Many years ago I wrote a blog post detailing one of the most common deck defects that I find: improper joist hanger nails. The issue detailed in that blog post was the use of 1-½" nails in places where full-size 10d nails are supposed to be used. The image below shows the difference between these two nails.
The details on improper joist hanger nails
It's acceptable to use these shorter nails to fasten the joist hanger directly into the ledgerboard, but most joist hangers also require nails to be driven at a 45-degree angle through the joist and then into the ledgerboard. When short nails are used at the joist they don't make it into the ledgerboard, so no support is added.
The easiest way to determine if short nails were used at this location is to look underneath the joists. In most cases there will be a few joists that aren't butted up tight to the ledgerboard, and the tips of the nails will be visible if the short nails were used:
The other way to tell is to simply pull a nail out. When the correct nails are used, they're really tough to get out. When the wrong ones are used, they come out pretty easily; I just use a mini pry-bar that I carry around in my tool pouch to pry a short nail.
What to do when this defect is found
The reason for my update on this blog post is that the largest manufacturer of joist hangers, Simpson Strong-Tie, has put out a letter giving direction on how to correct this particular defect. In short, the fix is to remove the short nails going into the joist and install Simpson's #9x2½" SD Connector screws.
Click the following link to see the full document: Repair of LUS Joist Hangers Installed with 10dx1-½" Nails. This document also lists the newly calculated load capacities, which I can't imagine myself ever using as a home inspector. I leave that stuff up to the engineers.
Is this defect a big deal? No, probably not ... but as you can see, the repair isn't a big deal either. For an average deck, the repair will probably take a $20 box of screws and about an hour of someone's time.