How to Estimate Roof Flashing
Flashing seals the joints where the roof meets walls, chimneys, skylights, valleys, and roof penetrations. Most roof leaks are flashing failures, not shingle failures, so accurate estimating and proper installation are critical.
Drip Edge
Drip edge L-shaped metal flashing is required (IRC R905.2.8.5) along every eave and rake. Standard 10 ft pieces overlap 1/4 inch. Total pieces equal eave plus rake perimeter divided by 10, plus 5% waste.
Drip Edge Pieces=Perimeter10×(1 + 5%)
Step Flashing
Step flashing installs at every shingle course where the roof meets a vertical wall (sidewall of a chimney, a dormer, or a wall intersection). Each piece is 5 inches wide x 7 inches tall with about 5 inches of horizontal exposure:
Step Pieces=Wall Length×125
Flashing Reference Table
| Location | Type | Material |
|---|---|---|
| Eaves & rakes | Drip edge | Aluminum or steel |
| Valleys | W-valley flashing | Aluminum or copper |
| Chimney sidewall | Step + counter | Aluminum or copper |
| Vent stack | Boot / pipe collar | Rubber + metal |
| Skylight | Skylight kit | Aluminum (mfr supplied) |
Replace Vent Boots Every Reroof
The rubber collar on plumbing vent boots dries out and cracks in 10 to 15 years - the most common cause of attic water stains. Always replace vent boots when reroofing, and consider lifetime metal Oatey-style boots for long-term reliability.