Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ

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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
1999

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, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
1999

Try using a rust converter ⇦ this on eBay or Amazon [affil links] 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
1999

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
2004

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.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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