Randomly disbursed throughout the crawlspace, are corroded Truss Plates. Lumber is testing at acceptable moisture levels. Moisture barrier in crawlspace has moisture on the underside, but dry on top, no gaps, and no signs of pooling water. Light fungal growth on some trusses, but nothing crazy. Vents only on the rear side of the structure. Any suggestions on the source of the random corrosion?
I have no idea on the cause of the corrosion, but it doesn’t look like they can perform as intended for much longer.
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Moisture, elevated humidity. Source unknown, but you have mold and corrosion.
Mainly living space. There is a bathroom, and a laundry room on that level, but the corrosion is spread randomly throughout the crawlspace, not just under water areas.
I would guess a bad batch of truss plates that was not galvanized properly. Regardless the trusses plates need to be fixed.
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Damn, I was so focused on the ones that were corroded, I did not notice the ones that were not corroded. Thanks @rmayo for bringing that into focus.
I must say however, to me that is a lot of mold/fungal growth.
Also, this sure look wet, hence @jjonas point.
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How old is this dwelling?
2004
I was wondering about possibly a bad batch of them as well, since they’re sprinkled randomly throughout the crawlspace, I’d say 30% look like this. You have good ones and bad ones right next to each other, even on the same truss.
I was thinking moisture as well, but the wood tests 10%.
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It looks like the trusses may have been exposed to a corrosive chemical? Maybe road salt in the storage yard? Maybe Salt caught in the webs from winter snow removal? I am just throwing it out there.
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Wow… one of the crazier things I’ve seen. Usually, when things are random like that I conclude it happened to the materials before they were placed but the leaching/staining around some plates pretty much rule that out. There don’t seem to be any other signs of moisture in the space. Maybe someone is just trying to screw with the HI? 
At the end of the day we don’t need to conclude why/how so I agree to just recommend someone else look at it. Of course, we all want to know how/why but we may never in this case.
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Or maybe?
Bleach: pH 11-13 . Bleach is one of the most common cleaning supplies in households and commercial settings. This particular product has a pH between 11 and 13. Its high level of alkalinity is what makes it corrosive.
I’m with Randy on this one.
Iron oxides in direct contact with wood will absorb moisture out of the air and transfer it to the wood leading to discoloration like that…
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Just sayin…
But there is no evidence of moisture staining on the floor sheathing.
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I agree with Randy on this, but the only other thing that may come to mind if there were any DWV venting leaks. I have seen sewer gasses cause all sorts of odd corrosion on copper and other metals that would not have been able to happen any other way. The sulpher gas can be highly corrosive…Just something to consider as an additional option. Generally the corrosion from sulpher will be very black in color.
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Possibly left out in the rain and are old stock. I would question whether it is anything going on in the crawlspace because it would have to be extremely location sensitive since the joist about 2 feet away are not affected. But I agree with others that they are severely compromised and will probably need a repair or outright replacement.
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Gusset plates badly corroded at several locations on the floor joists in the crawl space. This could eventually lead to structural failure. Recommend further evaluation and repairs as necessary by a qualified contractor.
Thank You All for the information!
I was really trying to understand what the root cause could be. I couldn’t figure out the randomness of the affected plates.
I basically had ended up recommending the same as what William said, … just not quite as eloquently 
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Hi Robert,
I was thinking condensation till you mentioned it was random. Most likely the root cause of it is what Randy mentioned, bad batch of plates combined with either intermittent sewer gases or humidity. You did mention sporadic organic growth so more likely the latter. The intermittent humidity could be from a leak some distance away, possibly difficult to see or barely adequate venting. Locally vapor barriers are normally installed for Radon mitigation, not humidity control.
Listen please. A lot of good reasons why they went bad it. However ,
Just called out what you see and move on. That’s what we do as home Inspectors.
We don’t know or nor need to know. Why it happened!
Just simply write-up what you see and move on.
Make your life easy…
We noted several gusset plates show extensive corrosion. Recommend further evaluation repair by a licensed professional
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