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Discussion Starter #1
I am a homeowner getting ready to replace the old Masonite siding on my home. I will be removing the old siding, installing Tyvek weather barrier, and installing Hardiplank siding. Planning to do all work myself.

For the most part seems to be a straighforward task. However, I question what to do at the bottom. The old siding does not overlap the foundation like normal. Instead the outside edge recessed about 1/2" from the edge of the foundation. Currently there is no flashing b/w the siding & foundation, but my concern is that flashing should be there to direct water away from the sill plate. There hasn't been any water issues yet, that I am aware of, but I want to do it right. Anyway, my real question is on caulking. Should I caulk behind & under the flashing, or leave it open? I assume that I should not caulk the front/top side as I need a drain path on the outside of the weather barrier. Also, I've read that cement siding should not contact aluminum flashing. Is this really an issue?

Any other suggestions for my installation would be appreciated.
 

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Thanks Grumpy. How about caulking. Should I caulk behind & under the flashing? Also, is it really a problem for fiber-cement siding to contact aluminum?
 

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NormZ said:
Also, is it really a problem for fiber-cement siding to contact aluminum?
That's the first I am hearing this. I just zipped an e-mail to certainteed asking for clarification. If it is a problem I am screwed since I've installed many jobs with aluminum trim.

As far as caulking goes, it wouldn't hurt to caulk behind, but I wouldn't bother with caulking under. INfact that may be counter productive. If any water makes it's way behind the siding at a higher point it will not be able to drain out if you caulk under.

What is critical IMO is to make sure to install the foundation flashing so the tyvek overlaps it.
 

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Discussion Starter #7
"Typical mill-aluminum products...will corrode/oxidize when placed in contact with Fiber Cement. The alkalinity of fiber cement will break down the bare aluminum." (http://www.tamlyn.com/literatures/donotuse.pdf). I have seen similar statements made elsewhere as well but only by manufactures of vinyl products, not by manufactures of fiber cement.
 

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Most aluminum is not mill finish. Most aluminum has a baked on enamle paint finish.

I've never read this before, it's news to me. I wonder how long it takes the oxidation process to occur. For example placing aluminum and copper together will create a galvanic reaction but it takes 40 years for any noticeable degredation. At the same time placing copper and steel together will create a reaction that will be noticeable within a year.

This is chemistry talking - not marketing !
LOL it's definetly marketing.
 

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I'm not disputing the fact that aluminum will corrode in contact with cement. Infact I have heard that be told in the past many times, in relation to aluminum flashing and masontry chimneys. HOWEVER the time that it takes for the corrosion to actually appear is what I am questioning.

Around here aluminum flashing is pretty much the standard for most flashings, inccluding chimney, apron and step flashings; all of which are in direct contact with the mortar of the walls they are flashing. From experience I know I have never seen any unusual weathering of that aluminum even on 20+ year old flashings.

So my question to the chemistry experts, since I am nost definetly not one, is how long does it take for these chemical reactions to occur.

MikeW, I read that web page you posted and found it interesting but didn't see anything about time frames of these chemical reactions. For all we know it can be almost instantaneous or take centuries. That's really what I am questioning.
Milke
 

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Grumpy,
I am glad that you found the information interesting. Construction is all about materials, and materials are all about chemistry. If you're really interested in learning more about the role of chemistry in construction, PM me and I’ll help you out.
 

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I put pieces of flashing behind Hardie Plank all the time as I hate to caulk the joints, too time consuming. Been doing it this way 4 years now no problems.

2b2s
 
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