Valley Flashing Takeoff (Metal, Pieces, Ice Barrier)
The valley flashing metal a roof needs, in linear feet, stock pieces, and square feet.
Example
You enter
- Plan run of ONE valley (ft) 12
- Number of valleys 2
- Roof pitch (rise per 12) 6
- Valley metal width (in) 24
- Stock length (ft) 10
- Lap at each joint (in) 6
- Waste (%) 10
You get
- Valley multiplier 1.5
- Valley length (ft) 18
- Total valley (LF) 36
- Effective piece (ft) 9.5
- Pieces 5
- Metal area / valley ice barrier 72
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method.
Details, formula, and sources
ScopeA valley is the longest run on the roof for its plan dimension because it goes diagonally AND uphill: the multiplier carries 288 under the radical where a common rafter carries 144, so a 12-ft plan run at 6:12 is exactly 18 ft of valley against 13.4 ft of common. Ordering off the plan comes up badly short.
Formula or tablevalley multiplier = sqrt(pitch^2 + 288)/12 (the framing-square 17-inch rule) against a common rafter's sqrt(pitch^2 + 144)/12; valley length = plan run x multiplier; pieces = ceil(LF x (1 + waste)/(stock - lap/12)); metal area = LF x width/12.
Source and editionValley geometry by the framing-square 17-inch rule - public geometry, no table reproduced.
Free accessThe 17-inch rule is in every framing reference and on the framing square itself.
What governsEstimate. AHJ and licensed professional govern.
API field namesvalley_run_ft, valley_count, pitch_rise_per_12, metal_width_in, stock_length_ft, lap_in, waste_pct, valley_multiplier, valley_length_ft, total_valley_lf, effective_piece_ft, pieces, metal_area_sf
- 17-inch rule valley multiplier sqrt(pitch^2 + 288)/12; the fuzzer pins agreement with the hip-valley-rafter tileframing-square geometry
- Open valley exposed metal; closed-cut and woven valleys use shingles and are out of scopestated scope limit
- Valley ice barrier reported as an allowance at the metal width; the eave ice-barrier tile does not include valleysstated gap in the sibling tile