Wood Screws Breaking - Not Sure Why

[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]

I would use a #9 or #10 and probably longer.

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.

[coppercone2]

check it with a strait edge

[IanB]

I believe deck screws are quite brittle, so they snap easily.

Also, you need to consider shear stress (sideways stress). If the panels expand due to moisture at a different rate than the runners, then the screws would be under significant stress due to differential expansion.

Have you built your fence according to a recommended design pattern? For example, when I think of "fence panel", then I think of a complete, self supporting panel that goes between the posts. Typically, panels are fixed directly to the posts, and do not have separate runners.

An alternative design is without panels, where you install the runners, and then fix vertical boards to the runners.

[bostonman]

Just to elaborate, this wood has been outside under tarps and slowly removed for a few months as the fence was being built.

The 2"x4"s were fully installed for about a week or two before the panels began being installed. Most of the wood (in my opinion) was acclimated and the outside temperature has remained within 20 or so degrees (we are in the winter months in the Boston area).

Stress seems the obvious issue as if the screws were a cheap grade of chemical protection against treated wood, then I'd assume it would take longer before the chemicals ate through them.

[bostonman]

An alternative design is without panels, where you install the runners, and then fix vertical boards to the runners.

.

This is what I did, correct?

[IanB]

This is what I did, correct?

I see. Now when the boards expand and contract, they will push the horizontal runners up and down. Are the runners free to float in the 4x4s, or are they fixed tightly? If they are fixed tightly so they cannot move, then when the boards expand, something will have to give. If the runners cannot give, then maybe the screws will break instead?

[wraper]

Boards look packed with no gap at all. Might be due to that as the only direction left to expand is outwards.

[bostonman]

The panels aren't spaced; they are laid against each other.

The first panel is against the inner side of the 4x4, six screws (two top/middle/bottom), and then the next panel is pushed against the first, etc... sixteen panels per section.

[Kim Christensen]

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.

They are actually #8 screws... The #2 on the box is the Philips screwdriver size.

Bostonman has the boards installed with no space between them. They will expand once out in the rain for months at a time. You need a slight gap between each board to allow for expansion for when they get wetter after installation. With dry wood, 1/4" should do it. If already very wet, you could space them less since they'll shrink in the summer.

[Muttley Snickers]

One of my best mates is a fencer and what you call runners we call rails. As others have mentioned the fence will take some time to settle so expect a bit of movement.

Also, can I suggest you grab an off-cut of the rail and place a test screw using the same technique you placed screws in previously, same depth, speed, torque etc. After installing the screw check its head immediately with your finger to see how warm or hot it got, if it's hot to the touch then you may have altered its temper. Dishwashing liquid or soap is always your best friend when putting screws in timber.   

[Bud]

You can try to pre-drill the runners for a few screws to see if those screws will experience same behaviour. If they also snap later then it is shear stress.

[vk6zgo]

One of my best mates is a fencer and what you call runners we call rails. As others have mentioned the fence will take some time to settle so expect a bit of movement.

Also, can I suggest you grab an off-cut of the rail and place a test screw using the same technique you placed screws in previously, same depth, speed, torque etc. After installing the screw check its head immediately with your finger to see how warm or hot it got, if it's hot to the touch then you may have altered its temper. Dishwashing liquid or soap is always your best friend when putting screws in timber.

If it was Jarrah, at the least, I would rub the screws on a wet cake of soap, or preferably, drill smaller pilot holes, first.

[RoGeorge]

If all else fail, try iron nails instead of screws.

[mikerj]

That style of woodscrew can be quite brittle, IME it takes relatively little bending stress to fracture them.  I'm with RoGeorge, nails are superior in this kind of application where significant movement from wind and moisture is inevitable.

[vk6zgo]

If all else fail, try iron nails instead of screws.

Even better, nuts & bolts.

[Marco]

Maybe you just got unlucky and got a really well dried batch and it's expanding, usually it will just shrink a little over time but expansion with no gap will be trouble.

[jpanhalt]

Wood moves perpendicular to the grain most with changes in weather (temperature and humidity).  Storing it under a tarp might be equated to seasoning, but it doesn't change that fact.*  Even with kiln dried wood seasoned indoors for years, one still gets movement with weather changes.  Ask anyone who installs hardwood floors.  Movement with packed boards can sums those effects. Wind is also a greater factor with packed boards. 

Observations:
Screws are breaking.
You used #2 screws.
The boards are closely packed.
Treated wood was not dried after treatment (perhaps irrelevant for swelling but not for the twisting and warping).

Assumption:
Wood is not stressed.

Conclusion:
Fence was constructed without compensation for swelling during wet months.  The wood is swelling/contracting, warping and twisting.
The screws used initially were too small. (I only use #2 for indoor trim that may need to be removed or toys.)

Recommendation:
Use at least #8, but I recommend #9 or #10 (coarse).
Consider ripping about 1/8" to 1/4" off every 4th board or so.  Pressure exerted by wood swelling when wet can be used to split stone.
I do not think the entire fence needs to be rebuilt.  If you move the boards, you will have some screws only slightly offset from the original and that is weak.  Reinstalling larger screws in the same holes will be stronger.

*As a general rule, wood stickered and stored in a dry barn, air dries at about 1" per year.   That is barely adequate and requires good airflow around the wood.  Wood simply stacked under a tarp does not qualify as seasoned or air dried.

Edit:  While nuts and bolts may be stronger, they typically leave the nut and a portion of the bolt exposed.  That snags people and clothes.

[Marco]

If the wood was still relatively high humidity and was drying, it wouldn't really be a problem. It would create its own gaps.

[jpanhalt]

That's why I added "perhaps irrelevant."  It's warping and twisting, but shrinking can also produce stress.  Fall in our NE, as elsewhere in the US, is a wet season.  Despite being "wet" compared to dried wood by definition, treated wood still expands when put in the rain.

[bostonman]

Just as another note, I pre-drilled all the holes before installing the screws. Not a very large hole, but enough to reduce the chances of the wood splitting when installing the screws.

The winter months are upon us, so unfortunately all I can do is wait to see if, and how many more screws break. This is a bit of a scary situation as the fence wasn't completed and already had screws breaking.

I should have called the "runners" rails instead, seems like the correct term.

[jpanhalt]

Bundle up.  Here in the Western Reserve we are 50°F today.  I suspect the screws will be harder to remove next year than they are today.

In the 1980's, I worked outside in Minnesota at temperatures as low as 10°F.  That's when I learned about 3M Thinsulate coveralls with nylon(?), fire resistant outer and inner layers.  Today's Carhartt and Dickie are OK, but more bulky.  I am not suggesting that you go to that extreme, but East coast cold should not be too much of an impediment.  Besides, it can be fun.