak so often?

Because it does three things at once: it joins two materials, it sits across a movement differential, and it has to hold water out. The field of the roof is continuous so water only passes over it; at a flashing the concrete and the metal differ in thermal expansion by three to five times, so they must move relative to each other and a mastic joint gets stretched hundreds of times until it fails. The answer is not more sealant but a top edge set into a groove, enough upstand height (150 mm minimum, 250 mm where ponding is possible) and a drip at the bottom edge.

### How thick should flashing be?

0.6 mm galvanised sheet is the general-purpose choice. Use 0.8 mm on large roofs or where maintenance traffic is expected, and 1.0 mm in high-wind coastal or tall-building situations. 0.5 mm suits only small areas with no foot traffic because it is soft enough to buckle with thermal movement. On coated steel roofs use matching coated sheet - galvanised and aluminium in direct contact form a galvanic couple when wet and the aluminium is consumed.

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