What Is Valley Flashing?
Valley flashing is the sheet metal installed in the valley of a roof — the V-shaped channel where two sloping roof surfaces intersect. Water from both slopes converges at the valley, which makes it one of the highest-volume drainage points on any complex roof. Properly designed valley flashing channels this concentrated flow to the eave without allowing it to penetrate beneath the roofing material.
Valley flashing is used in open valley applications, where the metal is left visible in the center of the valley between trimmed roofing material edges. On standing seam metal roofs (SSMR), TPO, and modified bitumen systems, valley flashing geometry and width must be sized for the specific roofing system and drainage area — the SMACNA Architectural Sheet Metal Manual provides minimum width tables by drainage area and pitch.
Valley Flashing Profile Types
W-Valley (W-Profile)
The W-valley profile has a raised center rib (the crossbar of the W) that divides the valley into two distinct channels. This center rib performs two critical functions: it prevents high-velocity water from one roof slope from crossing over to the other slope during heavy rainfall, and it prevents debris accumulation in the valley center that can create ice dams and water backup. W-valley metal is the preferred profile for most commercial and light-commercial applications, steep pitches, and high-drainage-area conditions.
V-Valley (V-Channel)
The V-valley is a simpler profile without a center rib — a plain V-shape with flat flanges on each side. It is easier to fabricate and has lower material cost, but provides no cross-flow prevention. Use V-valley metal on shallow-pitch valleys where water volume is low and cross-flow is not a risk, and where simplicity of installation is a priority.
Flat-Pan Valley
A flat-pan valley is a wide flat sheet with formed flanges or cleats at each edge. It is used primarily on low-slope and nearly flat valley conditions where a V or W shape would be too steep and create ponding at the valley edges. Also used on certain TPO and modified bitumen systems where the flat profile allows the overlying membrane stripping-in to lay flat across the valley.
Valley Width Selection
| Roof Pitch | Annual Rainfall | Recommended Width |
|---|---|---|
| 4:12–6:12 | Low–moderate (<40"/yr) | 18" W-valley |
| 4:12–6:12 | High (>40"/yr) | 24" W-valley |
| 6:12–12:12 | Any | 18"–24" W-valley |
| 12:12+ | Any | 24" W-valley minimum |
Material Selection
| Material | Thickness | Notes |
|---|---|---|
| Galvalume steel | 26 ga. | Standard for most commercial sloped roofs; compatible with TPO, EPDM, modified bitumen, and metal panel systems |
| Aluminum | .040" | Good for coastal environments; compatible with most roofing membranes; do not pair with copper fasteners |
| Aluminum | .032" | Light-commercial and metal panel applications where lighter gauge meets the span |
| Copper | 16 oz. | Long-service-life specifications and historic building restoration |
Material compatibility note: Galvanized steel should not be used under asphalt-based roofing products — asphalt chemistry accelerates zinc coating degradation. Use Galvalume (55% aluminum / 45% zinc alloy coating) or aluminum instead. On SSMR projects, confirm valley metal material matches panel material for galvanic compatibility.
How Trimgy Handles Valley Flashing
Valley flashing on Trimgy is drawn as a cross-section profile. For a W-valley, you draw the left flange, the V-channel, the center rib, the second V-channel, and the right flange. The grid makes it straightforward to set the total width (18" or 24"), the channel depth, and the center rib height. For a V-valley, it's simpler: two legs meeting at the center plus flat flanges on each side.
Contractors ordering custom-width valley flashing — for non-standard valley conditions, wide drainage areas, or project-specific material specifications — benefit from Trimgy's ability to order any width and profile without a shop custom-order premium. LTL freight is calculated automatically from your delivery zip code.
Draw Your Valley Flashing on Trimgy
W-valley, V-valley, or flat-pan in any width. Galvalume, aluminum, or copper. Instant pricing with LTL freight.
Start Drawing Free →Frequently Asked Questions
What is a W-valley vs a V-valley?
A W-valley has a raised center rib that divides the valley into two channels, preventing cross-flow and debris accumulation. A V-valley is a simple V-shape without the center rib — simpler but provides no cross-flow prevention. W-valleys are preferred for most commercial applications, steep pitches, and high-rainfall locations.
How wide should valley flashing be?
Minimum 18" for most commercial conditions. Use 24" in high-rainfall areas, on steep pitches generating high water velocity, or on valleys with a drainage area over 1,000 sq ft per side. The flanges should extend at least 4" beyond the roofing material trim lines on each side.
Should valley flashing go over or under the underlayment?
Over the underlayment for open valleys. Underlayment runs first, then valley metal over it, then roofing material on each side with the valley metal exposed in the center. This ensures any water penetrating under the roofing hits the underlayment before reaching the valley metal joints.
What material is best for valley flashing?
Galvalume 26 ga. for most commercial sloped roofs — excellent corrosion resistance and compatible with TPO, EPDM, modified bitumen, and metal panel systems. Aluminum .040" for coastal environments. Copper for long-service-life and historic specifications. Avoid galvanized steel under asphalt-based roofing products.
What is the typical price for custom valley flashing?
26 ga. Galvalume W-valley: $4–$7/LF. Aluminum .040": $5–$9/LF. Copper 16 oz.: $12–$18/LF. LTL freight is calculated from your delivery zip code and order footage in the Trimgy checkout.