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topic 3766

New steel over old galvanized roof, sealer available?


I'm in the roofing business recently..... I have a concern about covering an old galvanized roof with new steel roofing panels (painted) even though there is a primer on the bottom surface. Because there is some rusting on the old roof. Is there a sealant that might provide a barrier in these spots? Or maybe I'm overly concerned about early corrosion. Any opinions?

Dick Shipman 
- Port Townsend, Washington


Galvanizing is zinc, which is sacrificial to steel. So, while the galvanizing on the old roof will realistically do infinitely little to retard eventual rusting of the new roof, it will certainly not accelerate it.

Rust is not 'contagious' per se, but it is porous and spongey and can tend to attract and retain water, so I would agree with your intuition that it would be a good idea to brush away any loose rust and then apply some primer paint to those spots.

The above is theoretical "book knowledge", not practical roofing experience, so take it only for what it's worth.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey


Try using a Rust Converter [linked by editor to product info at Amazon] to stop the rust. Rust inhibitors only "inhibit" not stop the rust. Once the existing rust is converted, there usually is no further oxidation. Pete.

Pete Bagley
- Reno, Nevasa


I would like to know what I can do about my galvanized roof cap on the chimney of my home. The galvanized piece at the top appears to be breaking down and running. I need to coat or replace it and clean up the stove that is a mess. I can't seem to get an answer from anyone. Any suggestion would be greatly appreciated.

Delores Bunkoff
- Schenectady, New York

February 25, 2009

Hi, Delores. These chimney caps are often "stock" items in hardware stores. Probably easier and as cheap to replace it than refinish it.


Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

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