Is galvanized steel good for roof flashing?
Short answer. Yes, galvanized steel is one of the most common roof flashing metals and meets the building code when it is at least 0.019 inch thick, about 26 gauge. It is strong, inexpensive and easy to bend; its weaknesses are rust once the zinc coating wears or is cut, and corrosion where it touches copper or treated lumber.
How it compares
- Galvanized steel: zinc-coated steel that is stiff and durable, used for step flashing, valley metal, chimney flashing and drip edge. Heavier zinc coatings last longer than thin ones.
- Aluminum: lighter, does not rust, easy to bend, and available prefinished in colors. It should not be embedded in fresh mortar or placed against copper.
- Copper: very long lived and solderable but expensive, and its runoff corrodes galvanized steel and aluminum downhill from it.
- Prepainted steel: galvanized or Galvalume steel with a paint finish, common for trim and on metal roofs.
- Lead: soft and formable, used for pipe boots and some chimney work on shingle roofs.
What the code says
The International Residential Code, which Georgia adopts with state amendments, requires metal flashing to be corrosion resistant and at least 0.019 inch thick, listed as No. 26 galvanized sheet. Valley linings and drip edge have their own requirements in the same roofing chapter.
Getting long life from galvanized flashing
- Use new metal at every reroof; old step flashing is usually rusted at the bottom where water sits.
- Keep it from direct contact with copper and pressure-treated wood by using a membrane separator.
- Place cut edges where they are covered, since they rust first.
- Fasten with galvanized or other corrosion resistant nails, never bare steel.