Valley Flashing Calculator

About the Valley Flashing Calculator

Roof valleys are the V-shaped channels where two sloping roof planes meet. They channel large volumes of water and are one of the most leak-prone areas on any roof. Valley flashing provides a durable, watertight channel that directs runoff safely to the gutter.

Open valleys use a visible metal channel (typically W-shaped galvanized steel, aluminum, or copper) with shingles cut back from the center. Closed valleys weave or laminate shingles across the valley with ice and water shield underneath. Both methods require careful material estimation.

This calculator determines the linear feet of valley metal, ice and water shield underlayment, and the number of metal sections needed based on your valley lengths. Proper valley installation is critical — valley leaks account for a significant percentage of all residential roof failures.

When This Page Helps

Valleys channel more water per square foot than any other part of the roof. Proper flashing material estimation ensures continuous protection along every valley. This calculator accounts for overlap at metal joints and ice/water shield coverage.

How to Use the Inputs

1. Measure the length of each valley on the roof (follow the slope, not horizontal).
2. Enter the total combined valley length.
3. Select the valley type (open metal or closed/woven).
4. Review the material needed: metal sections, ice/water shield, or both.
5. Add 5–10% for overlap at metal section joints.

Example Calculation

Tips & Best Practices

• Always install ice and water shield under valley metal for a double layer of protection.
• Open valleys should be 4–6 inches wide at the top and widen toward the eave (1/8 inch per foot).
• W-valley metal is the preferred profile — the center ridge prevents water from overshooting.
• Overlap metal sections at least 6 inches, with the upper piece on top of the lower piece.
• Clip shingle ends at a 45° angle at the valley to direct water into the channel.
• Use roofing cement (not caulk) to seal the metal overlap joints.

Open Valley Construction

In an open valley, metal flashing is installed over ice and water shield, and shingles are cut back from the valley center on both sides. The exposed metal channel widens as it descends (typically 1/8 inch per foot) to handle increasing water volume. The edges of the shingles are sealed with roofing cement.

Closed Valley Construction

A closed-cut valley weaves the shingles from one roof plane across the valley and then cuts the shingles from the other plane in a straight line. A woven valley alternates shingle courses from each plane across the valley. Both methods rely on ice and water shield underneath as the primary waterproofing.

Valley Maintenance

Valleys should be inspected annually for debris accumulation, metal corrosion, shingle deterioration, and sealant failure. Clear leaves and pine needles from open valleys. In cold climates, check for ice dam formation at valley intersections where water volume is highest.

Frequently Asked Questions

• What is a W-valley?

  W-valley metal has a raised crimp (ridge) running down the center of the V-shaped channel. This center ridge prevents high-volume water on one side of the valley from overshooting across to the other side. It's the standard profile for open valley installations.

• Is open or closed valley better?

  Open valleys with metal flashing are generally more durable, easier to maintain, and less prone to debris clogs. Closed (woven or laminated) valleys have a cleaner appearance but can trap debris and are harder to inspect. Most roofing experts prefer open valleys for longevity.

• What width should valley metal be?

  Standard valley metal is 24 inches wide (12 inches on each side of the center). In high-volume areas or on steep roofs, 36-inch-wide metal provides extra margin. The exposed metal channel should be at least 4 inches wide at the ridge end.

• What material is best for valley metal?

  Galvanized steel (painted) is the most common and cost-effective. Aluminum is lightweight and rust-proof. Copper is premium and lasts 100+ years with beautiful patina. Avoid galvanized-to-copper contact (galvanic corrosion).

• Do all valleys need ice and water shield?

  Building codes in most jurisdictions require ice and water shield in all valleys, regardless of climate. It provides critical secondary protection if the valley metal or shingles fail. The membrane should extend at least 18 inches on each side of the valley centerline.

• How long are valley metal sections?

  Standard valley metal comes in 8-foot or 10-foot sections. Overlap at joints should be at least 6 inches. Some contractors have valley metal custom-bent to the full valley length, eliminating joints entirely.