[duffy mohawk]

I have used an aluminum flashing held down with galvanized screws through metal roofing. The flashing and roofing touch at the ridges every 12" or so. Should I be concerned about corrosion? If so, what would remedy the situation?

 

[Tom Struble]

yes you should be,never mix metals on a roof,change the flashing to galvinized or stainless steel

 

[Grumpy]

Yes and No. Tom is right. Dis-simmiliar metals will cause corrosions. It's called a galvanic reaction. However if you go to a gutter supply shop and ask for "gutter nails" you will get galvanized screw shank nails. I bring this up because most gutters are aluminum. Now that I think about it, most gutter zip screws and many rivets are painted galvinized. So will you get some corrosion? Yes, over time, not immediately. Is it worth freaking out over now? Probably not, In My Opinion. In 20 years it will certainly be a cause of concern. Now if you had copper mixed with galvanized that'd be a reason to freak out immediately.


I'm not sure why you'd have one flashing with another type of roof. Maybe some pictures would help.

 

[Grumpy]

here is a little something that may put you at ease... It is a scale of galvanic actions of metals.

The following metals are arranged in order of galvanic activity. The further apart metals are placed on this scale the faster or more corrosive effect they will have on each other...

Aluminum
Zinc
galvanized iron
tin on steel
lead
stainless steel
copper


As you can see aluminum and galvanized are only 1 or 2 places apart. I say 1 or two because I'v always belived galvanized to be a zinc based coating applied to the steel or other metal material... such as Aluminum. For example: http://www.galvalume.com/ Galvalume is an aluminum zinc allow. So if the galvanized screws are zinc based galvanized, you really have nothing to fear, again in my opinion. I'd still like to see pics

 

[Tom Struble]

hmmm i don't know Grumpy,anytime i see galv roofers in aluminum i see rusted nail heads,maybe the op could get away with an isolation spacer and ss screws

 

[seeyou]

hmmm i don't know Grumpy,anytime i see galv roofers in aluminum i see rusted nail heads,maybe the op could get away with an isolation spacer and ss screws

I thinks that's more of an issue with the galvanization getting damaged when the nail is driven. Also, if it's in the gutter, it's probably staying wet a lot longer than a fastener at the ridge. I'd blame what you're seeing more on that than electrolysis. There's not much electrolysis between alum and zinc as Grumpy noted.

If both metals are anodized or painted, I'd be less concerned than if both were raw. And I wouldn't be too concerned either way.

 

[Tom Struble]

i submit to the experts....but i wouldn't do it:wink:

 

[Grumpy]

Yes Tom, I agree. Nothing you are saying is wrong. But it's also easier said than done. FWIW I am no expert, just recycling information I have gathered through my research. LOL It could be wrong for all I know

A point that hasn't really been made is why would the roofer use aluminum flashing on a steel roof? That's why I asked for pics. It's a red flag for me that maybe this "roofer" wasn't so much of a roofer at all.