Rusty Roof, Rusted.

August 2, 2009 1:22 PM

House help! What kind of metal; and how do I stop it from rusting?

The porches on my home have a metal roof that is rusting. Can you please help me identify:

1. What kind of metal these are.
2. What course of action should I take to un-rusty them.

The paint store dude said I needed to sand it down, and prep it and then paint it.

The home inspector said there was some stuff you could paint over it and it would take care of the rust, and keep it awesome.

The very special guy at the hardware tried to sell me cans of rustoleum.

Photo of roofing in question:
http://www.flickr.com/photos/sirstan/3781685731/

posted by SirStan to home & garden (10 comments total)

If it's rusting, it's some kind of iron. Probably steel.

There's a reason Rustoleum is called Rustoleum.
Sand-prep-Rustoleum is pretty standard rust prevention technique.
posted by willpie at 1:24 PM on August 2

Looks like galvanized steel, I think.
posted by orme at 1:30 PM on August 2

Is it standard operating procedure to just install this and not prep and cover it, or is it just really old? Should I consider replacement? If I leave it as-is will the rust be detrimental, and what sorta timeline might that happen under?
posted by SirStan at 1:32 PM on August 2

I agree with orme - that looks like galvanized steel. Here is a link from Rona on how to paint galvanized steel. There are many other sites out there with similar suggestions.
posted by troll on a pony at 1:33 PM on August 2

Looks like steel or galvanized steel. About all you can do it powerwash it, prime it with a high quality rust preventing primer, and paint the whole thing. And start saving for a new roof. The sheeting is pretty thin and if it's rusting it doesn't have a lot of (leak preventing) life left in it, painting it will only be a temporary measure.
posted by Ookseer at 1:35 PM on August 2 [1 favorite has favorites]

yeah I agree with orme, that's galvanized. Once the water wears through the zinc, it's toast. you could try Naval Jelly or something similar but it's probably not gonna last all that long even with paint on it. I managed to save a metal shed that was suffering from rust cancer with it but it only held out another year or so.
posted by jtoth at 1:36 PM on August 2

Yep, galvanized steel. The rust will eat through the metal eventually. It depends on how much it has rusted through already, and how structurally sound it still is. By the looks of it, you should be able to sand and paint. But, really, make sure that there is no standing water on it after you prep, prime, and paint. The water should run off, not pool.
posted by Vaike at 1:37 PM on August 2

"Galvanization" means coating steel with zinc to prevent it from rusting. How quickly the coating degrades depends on the environment; see e.g. this page for some discussion.

The three experts you checked with before posting gave your proper advice.
posted by effbot at 1:42 PM on August 2

You may want to look into a "rust converter" product. These supposedly convert the rust to a rust proof coating. If you check with an industrial supply store, they should be able to give you some options. There are also "rust killers" that work as primer as well, and others where the film should be removed.

If you're looking for a one step solution, look for an "encapsulating" paint or primer. Rustoleum apparently makes a fairly good one that is readily available and fairly cheap. They also have a pro or industrial version that may be a little better and definitely more expensive. We use a Hammerite brand that may be harder to find and costs quite a bit more, but often one coat is sufficient if it is painted under proper conditions.

Although no paint is absolutely safe, you should really pay attention to the safety info and precautions for these products. Most of the rust treatment chemicals will be strong acids, and while they should be OK when they dry, the runnoff from these may be dangerous.

Each of these should have environmental conditions on the can or MSDS sheet which show what conditions they should be applied in. A lot of times the humidity is fairly important for them to cure properly. This is particularly important with the encapsulating paints if you use multiple coats or put it on to thick, since the outside may dry and prevent the inner layers from drying properly.
posted by Yorrick at 3:10 PM on August 2

Since no one else has pointed it out, this was once a galvanized steel roof. That means it was a steel roof with a layer of zinc adhered to it. The rusting reaction is a form of electrolysis and by acting as an anode. This means that initially, rather than rusting the steel the water and oxygen react with the zinc instead. This means that a thin layer of zinc can protect the underlying metal even if pinholes or blemishes are in it.

On the other hand, you have a large area of exposed and rusting steel. You do have the option of sanding off the rust, applying a new zinc primer and re-painting it. Another option is to use a product like Rustoleum which combines these steps, a bit less effective, but might work for your purposes. The final answer would be to replace the roof.

I'd base it on the remaining thickness of sound steel, as well as how long you want the roof to last. If the steel still seems very sturdy you might be okay with the re-coat, although you are probably getting more like seven to ten more years instead of the twenty or more you should expect from a metal roof. If you do decide to keep this material make sure you take it up and look at the underside as well. It should also get sanded, zinc coated and painted, otherwise just coating the top is only concealing the problem and your next warning sign will be when it gives out.
posted by meinvt at 7:44 PM on August 2