How to Specify Apron Flashing (Complete Roof-to-Wall Guide)
Apron flashing is part of a system — not standalone metal.
Complete Roofing & Transition Engineering Guide
Apron flashing is installed:
- ✔ At roof-to-wall intersections
- ✔ At chimney bases
- ✔ At dormer fronts
- ✔ At parapet transitions
It serves to:
- ✔ Direct water onto roof surface
- ✔ Prevent water intrusion at wall joint
- ✔ Protect cladding interface
Apron flashing is part of a system — not standalone metal.
1️⃣ What Defines Apron Flashing?
Apron flashing is defined by:
- ✔ Roof pitch
- ✔ Roof flange length
- ✔ Wall leg height
- ✔ Kick-out detail
- ✔ Hem detail
- ✔ Thickness
- ✔ Material
- ✔ Counter flashing interface
Without wall and roof detail, apron cannot be specified properly.
2️⃣ Roof Pitch (Critical)
Roof pitch determines:
- ✔ Angle of bend
- ✔ Water flow direction
- ✔ Required overlap
Common pitches:
- 5°
- 10°
- 15°
- 18°
- 22.5°
- 30°
Incorrect pitch angle causes:
- Gaps
- Water pooling
- Sealant failure
Always confirm pitch before forming.
3️⃣ Roof Flange Length
Roof flange sits on roofing panel.
Common lengths:
- 100 mm
- 150 mm
- 200 mm
Longer flange improves:
- ✔ Water shedding
- ✔ Wind resistance
- ✔ Protection against backflow
Short flange increases leak risk.
4️⃣ Wall Leg Height
Vertical wall leg covers wall surface.
Common heights:
- 100 mm
- 150 mm
- 200 mm
- 300 mm
Wall leg must extend:
Above potential water line
Under counter flashing
Height depends on:
- Rainfall intensity
- Wall type
- Cladding design
5️⃣ Counter Flashing Interface
Apron flashing typically works with:
- ✔ Counter flashing
- ✔ Reglet detail
- ✔ Masonry cut-in
- ✔ Wall cladding overlap
Specify:
Whether counter flashing is separate profile
Or integrated bend
System detail is critical.
6️⃣ Kick-Out / Diverter Detail
Some apron flashings include:
- ✔ Kick-out at lower end
- ✔ Diverter tab
- ✔ Extended drip
Kick-out prevents water from entering wall cavity.
Important in:
Heavy rain regions.
7️⃣ Hem Detail
Edges may include:
- ✔ Safety hem
- ✔ Double hem
- ✔ Reinforcement fold
Hem improves:
- ✔ Stiffness
- ✔ Safety
- ✔ Wind resistance
Hem increases developed width and forming stations.
8️⃣ Thickness Range
Common thickness:
- 0.45 mm
- 0.50 mm
- 0.60 mm
- 0.75 mm
Commercial buildings may require thicker flashing.
Thickness should match roofing system.
Too thin = oil canning.
Too thick = unnecessary cost.
9️⃣ Material Type
Common materials:
- Galvanized steel
- Galvalume
- Prepainted steel
- Aluminum
Material must match roof and wall system.
Mixing materials may cause galvanic corrosion.
🔟 Wind & Rain Exposure
Roof-to-wall intersections are:
High water concentration areas
Exposed to wind-driven rain
In high wind zones:
- ✔ Increase flange length
- ✔ Increase thickness
- ✔ Improve fastening detail
Apron flashing must not rely only on sealant.
1️⃣1️⃣ Installation Sequence
Apron flashing is installed:
✔ Before panel installation (in some systems)
✔ After panel installation (panel lapping over)
Sequence affects:
- Flange design
- Fastener placement
- Sealant detail
Specify installation method early.
1️⃣2️⃣ Overlap & End Detail
Apron flashing sections overlap.
Typical overlap:
100–200 mm
Lower end may require:
Kick-out
Diverter
Must be specified.
1️⃣3️⃣ Typical Coil Width
Coil width =
Roof flange + wall leg + hem + bend allowance.
Example:
150 mm roof flange
200 mm wall leg
350 mm + bend allowance → approx. 380–420 mm
If hem included, add additional allowance.
Exact developed width must include:
- ✔ Bend radii
- ✔ Thickness compensation
- ✔ Springback correction
Never approximate coil width.
1️⃣4️⃣ Machine Engineering Requirements
Apron flashing can be produced on:
- ✔ Trim roll forming machine
- ✔ Press brake
- ✔ Multi-profile flashing line
Typical roll former:
-
6–14 forming stands
-
40–70 mm shafts
-
5–15 kW motor
-
Hydraulic cut
More complex hems require additional stations.
1️⃣5️⃣ Production Speed
Typical speeds:
10–25 m/min
Simple geometry allows moderate production speed.
1️⃣6️⃣ Tolerance Requirements
Typical tolerances:
- Angle ±1°
- Leg length ±1–2 mm
- Length ±2–3 mm
Angle accuracy critical for proper wall seating.
1️⃣7️⃣ Common Specification Mistakes
- ❌ Not confirming roof pitch
- ❌ Not defining wall leg height
- ❌ Ignoring counter flashing interface
- ❌ Using too thin material
- ❌ Not including kick-out
- ❌ Guessing coil width
Apron flashing failures often appear during heavy rainfall.
1️⃣8️⃣ Developed Width Reminder
Developed width must include:
- ✔ Roof flange
- ✔ Wall leg
- ✔ Kick-out (if any)
- ✔ Hem allowance
- ✔ Bend allowance
- ✔ Thickness compensation
- ✔ Springback correction
Accurate flat pattern is essential.
1️⃣9️⃣ Final Apron Flashing Specification Checklist
Before tooling or machine approval:
- ✔ Confirm roof pitch
- ✔ Confirm roof flange length
- ✔ Confirm wall leg height
- ✔ Confirm counter flashing detail
- ✔ Confirm kick-out requirement
- ✔ Confirm hem detail
- ✔ Confirm thickness range
- ✔ Confirm material type
- ✔ Confirm coating
- ✔ Confirm wind/rain exposure
- ✔ Calculate developed width
- ✔ Confirm coil availability
- ✔ Confirm production speed target
Only then proceed.
FAQ Section
Is apron flashing the same as base flashing?
Often similar — terminology varies by region.
Does wall leg need to be tall?
Yes — must extend above potential water line.
Should it match roof thickness?
Yes — or slightly thicker for durability.
Is counter flashing required?
Yes in most wall intersections.
Can it be press-braked?
Yes for small volume production.
Is coil width wide?
Yes — typically 350–450 mm depending on design.