What Coil Size Do I Need for Valley Flashing?

Valley Flashing Coil Size Guide (Before Forming)

Valley flashing is installed in the internal valleys of a roof, where two roof slopes meet and water is concentrated and directed downward. It is one of the most critical flashing components for preventing leaks and ensuring proper drainage.

The most important starting point in production is selecting the correct coil size before forming — especially the coil width, which determines the valley width and water flow capacity.

For valley flashing, coil size directly affects:

Water drainage capacity
Coverage area in the valley
Profile geometry and depth
Installation fit
Long-term durability

This guide explains exactly what coil size you need before forming valley flashing.

What Is Included in “Coil Size”?

Coil size includes:

Coil width (most critical)
Thickness (gauge)
Coil weight
Coil inside diameter (ID)
Coil outside diameter (OD)
Material type and coating

All must match both flashing design and forming equipment.

1. Coil Width (Primary Factor)

The most important factor is the coil width before forming, based on the developed strip width.

For valley flashing, this includes:

Center valley width (water channel)
Side flanges (for fixing under roof panels)
Hemmed edges (if included)
Bend allowances

Typical coil width ranges:

400 mm – 600 mm → small residential valleys
600 mm – 900 mm → standard roofing systems
900 mm – 1500 mm+ → large or commercial valleys

👉 Coil width must allow for adequate water flow + overlap + hems

2. Thickness (Gauge)

Valley flashing must handle concentrated water flow and exposure.

Typical ranges:

0.4 mm – 0.6 mm → standard steel flashing
0.5 mm – 0.7 mm → aluminum flashing
0.7 mm – 1.2 mm+ → heavy-duty applications

Thickness affects:

Durability
Resistance to deformation
Longevity under water exposure

3. Coil Weight

Coil weight depends on production setup.

Typical coil weights:

0.5 – 2 tons → small production
2 – 5 tons → standard roll forming
5 tons+ → high-volume systems

4. Coil Inside Diameter (ID)

The coil ID must match the uncoiler.

Common sizes:

300 mm – 400 mm → small coils
508 mm (20”) → standard
610 mm (24”) → larger systems

5. Coil Outside Diameter (OD)

OD depends on coil weight and thickness.

Typical OD:

800 mm – 1500 mm+

Must match:

Uncoiler capacity
Handling systems
Production space

6. Material Type and Coating

Material selection is critical due to constant water exposure.

Common materials:

Pre-painted galvanized steel
Aluminum (very common for corrosion resistance)
Zinc or coated steel

Material affects:

Corrosion resistance
Water durability
Appearance

Why Coil Size Is Critical for Valley Flashing

Valley flashing handles high water volumes and must be precisely sized.

If coil size is incorrect:

Water channel may be too narrow
Overflow or leaks may occur
Profile may not fit correctly under panels
Hems may not form properly
Installation issues will arise

Accuracy is critical for drainage performance and leak prevention.

Water Flow and Valley Geometry (Key Feature)

Valley flashing includes:

Central channel for water flow
Side flanges for fixing
Optional hems for strength and safety

These require:

Sufficient coil width
Correct bend positions
Consistent forming

Incorrect coil size affects:

Drainage capacity
Roof performance
Installation quality

Common Coil Size Mistakes

Coil too narrow

Insufficient water flow capacity
Risk of overflow

Incorrect width calculation

Poor fit under roof panels

Wrong thickness

Too flexible OR difficult to form

Incorrect material

Corrosion or premature failure

Poor slitting quality

Edge defects and finish issues

Machine Matching (Critical)

Before ordering coil, confirm your machine supports:

Coil width range
Thickness range
Maximum coil weight
Hemming capability (if required)
Material compatibility

👉 Coil size must match machine + valley design

What Buyers Must Confirm Before Ordering Coil

Always confirm:

Valley flashing design (channel width and flanges)
Developed strip width
Thickness
Material type and finish
Coil weight
Coil ID / OD
Machine limits

Never estimate — always ensure water flow requirements are met.

Final Thoughts

For valley flashing, the coil size before forming defines drainage capacity, fit, and long-term performance.

Getting the coil size right ensures:

Proper water flow
Accurate profile dimensions
Reliable leak protection
Clean hems and edges
Efficient production

Valley flashing is one of the most critical roofing components — coil size accuracy is essential.

FAQ

What coil size do I need for valley flashing?

You need the correct coil width, thickness, weight, ID/OD, and material type based on the valley design.

What is the most important factor?

Coil width — it determines water flow capacity.

Why is valley flashing critical?

It handles the highest water concentration on the roof.

Does thickness affect durability?

Yes — thicker material improves lifespan and strength.

Can coil size be adjusted during production?

No — it must be correct before forming.