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new fh article on pressure treated wood

| Posted in General Discussion on December 10, 2003 12:47pm

We are building a deck with Natural Select pt wood. Wolman says this stuff is only slightly more corrosive than the old cca stuff, although they do recommend using only hot-dipped galvanized fasteners, which are not readily available in my area.

The new FH article includes this type of pt wood with those that it states are roughly 5 times as corrosive as cca.

I’m not sure exactly what my question is, but we’ve already put up 90 regular galvanized joist hangars with grip-rite galvanized, but not hot-dipped nails. I’m pretty sure we’re going to rip them off and use Simpsons Zmax joist hangars and Maze hot dipped nails, but I guess I just was wondering what everybody thought about the discrepancy between FH and the manufacturer regarding the level of corrosiveness of this Natural Select stuff.

Thanks all.

 

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Replies

  1. RW | Dec 10, 2003 02:07am | #1

    It seems like there's a whole lot of contradictory information out there. Every source I've looked at says something different than the next. This is only 10% as corrosive, no it's three times . . . I don't know that anyone has really been able to sort through it all. I do know that Simpson had some test results on their website some time ago where they had attempted to test everything they sold fastener-wise with virtually every kind of treated wood. The concensus was that everything was corrosive. How much is the catch. I think your idea of going to hot dipped as a minimum is a good idea. The ZMax is supposedly even more resistant than that. I've gone stainless hunting myself. Anything I can get in stainless I'll use. Anything I can find, I'm still in hot dipped territory. Last deck I did the only galvanized I used was carraige bolts. The rest was SS. But it's pricey. I guess I looked at it as a liability thing. I don't need a $60K project to fall over after two years.

    "The child is grown / The dream is gone / And I have become / Comfortably numb "      lyrics by Roger Waters

    1. RWELLS19 | Dec 10, 2003 03:48am | #2

      We built a 70' P.T. deck on the front of a 90' house. The P.T. joists

      were nailed with hot dipped galvanized nails and then galvanized hangers were fastened with hot dipped hanger nails.

      The house was then bricked up to and surrounding the joists.

      After a couple months we are seeing rust coloured stains from the weeper holes. I'm wondering if the new P.T. is reacting with the mortar or the hot dipped galvanized hangers and nails. After reading the article I'm leaning towards the galvanized reaction theory.

      Any opinions?

      1. JohnSprung | Dec 10, 2003 05:05am | #3

        Can you get access to check on the nails, maybe even pull one to look at?

        -- J.S.

      2. RW | Dec 10, 2003 05:43am | #5

        Without knowing what PT you used, no. I do recall reading (probably FHB or JLC) that part of the problem with ACQ was metallurgy - too much copper in the treatment process started a reaction betwixt dissimilar metals. Even hot dipped.

        As to not using stainless because it's not strong enough? On second thought, that doesn't even really rate a comment.

        "The child is grown / The dream is gone / And I have become / Comfortably numb "      lyrics by Roger Waters

    2. PKCalabogie | Dec 10, 2003 05:06am | #4

      I'd think long and hard before using stainless in any structural application.  My understanding is that all stainless is subject to metal fatigue under cyclical loading; i.e., it's not a matter of whether it will break, it is only a matter of WHEN it will break.  I'd take galvanized steel over stainless any day.

      Paul 

      1. UncleDunc | Dec 10, 2003 06:20am | #6

        >> ... all stainless is subject to metal fatigue under cyclical loading ...

        I'm no metallurgist, but my understanding is that all metals are subject to metal fatigue under cyclical loading. And that fatigue is only an issue when the metal in question is flexing under load. In a properly engineered deck frame, I'd think the nails are in a fairly static situation.

        >> ... it is only a matter of WHEN it will break.

        That is indeed the question, or more specifically, when it will break relative to the life span of pressure treated deck components. If I'm not mistaken, the makers of joist hangers accept the use of SS nails. If their engineers are wrong, if there is evidence of SS nails not performing in the real world, I'd like to hear about it.

        Galvanized nails aren't eternal, either. IIRC, the Forest Products Laboratory test of deck fasteners showed that in a rainy climate, galvanized nails, even hot dipped galvanized, rust through in a surprisingly short time, while the SS nails still looked new.

        1. PKCalabogie | Dec 23, 2003 05:22am | #7

          You are quite right, all metals are subject to fatigue under cyclical loading.  In fact all materials are subject to fatigue.  I guess I was a little imprecise in my last posting.  What I should have said was there are two broad classes of materials which behave very differently under cyclical loading.  For one class of material (including steel & wood) the strength of the material decreases with each sucessive load until it levels off at what is known as the fatigue limit.  If the part or structure is designed such that the fatigue limit is not exceeded, it will bear the load indefinitely.  It does not matter whether the load is bending, shear or torsional.  For the other class of material (including stainless steel, aluminum, copper, and all plastics) the strength of the material continues to decrease with each successive load until the part fails.  When working with such materials engineers must define an endurance limit beyond which the part should be replaced.  That is the thinking behind my statement that it is not a matter of if it will break but when it will break.

          Your other point that the material selection is dependant upon the entire system and that the wood will probably be gone long before the endurance limit of the stainless joist hanger is reached is probably right.  (As long as the guy who rebuilds it in 30 years doesn't say "hey these joist hangers still look good, lets re-use them").  Endurance limits are measured in millions of cycles.  From 1st year metallurgy I guess I've just developed knee jerk reaction to any suggestion of using stainless in a structural application.  I'm kinda old fashioned and like simple materials like wood, masonry, steel and paint.

          1. UncleDunc | Dec 23, 2003 06:40am | #8

            It's still not clear to me that just repeated loading will inevitably fatigue a stainless steel object to the point of failure. Doesn't the load have to be large enough to actually flex the object before it will cause fatigue? If I repeatedly lay one stainless steel washer on top of another one, my intuition tells me that it will be a long, long time before the bottom washer crumbles under the load. Same thing if I repeatedly load a 1' long by 1" square channel in bending with a one ounce load, I foresee my lab being lost to geological subduction before the channel breaks.

            >> I'm kinda old fashioned and like simple materials like wood, masonry, steel and paint.

            But the old fashioned materials have to be engineered, too. For instance, in my experience, galvanized steel that is subject to repeated wetting, like a joist hanger under a deck, will eventually rust to the point of failure. Why does it make sense to reflexively reject stainless steel fasteners because they'll eventually break, but accept galvanized steel hangers which will eventually rust out?

            Rejecting stainless steel fasteners without analyzing the loads and duty cycle isn't engineering, it's voodoo.

          2. JohnSprung | Dec 23, 2003 10:34pm | #10

            Does anybody know of examples of stainless that has been in use for a long time?  The only one I can think of is the top of the Chrysler building in NYC.  Is stainless holding up, or are there problems?

            -- J.S.

          3. dmac15075 | Dec 24, 2003 04:03am | #11

            I don't have a Machinists Handbook nearby, but there are many different grades of stainless steel available. For the most part the ones that we see are 308 or 316 grade, which do have a shear strength much less than that of a similar galvanized steel fastner.

            Now think of that in terms of what we use. I've snapped the heads off of more #10 SS deck screws than I care to admit, but a regular steel screw would have just keeps right on going into the wood. When you are attaching beams to a post with 3/8" SS carriage bolts you are probably well within the fatigue limits of the fastner, and easily meet the building codes. Sure a galvanized fastner would be stronger, but does it need to be?

            Going to the comment about the stack of washers, shear strength is not the same as compressive strength. It may take a long time for that washer on the bottom to compress, but only support half of the washer and stack the weight on the unsupported side and that's the shear force and a whole different thing.

  2. AndyEngel | Dec 23, 2003 03:35pm | #9

    The new PT is five times as corrosive to mild steel as the old, and twice as corrosive to galvanized. The new Z185 hangers have three times the zinc on them as the old standard Z60 hangers. Given the deck collapse we saw last summer in Chicago (I'm not suggesting its cause, just reminding everyone of its consequences), I'd replace the hangers. I might even build my next deck supporting the joists on a 2x4 ledger.

    What we learned: Copper flashing, minimum of hot dipped galvanized nails (Note - Hot galvanized is not the same as hot dipped, and it's not as good. Read the label!), stainless preferred. The code exempts bolts of 1/2 in. or greater from the galvanization requirement, but that's based on testing performed with the old PT. I'd err on the side of caution and use HD bolts.

    Andy Engel, The Former Accidental Moderator

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