Aluminum Roof Panels over Steel Type B Deck

[kmart30]

Should there be a concern with an aluminum standing seam roofing over a steel deck in regards to corrosion? The panel is mechanically fastened to clips which are fastened to the steel deck. The fasteners dont penetrate the alum. panels so they are just resting on the steel deck. Should there be some type of barrier like iso board or something similar?

 

[ShawThing]

Yes, I think it would be a good idea to isolate the dissimilar metals to avoid galvanic corrosion issues.

 

[Ron]

Yes. You need to isolate the two. I did a failure investigation on similar a few years back. Corrosion occurred at connections within about 3 years. Perforated the aluminum. Will see if I still have photos.

Project was in Florida panhandle. Where are you located in Florida?

Even separating the aluminum panels from steel deck is not the only thing you'll need to do. The attachment clips must have the correct fasteners. In the case I did, the roofer used epoxy coated fasteners....good idea and recommended by manufacturer; however, no one took into account that the fasteners, because of the clip design, would rub against the aluminum panels. Due to differences in thermal expansion and the insulation between the aluminum and steel, there was a fair amount of differential movement. This eventually wore the coating down so that there was contact. Further, if they breach the coating during fastener installation, the same thing happens...contact.

You will need stainless steel fasteners and I would suggest an additional separator of rubber or other membrane between them as well.

 

[hokie66]

Ron is correct. But why would you not use steel roofing instead?

 

[kmart30]

Project is in the Tampa area and close to the coast which is the main reason why it made me think of a higher potential for corrosion with dissimilar materials. I am guessing they want aluminium panels because of the corrosion benefits but now they have this problem.

Ron, that would be great if you could send some pictures; I am interested to see what the corrosion looked like.

 

[Ron]

km30....really makes little difference in the panels. Yes, aluminum is better for the exposure, but as noted, the connections can be problematic. I'll continue to look for photos and will post when I find them. Which manufacturer's panels are proposed?

One alternative is galvalume panels....slightly less differential corrosion potential than steel with aluminum.

 

[hokie66]

Now that BlueScope Steel operates in North America, I believe that Colorbond steel roofing would be the best choice. There are different grades, and the highest grade is for subtropical coastal areas like Tampa. But I don't know how the supply chain works in Florida.

 

[Ballsack]

Aluminum is an anode so it is recommended to prevent contact to stainless steel or other cathodes to prevent corrosion of the anode.
Ex. use of top hat washer to prevent contact of stainless steel bolt and galvanized steel section.

If the anode is a much larger component compared to the cathode, corrosion do not usually occur.
However, if it's the other way around (small anode, large cathode), then corrosion would occur on the anode.

Reference:

http://www.worldstainless.org/Files/issf/non-image-files/PDF/Euro_Inox/Contact_with_Other_EN.pdf

 

[Ingenuity]

Now that BlueScope Steel operates in North America, I believe that Colorbond steel roofing would be the best choice. There are different grades, and the highest grade is for subtropical coastal areas like Tampa. But I don't know how the supply chain works in Florida.

ColorBond (the Aussies spelt "color" like the yanks, just for you hokie!) has a Zincalume base (aluminium, zinc and silicon) and the US version of this is Galvalume - and I think Zincalume is made under license granted by the US owner/patent holder of Galvalume.

However, surprisingly, there is little Galvalume products I see in my area except some flashing and rain gutter products...and have never seen Colorbond products.