How to Specify Valley Flashing (Complete Roofing Engineering Guide)
Valley flashing is not decorative — it is hydraulic protection.
Complete Roofing & Manufacturing Guide
Valley flashing is installed:
- ✔ Where two roof slopes meet
- ✔ Beneath roofing panels
- ✔ In high-flow drainage zones
It must:
- ✔ Handle concentrated water flow
- ✔ Resist uplift
- ✔ Prevent debris damming
- ✔ Match roof pitch
Valley flashing is not decorative — it is hydraulic protection.
1️⃣ What Defines Valley Flashing?
Valley flashing is defined by:
- ✔ Overall width
- ✔ Center valley depth
- ✔ Side flange length
- ✔ Roof pitch
- ✔ Hem detail
- ✔ Thickness
- ✔ Material
- ✔ Coating
Without water flow and pitch data, valley cannot be specified properly.
2️⃣ Valley Type
There are two main types:
Open Valley
- ✔ Exposed metal channel
- ✔ Common in metal roofing
- ✔ High water capacity
Closed Valley
✔ Covered by roofing panels
✔ Less common in metal systems
Metal roofing primarily uses open valleys.
Always define valley type.
3️⃣ Overall Width
Common valley widths:
- 400 mm
- 450 mm
- 500 mm
- 600 mm
- 900 mm (wide commercial)
Wider valley handles:
- ✔ Higher rainfall
- ✔ Steeper roofs
- ✔ Large roof areas
Undersized valley causes overflow in heavy rain.
4️⃣ Center Channel Depth
Valley may include:
- ✔ Flat center
- ✔ W-shaped valley
- ✔ Raised diverter ribs
Depth improves:
- Water flow control
- Debris guidance
- Overflow resistance
W-valley designs reduce splash and crossflow.
5️⃣ Side Flange Length
Side flanges tuck under roof panels.
Common lengths:
- 75 mm
- 100 mm
- 150 mm
Longer flanges improve:
Wind resistance
Water backup protection
Must match panel overlap design.
6️⃣ Roof Pitch Compatibility
Roof pitch affects:
- Water velocity
- Flow volume
- Fastener positioning
Steeper roofs increase:
Water speed
Erosion risk
Low slope roofs increase:
Water pooling risk
Always confirm pitch before sizing valley.
7️⃣ Thickness Range
Common thickness:
- 0.45 mm
- 0.50 mm
- 0.60 mm
- 0.75 mm
Heavy commercial roofs may require thicker flashing.
Thickness improves:
- Dent resistance
- Wind stability
- Long-term durability
8️⃣ Material Type
Common materials:
- Galvanized steel
- Galvalume
- Prepainted steel
- Aluminum
Material should match roof panels to prevent galvanic reaction.
Coastal zones may require AZ coating or aluminum.
9️⃣ Coating & Finish
Valley flashing may be:
- ✔ Same color as roof
- ✔ Neutral metallic
- ✔ High-corrosion coating
Coating must match environmental exposure.
Valleys see concentrated water — corrosion protection is critical.
🔟 Ice & Snow Considerations
In snow regions:
✔ Ice dams form in valleys
✔ Backflow risk increases
Consider:
- ✔ Wider valley
- ✔ Higher side flange
- ✔ Thicker material
Snow load increases stress in valley zone.
1️⃣1️⃣ Wind Uplift Considerations
Valley is partially protected but:
✔ High wind can drive rain upward
✔ Fastener spacing must be adequate
Longer flanges and hems improve resistance.
1️⃣2️⃣ Typical Coil Width
Coil width =
Center flat + 2 side flanges + hems + bend allowance.
Example:
300 mm center
100 mm flange ×2
300 + 200 = 500 mm
Add bend allowance → approx. 520–560 mm
If W-valley design:
Add additional bend allowances.
Exact developed width must include:
- ✔ Bend radii
- ✔ Thickness compensation
- ✔ Springback correction
Never approximate coil width.
1️⃣3️⃣ Machine Engineering Requirements
Valley flashing can be produced on:
- ✔ Trim roll forming machine
- ✔ Press brake
- ✔ Dedicated flashing line
Typical roll forming setup:
-
6–12 forming stands
-
40–70 mm shafts
-
5–15 kW motor
-
Hydraulic cut
W-valley designs require additional forming stations.
1️⃣4️⃣ Production Speed
Typical speeds:
10–25 m/min
Simple geometry allows efficient production.
1️⃣5️⃣ Tolerance Requirements
Typical tolerances:
- Width ±2 mm
- Angle ±1°
- Length ±2–3 mm
Angle accuracy critical for correct roof seating.
1️⃣6️⃣ Debris Management
Valleys collect:
- Leaves
- Dust
- Sand
- Snow
Design should:
Avoid flat stagnant zones
Promote self-cleaning water flow
W-valley design often preferred.
1️⃣7️⃣ Common Specification Mistakes
- ❌ Undersizing valley width
- ❌ Not considering rainfall intensity
- ❌ Ignoring snow zone
- ❌ Using too thin material
- ❌ Not matching roof pitch
- ❌ Guessing coil width
Valley flashing failure often results in severe water damage.
1️⃣8️⃣ Developed Width Reminder
Developed width must include:
- ✔ Center flat
- ✔ Side flanges
- ✔ Hem allowance
- ✔ Bend allowance
- ✔ Thickness compensation
- ✔ Springback correction
W-valleys require multiple bend allowances.
1️⃣9️⃣ Final Valley Flashing Specification Checklist
Before tooling or machine approval:
- ✔ Confirm valley type (open or W-valley)
- ✔ Confirm overall width
- ✔ Confirm center depth
- ✔ Confirm side flange length
- ✔ Confirm roof pitch
- ✔ Confirm thickness range
- ✔ Confirm material type
- ✔ Confirm coating
- ✔ Confirm snow/wind zone
- ✔ Calculate developed width
- ✔ Confirm coil availability
- ✔ Confirm production speed target
Only then proceed.
FAQ Section
Is wider valley always better?
Only if rainfall and roof area demand it.
What is a W-valley?
A valley with raised center rib to prevent crossflow.
Does thickness matter?
Yes — valley sees concentrated water and debris.
Should valley match roof material?
Yes — to avoid galvanic corrosion.
Can valley flashing be press-braked?
Yes for small production volumes.
Is coil width wide?
Yes — valley flashing often requires wider coil than other trims.