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Roof Flashing Calculator

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

Quick answer

With the default inputs: Parapet 40 m · Gutter 20 m · Pipes through roof 4 pcs etc.Total flashing run 62.4 m

Use your own dimensions ↓

Inputs

pcs
%

Result

Total flashing run
62.4m
Parapet / gable40.0m
Gutter / valley20.0m
Pipes (perimeter total)2.4m
Developed area incl. laps20.6m²
Sheet unit weight4.71kg/m²
Total sheet weight97.0kg
Fixings (two rows)418pcs
How this is worked out

Flashing is the waterproofing at junctions between the roof and vertical elements - parapets, gables, pipes and rooflights. About 70% of roof leaks come from these junctions rather than the field of the roof. Measure each run separately: parapets and gables by length, gutters and valleys by length, and pipes by perimeter times number. Developed width is the flat width before folding; parapet flashing is normally 250-300 mm (150 mm upstand plus 100-150 mm on the flat) and gutter flashing is wider. Sheet unit weight is thickness × 7850 kg/m³, so 0.6 mm is about 4.71 kg/m². Allow 10% for laps of 50-100 mm per length, with corners and stop ends counted separately. Fixings are estimated at one every 300 mm in two rows, plus extra at corners and ends. The top edge must be set into a groove or held by a cover bar and sealed - fixings alone with an open top edge let water track behind the sheet.

Next up: Roof Tile Calculator · Roof Sheathing Calculator

Formula

Total runL = parapet + gutter + pipes × perimeter
Developed areaA = L × width × (1 + lap)
Unit weightq = thickness(mm) × 7.85 (kg/m²)
Total weightW = A × q
Fixingsn = (L / 0.30 + 1) × 2
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.

Basis

FAQ

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.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Roof Flashing Calculator. 2026-10-25. https://jianzhujisuan.com/en/roof-flashing

Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.