Roof flashings are the sheet metal pieces that seal every transition, penetration, and termination in a roofing system. Most roof leaks don't happen in the middle of the field — they happen at flashings: sidewall flashings that weren't lapped correctly, valley flashings undersized for the pitch, head flashings with insufficient height. This guide covers every common flashing type, its function, and the minimum specifications that determine whether it will perform correctly for the life of the roof.
Sidewall Flashing
Sidewall flashing is the L-shaped sheet metal assembly at the continuous junction of a sloped roof field and a vertical wall — along parapets, mechanical room walls, chimneys, and dormers. Fabricated in overlapping sections, the horizontal leg integrates with the roofing field (membrane, underlayment, or panel system); the vertical leg mounts behind the wall cladding or counterflashing. Sections overlap a minimum of 6" at all lapped joints per SMACNA.
Function
Sidewall flashing intercepts water that would otherwise travel behind the cladding and into the wall cavity at the roof-to-wall junction. The horizontal leg integrates with the roofing field below; the vertical leg runs up the wall and is covered by cladding or counter flashing above.
Minimum Specifications
- Horizontal leg: 4" minimum onto the roof field — must extend far enough to allow membrane weld width or felt integration plus the required lap
- Vertical leg: 4" minimum above the roof surface (measured perpendicular to slope, not vertically)
- Section overlap: 6" minimum at all lapped joints; sections sized to accommodate thermal movement
- Material: 26 ga galvanized steel minimum; .040" aluminum; copper 16 oz minimum per SMACNA
- Hem: Open hem on vertical leg top edge to prevent water infiltration behind cladding
Most common failure mode: Horizontal leg too short — less than 4" — allowing wind-driven water to travel up under the roofing material and over the sidewall flashing edge.
Counter Flashing
Counter flashing covers the top edge of sidewall flashing and is the piece visible from the exterior. It may be embedded in a masonry reglet (a slot cut or cast into the wall) or surface-applied and sealed.
Function
Counter flashing prevents water from getting behind the sidewall flashing by covering its top edge. Without counter flashing, wind-driven rain hits the vertical leg of the sidewall flashing directly and can infiltrate behind it.
Minimum Specifications
- Wall leg (embedded): 1" into reglet minimum; 1.5" preferred
- Wall leg (surface-applied): 4" up the wall surface, sealed with urethane sealant
- Face leg overlap: Minimum 2" over top of sidewall flashing
- Counter flashing height: Must maintain 2" overlap with sidewall flashing at all points along the run, accounting for roof pitch
- Material: Match sidewall flashing material for galvanic compatibility
Head Flashing (Head Wall Flashing)
Head flashing is installed where a sloped roof meets a horizontal wall above — at the top of a shed-roof dormer, at a step in a parapet, or at the top of a vertical wall that rises above the roof surface. Water running down the wall concentrates at this junction.
Function
Head flashing captures water coming off the wall above and directs it onto the roof surface without letting it get behind the roofing membrane or under the shingles.
Minimum Specifications
- Vertical leg (on wall): 4" minimum up the wall above the roof surface
- Horizontal leg (on roof): 4" minimum onto the roof surface, integrated with the roofing field membrane or underlayment
- Hem: Open hem with drip at outer edge of horizontal leg
- Material: 24 ga galvanized or Galvalume; 0.032" aluminum in coastal locations
Valley Flashing
Valley flashing spans the internal angle (valley) where two roof planes meet. Water from both planes converges in the valley and must be managed at high volume and velocity.
Open Valley vs. Closed Valley
In an open valley, the flashing is visible and the roofing material stops 2" from the centerline on each side. In a closed-cut or woven valley, the roofing material covers the flashing. Open valleys allow visual inspection and replacement; closed valleys are preferred where minimal exposed metal is specified.
Minimum Specifications (Open Valley)
- W-valley width: 24" total (12" each side); wider for steep pitches or large drainage areas
- Valley centerline rib height: 1" anti-silt dam at center to prevent debris accumulation
- Edge hem: 1" open hem on each edge, turned under, to prevent water wicking under the roofing material
- Material: 24 ga Galvalume minimum; 0.040" aluminum; copper 20 oz for long-service-life and coastal applications
- Section length: 10' maximum per piece; overlap sections minimum 6"
Pitch and drainage area: For valleys with a drainage area over 1,000 sq ft per side or roof pitches below 4/12, increase valley width to 28"–36". Undersized valleys on large drainage areas are a code violation in many jurisdictions and a common source of ice dam damage.
Diverter Flashing (Kick-Out Flashing)
Diverter flashing installs at the point where a sloped roof edge meets a vertical wall — at the bottom of a sidewall where the eave terminates at the wall face. It diverts water from the roof into the gutter rather than letting it run against the wall cladding.
Why It's Often Missing
Diverter flashing is one of the most frequently missed pieces in commercial re-roofing. Without it, water from the roof surface runs down the wall face and wicks behind the cladding at the bottom, causing extensive moisture damage within 5–10 years — often not visible until it is severe.
Minimum Specifications
- Diverter height: Sufficient to direct water into the gutter — typically 3" rise above the eave edge
- Wall leg: 4" up the wall, integrated with sidewall flashing above
- Gutter leg: Should terminate inside the gutter, not over the outside face
- Material: Match sidewall flashing and valley materials for the project
Apron Flashing
Apron flashing (also called front-wall flashing) seals the junction at the base of a wall or chimney where it meets the roof — the downslope side. Unlike sidewall flashing on the sides, apron flashing is a single continuous piece that runs the full width of the chimney or wall segment.
Minimum Specifications
- Vertical leg: 4" minimum up the wall
- Horizontal leg: 4" minimum under roofing material downslope
- Side ends: Must integrate with sidewall flashing at each side — typically the apron is cut and bent to wrap the corners, or corners are lapped under the first sidewall flashing pieces
Flashing Material Comparison
| Material | Min. Spec | Lifespan | Best Use |
|---|---|---|---|
| Galvanized steel | 26 ga (0.0179") | 20–30 years | Standard commercial |
| Galvalume steel | 26 ga | 30–40 years | Coastal and humid climates |
| Aluminum | 0.019" (26 ga equiv.) | 30+ years | Coastal; incompatible with masonry direct contact |
| Copper | 16 oz (0.022") | 50–100 years | Historic; long-service-life; slate and copper roofing |
| Lead | 2.5 lb/sq ft | 50+ years | Penetrations; chimney tops; custom shapes |
Ordering Custom Roof Flashing
Standard stock flashing works for common pitches and simple geometries. When you need custom flashing — non-standard pitch, unusual roof-to-wall geometry, oversized valley widths, or materials that match an existing roof — you need a fabricator who can produce to your exact spec. The critical information for any custom flashing order is: flashing type, all leg dimensions, bend angles (not just pitch — the actual degrees), material and gauge, linear footage or piece count, and hem type on each exposed edge.
Trimgy's grid-based drawing tool lets you specify any flashing cross-section precisely, with any bend angle, and receive a confirmed price with freight included — without a phone quote. Order custom flashing to spec →
Frequently Asked Questions
What are the main types of roof flashing?
The main types of roof flashing are: sidewall flashing (continuous L-section assembly at roof-to-wall junctions, fabricated in lapped sections integrated with the roof field), counter flashing (covers the top of sidewall flashing, reglet-mounted or surface-mounted), valley flashing (W-shaped or V-shaped at roof valleys), head flashing (above horizontal penetrations like windows), diverter flashing (directs water from the wall at the eave termination of a sidewall run), apron flashing (at the base of a vertical surface where a slope meets it), and cap flashing (coping caps on parapet walls).
What material is best for roof flashing?
For most commercial applications, 24 ga. Galvalume or Kynar-painted steel is the standard for exposed flashing and coping. For sidewall and counter flashing, 26 ga. galvanized steel or .040" aluminum is common and meets SMACNA minimums. Copper is the premium choice for long-term flashing with a 50+ year service life. Avoid using dissimilar metals in contact — copper in contact with aluminum or galvanized steel causes galvanic corrosion.
What is the minimum gauge for roof flashing per SMACNA?
The SMACNA Architectural Sheet Metal Manual, 7th Edition, specifies: sidewall flashing and counter flashing — 26 ga. steel or .040" aluminum; gravel stops and fascia — 24 ga. steel or .040" aluminum; coping caps (face ≤ 12") — 24 ga. steel or .040" aluminum; coping caps (face > 12") — 22 ga. steel or .050" aluminum. The NRCA Roofing Manual aligns with SMACNA minimums for most flashing applications.
How is diverter flashing different from sidewall flashing?
Sidewall flashing is the L-shaped sheet metal assembly at the continuous roof-to-wall junction — fabricated in lapped sections that integrate with the roofing field and direct water downslope. Diverter flashing is a separate piece installed at the eave termination of the sidewall run — its purpose is to direct water away from the wall face and into the gutter. Without it, water runs down the wall and wicks behind the cladding. Diverter flashing is one of the most commonly missed details in commercial re-roofing work.