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galvanized flashing against concrete

galvanized flashing against concrete

We are having a very minor dispute as to the proper flashing material to use between concrete & wood. We are flashing between a rim joist & concrete porch. I say stainless because I believe in time galvanized steel will deteriorate as would aluminum. Any official advice would be appreciated.

RE: galvanized flashing against concrete

I am not a structural guy with formal experience in the building industry, but I believe that galvanized steel components are very commonly used as anchors for typing wood to concrete as well as for many similar applications. Stainless steel is not impervious to corrosion in crevice conditions.  The suscpetibility to corrosion will be related to how tight the corrosion, exposure time to water, whether the wood is treated with anything that promotes pitting of stainless, etc..  The corrosion rate for galvanized coatings exposed to water is reportedly the lowest in the pH range of 6-12.5 (per ASM Metals Handbook)and I imagine the pH range of water that may periodically collect in crevices between the concrete and the flashing will be in that range.  Not sure what pH the moisture between the flashing and wood. It may depend upon whether the wood is treated. The attached link is to an article describing use of zinc coating to protect rebar from corroding in concrete. The thermal-sprayed zinc will have different characteristics than galvanizing, but since galvanized coatings are also zinc the article may be a general indicator of performance.      

RE: galvanized flashing against concrete

Whatever happened to copper?  It would work well.  Galv should work OK also since the concrete will be at a high pH..  SS is prob. not needed, you aren't going for a 100 year design life and it is a pain to work with.  If this is in a location that can be exposed to road salt (northern) or ground salt (coastal) then SS is not a good choice.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

RE: galvanized flashing against concrete

First, I'm sorry for the lousy editing on my first message. I should have read it over before clicking the "submit post" button.

Edstainless, I agree that copper could be considered, although the potential for unsightly staining from the initial corrosion of the copper should be considered, too.  Some homes in our area have bad discoloration of concrete from water running off copper roof components.

RE: galvanized flashing against concrete

This is a simple z flashing over the rim joist to under the concrete. Residential construction so 100 year design life is not out of the question. I have done a few repair jobs where I removed  concrete porches/patios & the flashing was ate up & the rim joist was rotted. I always assumed that galvanized flashing was used on these jobs but I couldn't tell. None of them wrapped the top of the rim joist. The tear outs were between 12 & 30 years old. I live in Illinois where people tend to salt their porches in the winter. We were planning to use 26 ga. galvanized, so their really isn't a workability issue with the stainless. I still don't believe that coating on light gauge galvanized is heavy enough to hold up for any lenght of time. Now that you've heard the rest of the story do you have any more enlightned recommendations for proper material & gauge? The basic issue at thejob is cost. The flashing will be 16" cut by 24'. There will be an L flashing on top of the concrete as well. Could we get away with aluminum  coil stock there?

RE: galvanized flashing against concrete

I would avoid stainless if salting is common. It is highly likely to pit in the crevice areas and will be quite expensive relative to other options.
Aluminum may not be much better in some cases. I've seen 10-15 yr. old corroded aluminum sliding door sills mounted on concrete foundations, and that is in "mild" California weather so your situation would probably be worse unless you search out and use one of the more corrosion resistant grades. ASM Metals Handbook indicates that aluminum in contact with fresh concrete may undergo some superficial etching from the high pH, but that the long term effect is oftem minimal.  It does also say that aluminum components (such as highway poles and railings)in contact with concrete in crevice areas where salts may accumulate are often coated witha sealing compound to minimize salt getting into the crevices. It further says that aluminum alloys of 2XXX series and 7xxx series are less resistant to chloride pitting than alloys of the 3xxx, 5xxx and 6xxx series, with the 6xxx being the least preferred of the three choices.  Typical seawater penetration rates are reportedly less than 0.001" in 10 years for the 3xxx and 5xxx alloys, but it is hard to relate sea water corrosion rates to what might happen if salt solutions get into your crevices and start to concentrate through evaporation.
Sounds like someone needs to invent and distribute some type of nonmetallic flashing if you want relaible 100 yr protection.

RE: galvanized flashing against concrete

Could you get someone to make the Z & L flashing out of plastic or GRP?

RE: galvanized flashing against concrete

The copper that I have seen installed was pre-washed with acid to generate the patina and minimize the staining.
I agree, don't use stainless.
There are two problems with galvanized.  You probably can't get heavy Zn, only the light stuff.  And the cut edges are all exposed

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

RE: galvanized flashing against concrete

I appreciate the advice, thanks. I'm now thinking about using 26 ga gal z flashing conered with peel & stick roof flashing.  any thoughts?

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