Battling the tropics: why thicker zinc coatings mean worry free roofing?

Imagine your steel roof under a daily barrage of salt spray and tropical downpours, conditions that speed corrosion by up to threefold compared to milder climates. Zinc coatings serve two valuable functions for roofs of steel: they act as a physical shield against moisture and oxygen, and preferentially corrode, forming a dense patina that buries scratches and inhibits subsequent attack on the substrate steel.

According to American Galvanizers Association’s Time to First Maintenance chart, the life of a 2.5 mils (≈63 µm) zinc coating under a tropical-marine environment is roughly 50 years before 5 % of the substrate rusts, considered the point at which maintenance is. Increasing the coating to 4 mils (≈100 µm) extends that time frame to around 60–65 years, indicating that every additional 1 mil of zinc can be roughly equivalent to one additional decade of useful life in extreme tropical environments.

For truly long‑term roofs under tropical conditions, a thickness of at least 90 µm (≈3.5 mils) of zinc is recommended. Within design and budget constraints, application of a duplex system, hot-dip galvanized substrate undercoated with a high-performance paint or powder finish coat, can delay patina formation further as well as provide aesthetic appeal. Although these coatings reduce corrosion rates significantly, it is nevertheless recommended to schedule an occasional cursory visual inspection every 8–10 years to determine early wear patterns, such as coating thinning or local rust spots, before they become costly repairs.

In conclusion, defining a thicker zinc coating appropriate for tropical environments is worth it in terms of longevity and saves lifetime maintenance costs. Having understood the thickness of service life equation, specifiers and building owners may make informed choices that keep steel roofs protective for decades in some of the world’s toughest environments.

References:

American Galvanizers Association. (n.d.-a). Performance of galvanized steel in different environments. Retrieved July 24, 2025, from https://galvanizeit.org/knowledgebase/article/performance-of-galvanized-steel-in-different-environments.

American Galvanizers Association. (n.d.-b). Time to first maintenance chart for hot dip galvanized coatings. Retrieved July 24, 2025, from https://galvanizeit.org/hot-dip-galvanizing/how-long-does-hdg-last/in-the-atmosphere/time-to-first-maintenance.

Portland Bolt. (n.d.). Galvanized coating lifespan. Retrieved July 24, 2025, from https://www.portlandbolt.com/technical/faqs/galvanized-coating-lifespan/.