Electronics > Mechanical & Automation Engineering

Wood Screws Breaking - Not Sure Why

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bostonman:
I built a fence, the vertical posts are 4x4s (pressure treated/ground contact) and between them are three 2x4s (top/middle/bottom) which I call "runners" (the horizontal pieces to hold the panels), and the panels are non-PT 5/8th thick.

Attached are the labels for the wood used and the screws.

I installed each panel with six screws (two on the top/middle/bottom) approximately 1.5" from the edges. In the last few days, after about four-weeks of installing the panels, I'm seeing the screws breaking (about eight so far). They are breaking at the point where they enter the 2x4", so I'm able to unscrew the broken piece (which is about 5/8th long - the part that is in the panel) and the rest remain inside the 2x4".

The screw box states they are made for all types of treated wood, I'm not over-tightening them, the panels aren't under mechanical stress, so I'm uncertain why these are snapping.

Does anyone have ideas on why these would be breaking?

Update: I forgot to add that I looked at these under the microscope. They don't appear to have pits due to chemicals eating them.

jpanhalt:
A #2 size for that application seems too skinny.  That's about the size used for drywall and they do tend to break.  I would use a #9 or #10 and probably longer.  I usually get TimberLok brand; although, the store brands may work too.  I consider a fence structural.

bostonman:

--- Quote ---I would use a #9 or #10 and probably longer.
--- End quote ---

Would stress due to being too thin cause them to snap after weeks without a load? I would have expected them to snap while screwing them in if that were the case.

As for being longer, they are going into 1.5" side of the 2x4"s, so I'm limited to this length due to wood depth and size options.

jpanhalt:
Treated wood is usually quite wet and changes after installation due to drying.  I wouldn't say they were without stress.

wraper:
Could be due to thermal expansion/contraction of what they're holding. Or due to wood moisture change as already said.

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