Corrosion Protection for Steel Frames Under Rooftop AC Units

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Steel frames under rooftop AC units corrode faster than most structures because they face constant condensation, rain, salt-laden air and vibration. The most reliable protection combines hot-dip galvanizing, a compatible powder coating or paint system, correct drainage and periodic inspection of coating damage.

Why rooftop AC frames corrode faster

A rooftop air conditioning frame sits in one of the harshest spots on a building. The condensate from the units drips onto the steel, rain collects in the same places, and the sun heats and cools the metal daily. In coastal areas such as Tel Aviv, salt in the air speeds up the process further. Corrosion usually starts where the coating has been scratched, where water pools on horizontal members, or where dissimilar metals touch each other.

Understanding these weak points is the first step in choosing a protection strategy that actually lasts.

Choosing the right base protection

For frames that carry AC units, hot-dip galvanizing is the most dependable base protection. The zinc layer protects the steel even where the topcoat is damaged, because zinc corrodes sacrificially. Galvanized steel should be finished with a coating system suited to the environment, usually a powder coating or a two-component paint designed for galvanized surfaces.

Specify hot-dip galvanizing for the structure
Confirm the topcoat is compatible with galvanized steel
Avoid bare steel exposed to condensate

Designing for drainage and airflow

Good design reduces corrosion before any coating is applied. Frames should avoid flat horizontal surfaces where water can sit, and members should be arranged so that water runs off rather than collecting in pockets. Closed profiles should have drainage holes so that trapped moisture can escape. Good airflow under the unit also helps the metal dry between rain events and reduces condensation.

Ask your fabricator to show how water will leave the structure before the frame is manufactured.

Preventing galvanic corrosion at connections

When steel meets aluminum, stainless steel or copper, moisture can drive galvanic corrosion at the contact point. Use insulating washers or gaskets, select compatible fasteners and avoid direct metal-to-metal contact between different materials. Bolts and fixings should be galvanized or stainless steel of an appropriate grade, and any drilled holes or cut edges should be treated with touch-up coating before installation.

Maintenance and inspection schedule

Even a well-specified frame needs regular checks. Inspect the rooftop frames at least once or twice a year, and after strong storms or seasons with heavy salt exposure. Look for blistering or peeling paint, white zinc corrosion products, rust staining below fixing points and cracked sealants where members meet the roof.

Small areas of damage are much cheaper to repair when found early. Clean the spot, remove loose material, and apply a suitable zinc-rich or compatible touch-up product.

Working with the fabricator and installer

Corrosion protection starts at the drawing stage. Share the exact location, the unit weights and the environmental conditions with your fabricator so that the material and coating specification matches the site. Ask for a written description of the coating system and surface preparation, and confirm how the frame will be fixed to the roof structure. For loads and fixing details, have the structural engineer confirm the values, since exact figures depend on the project and the Israeli standards in force.

Frequently asked questions

Is galvanizing alone enough to protect a rooftop AC frame?

Galvanizing gives strong base protection, but it works best combined with a suitable topcoat. The topcoat adds color, extra barrier protection and helps the zinc last longer in harsh conditions.

Why does rust appear under my AC frame even when it was painted?

Rust usually starts at scratches, cut edges, drilled holes or areas where water collects and the coating was thin. Paint alone rarely fails evenly, so damaged spots need prompt touch-up.

Can I put stainless steel and steel together in the same frame?

You can, but direct contact between them in a wet environment can cause galvanic corrosion. Use insulating gaskets or washers at the contact points and choose compatible fasteners.

How often should rooftop AC frames be inspected?

A visual inspection once or twice a year is a sensible minimum, with extra checks after storms or in salty coastal conditions. Early detection makes repairs simpler and cheaper.

Common mistakes to avoid

The most common mistake is painting over bare or poorly prepared steel without galvanizing, which lets corrosion creep under the paint. Another is leaving horizontal surfaces and closed profiles without drainage, so water sits and rots the coating from inside.

Our recommendation

Specify hot-dip galvanized steel with a compatible powder coating or paint system, design the frame so water drains freely, and schedule a yearly visual inspection with prompt touch-up of any damaged spots.

Summary

Protecting steel frames under rooftop AC units depends on galvanizing, a compatible topcoat, good drainage and regular inspection. Addressing these together gives the longest service life.

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