What Is Sidewall Flashing?
Sidewall flashing is the L-shaped sheet metal assembly installed at the continuous junction of a sloped roof field and a vertical wall — along parapets, mechanical room walls, stairwell walls, and building additions. The horizontal leg is lapped or adhered under the roofing field membrane or underlayment; the vertical leg rises up the wall behind the cladding, counterflashing, or reglet assembly. On low-slope systems (TPO, EPDM, modified bitumen), the horizontal leg integrates with the field membrane, typically heat-welded, torched, or cold-applied. On steep-slope systems (standing seam, metal panel), the horizontal leg runs under the panel seams.
Sidewall flashing at active roof-to-wall junctions is fabricated in sections that overlap at least 6" and are independently fastened, allowing the roof field to expand and contract thermally without transmitting stress to the wall attachment. A continuous one-piece L-angle run is not appropriate for thermal movement at junctions exceeding 20 linear feet — sectional sidewall flashing with lapped joints is the SMACNA-specified approach.
Where Sidewall Flashing Is Required
- Parapet sidewalls — where a sloped roof surface runs up to a parapet or interior parapet wall
- Mechanical room and stairwell walls — vertical wall faces penetrating through a sloped roof plane
- Dormer sidewalls — where a dormer's vertical wall meets the main roof slope
- Roof-to-addition junctions — where a sloped roof butts against a vertical wall at a building addition or connector
- Skylight sidewalls — curb-mounted skylights with sloped roof conditions on the side faces
Standard Sidewall Flashing Dimensions
SMACNA specifies minimum 4" horizontal and 4" vertical legs for sidewall flashing — typically fabricated in 8" × 8" or 10" × 10" sections. On low-slope systems the horizontal leg must extend far enough to allow membrane integration (heat-weld or adhesive) while maintaining the required lap. On steep-slope standing seam systems the horizontal leg must clear the seam ribs. Sections overlap a minimum of 6" at all lapped joints.
| Section Size | Horizontal Leg | Vertical Leg | Application |
|---|---|---|---|
| 8" × 8" | 4" on roof field | 4" up wall | Standard commercial, pitches up to 9:12; membrane or steep-slope systems |
| 10" × 10" | 5" on roof field | 5" up wall | Steep slopes (9:12–12:12) or where wider membrane weld width is specified |
| 12" × 12" | 6" on roof field | 6" up wall | Very steep slopes, masonry wall junctions, high wind exposure |
Diverter Flashing at the Eave Termination
At the bottom of a sidewall flashing run — where the roof-to-wall junction terminates at the eave — a diverter flashing is required. This piece at the last flashing position redirects water from the sidewall run into the gutter, rather than allowing water to run against the wall face below the junction. Per SMACNA and IBC flashing details, this diverter is required at all sidewall-to-eave terminations and is one of the most commonly missed details on commercial re-roofing projects.
Key installation note: The diverter flashing must terminate inside the gutter face, not over the outside edge. A diverter that ends at the outside of the gutter still allows water to track down the wall face at peak flow. Verify termination depth when inspecting completed work.
Materials
| Material | Thickness | Application |
|---|---|---|
| Aluminum | .040" | Standard commercial; corrosion-resistant; compatible with most cladding types |
| Galvalume steel | 26 ga. | Light commercial and metal building applications; good corrosion resistance |
| Galvanized steel | 26 ga. | Budget light-commercial; requires periodic inspection as zinc coating weathers |
| Copper | 16 oz. | Historic masonry buildings and long-life specifications; self-sealing at fasteners |
How Trimgy Handles Sidewall Flashing
Sidewall flashing is ordered by drawing the L-shape cross-section — two legs at the required dimensions — and selecting material and thickness. Trimgy prices by linear footage of the profile. Total footage needed equals the wall run length plus 10–15% for overlaps and end cuts. For sectioned runs, add footage for each 6" overlap joint.
For roofing and sheet metal contractors who run projects with sidewall junctions regularly, having the sidewall flashing profile saved in Trimgy means reordering for the next job takes seconds. Order exactly the footage needed for the specific wall run without a minimum-quantity premium.
See the complete roof flashing types guide →
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Start Drawing Free →Frequently Asked Questions
What is sidewall flashing?
Sidewall flashing is the L-shaped sheet metal assembly at the continuous junction of a sloped roof field and a vertical wall — at parapets, mechanical room walls, dormers, and building additions. The horizontal leg integrates with the roofing field; the vertical leg mounts behind the cladding or counterflashing. Fabricated in overlapping sections per SMACNA minimum geometry.
What is the difference between sidewall flashing and counterflashing?
Sidewall flashing is the primary waterproofing layer at the roof-to-wall junction — the horizontal leg integrates with the roof field membrane or system. Counterflashing installs above it, embedded in a masonry reglet or surface-applied, to cover and seal the exposed top edge of the sidewall flashing. The two pieces work as a system but move independently to accommodate thermal expansion at the wall.
What size should sidewall flashing pieces be?
SMACNA specifies minimum 4" horizontal and 4" vertical legs — standard sections are fabricated at 8"×8" or 10"×10", overlapped a minimum of 6" at all joints. For steep slopes (above 9:12) or metal panel roofs, 10"×10" or 12"×12" sections provide better coverage and membrane weld clearance.
What is a diverter flashing and when is it required?
A diverter flashing is installed at the bottom of the sidewall run where the eave terminates against the wall. It redirects water into the gutter rather than down the wall face. Required per SMACNA and IBC at all sidewall-to-eave terminations — one of the most commonly missed details on commercial re-roofing projects.
What is the typical price for sidewall flashing?
Aluminum .040" 8"×8" sidewall flashing runs $4–$8 per LF of wall run. Galvalume 26 ga. runs $3–$6/LF. Copper runs $10–$16/LF. Order by total footage based on wall run and piece dimensions.