Roof Flashing Calculator

How the roof flashing calculator works

Flashing is the metal that keeps water out where a roof meets an edge, a wall or another slope: drip edge along eaves and rakes, step flashing up a sidewall, and valley lining where two slopes meet. This calculator turns a run of flashing into a count of pieces. It adds your wastage allowance to the length, divides by what one piece covers, and rounds up. Enter feet or metres; the logic is the same.

The input that matters is coverage per piece, and it's not the same as the length on the label. Flashing pieces overlap where they join, so each one covers less than its full length. With the US defaults, 40 ft of flashing plus 10% is 44 ft, and at 9.83 ft of coverage per piece that's 4.47, so 5 pieces, $90 at $18 each. The metric default, 12 m plus 10% at 2.95 m per piece, is also 5 pieces, £75 at £15.

Drip edge: length minus the lap

Drip edge runs along the eaves and rakes of a shingle roof. The International Residential Code, section R905.2.8.5, requires adjacent pieces to overlap by at least 2 in (51 mm), and asks for the metal to be fastened at no more than 12 in centres; the City of Pacific, Missouri, publishes the 2018 IRC\'s R905 roof-covering requirements. So a 10 ft length covers 10 ft less 2 in, or 9.83 ft, which is the default here. In metric, a 3 m length lapped 51 mm covers about 2.95 m. The lap alone adds about 1.7% to the count; the wastage allowance is for the cut ends at corners and where the eave meets the rake.

A typical house might need 120 ft of drip edge around its eaves and rakes. At 5% wastage, 120 × 1.05 ÷ 9.83 = 12.8, so 13 pieces. The metric version, 30 m at 5% and 2.95 m coverage, is 11 pieces.

Step flashing: one piece per course

Step flashing isn't sold by the length. Where a roof slope meets a wall, a short bent piece of metal goes under each course of shingles and up the wall behind the siding. GAF's shingle installation instructions say to use one piece of non-corroding metal step flashing for each course, with each piece reaching 5 in (127 mm) onto the shingle and 5 in up the wall, and the pieces overlapping each other by 2 in (51 mm). The same sheet gives a shingle exposure of 5-5/8 in (143 mm).

So for step flashing, enter the shingle exposure as the coverage per piece: 0.47 ft or 0.143 m for a 5-5/8 in exposure, or whatever exposure your shingles use. A 12 ft sidewall with 10% wastage then needs 12 × 1.1 ÷ 0.47 = 28.1, so 29 pieces. A 3.6 m wall in metric needs 3.6 × 1.1 ÷ 0.143 = 27.7, so 28. Sidewall flashing also has minimum sizes in most building codes, so check the pieces you buy against yours.

Valleys and other runs

Valley flashing works like drip edge: enter the length of each valley, and use the piece length less the lap from the manufacturer's instructions as the coverage. The IRC section R905.2.8.2 says metal linings for open valleys must be at least 24 in (610 mm) wide, and valley linings in general go in according to the manufacturer's instructions. Measure valleys along the slope, not on the plan, because a valley is longer than its footprint.

Run the calculator once for each type of flashing. Mixing drip edge, step flashing and valley metal in one total would divide very different pieces by one coverage figure and give a meaningless count.

Checking the count against the roof

Multiplying the pieces back out shows how much spare the rounding leaves. The US default of 5 pieces at 9.83 ft covers 49.2 ft against the 44 ft needed with wastage, so there's about 5 ft in hand. The metric default of 5 pieces at 2.95 m covers 14.75 m against 13.2 m. For the step-flashing wall, 29 pieces at 0.47 ft cover 13.6 ft of wall against 13.2 ft with wastage. The 120 ft drip-edge example comes to 13 pieces covering 127.8 ft against 126 ft.

Short offcuts rarely help. Each join needs its own lap, and a piece has to be long enough to fix properly, so a pile of short ends won't replace one full length. That's why the calculator always rounds up to whole pieces.

Measuring the roof and choosing metal

Measure each edge where it will actually be flashed: eaves along the gutter line, rakes up the slope, sidewalls from the eave to where the roof stops against the wall. Check the metal and finish against what's already on the roof, because dissimilar metals in contact can corrode. Other countries use different details, such as lead or aluminium soakers and cover flashings on many UK and European tiled and slate roofs, but the counting works the same way once you know what each piece covers.