CSLB C-39 Roofing ContractorFlashing, Ventilation and InsulationMedium
A roofer plans to install aluminum step flashing directly in contact with existing copper valley flashing on a re-roof project. What problem is most likely to occur?
- AGalvanic corrosion accelerating deterioration of the aluminum
- BThermal expansion cracking of the copper flashing
- CUV degradation of the aluminum flashing
- DReduced fire rating of the roof assembly
Show answer & explanationAnswer & explanation
Correct answer: A. Galvanic corrosion accelerating deterioration of the aluminum
Aluminum and copper are far apart on the galvanic series; when dissimilar metals are in direct contact with moisture present, the less noble metal (aluminum) corrodes rapidly through galvanic action. Roofers must isolate dissimilar metals with a compatible underlayment or use like metals.
Why the other options are wrong
- B. Copper does not crack from contact with aluminum; this describes an unrelated failure mode.
- C. UV exposure affects aluminum minimally and is unrelated to contact with copper.
- D. Fire rating is unaffected by metal compatibility issues.
Galvanic Corrosion
Accelerated corrosion that occurs when two dissimilar metals are in direct electrical contact in the presence of an electrolyte (moisture).
- Less noble metal (e.g., aluminum) corrodes faster
- Copper is highly cathodic; avoid direct aluminum-copper contact
- Use isolation membranes or compatible metals to prevent it
Memory trick: Copper 'eats' aluminum when they touch and get wet.