Dear Builder’s Engineer,
I’ve noticed that some details call out screws, some call out nails, and some don’t make any callout at all. Is there really a difference in the strength of nails vs. screws?
I wish the answer was as straightforward as your excellent question, but unfortunately it is not.
Connector strength depends on a long list of variables: species of wood, diameter of connector, duration of load, moisture content of wood, thickness of members, length of penetration, edge distance, end distance, spacing of connectors, orientation of wood grain, and probably the installer’s brand of underwear.
Before we drill too deeply into this subject, here’s an interesting tidbit from the IBC’s referenced wood code, the NDS (National Design Specification for Wood Construction, by the American Forest and Paper Association and American Wood Council).
No reduction to design values is anticipated if soap or other lubricant is used on the wood screw. (188.8.131.52.)
So that soap or spit you’ve been using to keep screw heads from buggering does not compromise holding power and is okay by code. My dad used to run nails through his crewcut for the same effect. But I digress.
Nails and screws are intended to resist two types of loads: shear and withdrawal. Shear is the force that wants to slide the pieces being joined. Withdrawal, also known as pullout, is the force that wants to pull two members apart. When you hook on to a nail with the claw portion of your hammer and apply a prying action, for example, you’re subjecting the nail to withdrawal. That is if you still own an antique such as a claw hammer.
Let’s assume we’re connecting a 2x of Doug Fir. Here are some strength values for different types of connectors from the NDS.
|* 16d common nail||.162”||138 lb.||134 lb.||40 lb.|
|* 16d sinker||.148”||115 lb.||112 lb||Not listed in table.|
|* 16d box (nail gun)||.131”||95 lb.||93 lb.||33 lb.|
|* #6 screw||.138||71 lb.||70 lb.||141 lb.|
|* #8 screw||.164||90 lb.||89 lb.||168 lb.|
I highlighted the two rows corresponding to commonly used types of nails and screws.
In general, screws have much greater pullout power than similar sized nails but slightly less shear capacity.
An oddity I noted was that 16d sinkers are not even mentioned in the withdrawal portion of the code (I looked hard), but they are clearly shown in the shear part. I take that to mean sinkers may not be used to resist pullout. I also found it odd that the code would specifically allow soap or other lubricants on screws but is absolutely silent on lubricants (such as vinyl coating) for nails. When was the last time you actually used a 16d common nail? For me it was about 35 years ago and the experience was so harrowing I considered it a godsend that sinkers arrived on the jobsite the next week. (The green ones were way better than the black type.) This was before the advent of nail guns, of course.
At certain times the type of connector is critical, and sometimes it is not. When I’m worried about pullout, I spec screws and don’t allow substitutions. If the work is in cramped quarters such as a crawlspace, I usually spec screws because I assume they’ll be easier to install, but as long as pullout isn’t a concern, nails would be okay too. When the type of connector is not critical I’ll spec nothing.
Occasionally a builder will ask for clarification or a substitution on the connector I’ve spec’d. I welcome those calls because it’s nearly impossible for a guy in the field to know all that went into the design, and it’s tough for me to know what’s actually doable out there.
In summary, there is a difference between the strength of nails and screws but it may or may not be much depending on how the load is applied and a bunch of other variables. If in doubt, call your friendly neighborhood engineer—he or she should be happy to help you pick the best connector for the job.
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