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Is there a case where nails are better than screws, from a structural standpoint?

For instance: Say you used screws instead of nails on a backyard deck, when the design specifically called for nails. Would the result be inferior? Could it fail inspection? Would nails absolutely be preferable in this instance?

Assume the screws are at least as thick/long as the intended nails. Are there any rules of thumb about when to use screws vs. nails?

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Screws are a "superior" fastener over a nail (they have far superior tensile strength) - especially if you're talking about screwing down decking.

However there are many scenarios where a nail is the proper fastener for the application (attaching joists is one example - screws are brittle and will fail when subjected to the forces of a shear loaded application)

Using your deck example - you should use nails to attach the joists to the deck framing but use screws to fasten down the decking itself.

Also keep in mind that screws take a longer time to install - think of a nail gun versus a drill...

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That explains why the trellis over our back porch (rented duplex) is falling down. –  Wayne Werner Aug 4 '10 at 18:21
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Another point to consider is that sometimes fastening as much as possible is not necessarily a good thing. If working with wood (which can change its shape according to environment conditions) it can be better to allow a nail to move ( and help release built up forces) than to use a screw and have the wood crack. –  user1835 Jun 20 '11 at 2:01
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like for example in framing -- it is hands down faster to frame with 3" nails out of a pneumatic nail gun, especially if you are nailing at 45 deg angles when studs are in place (as opposed to assembling on the floor and raising) –  amphibient Feb 13 '13 at 5:23
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Learn to spec out structural wood screws. They are not brittle and are made to work in shear as well as or better than nails. They will also be created to cut their own path through the wood so they don't split your timber. Most people just go down and pull the cheap galvanized stuff out of the bulk bin and don't pay attention to the varied purposes for which fasteners are designed. –  Fiasco Labs Feb 13 '13 at 5:53
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Nails are considered an "elastic connection". They handle wood movement much better than screws. Many times if you have severe wood movement with nails you will see things like nails that tilt or seem to back out. This is actually a good thing. Many times if a screw had been used in that case it would have caused the wood to split as it moved.

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Technically when using joist hangers, a specific kind of hardened galvanized nails are supposed to be used. This is because regular deck screws probably don't have the necessary shear strength. For a deck though, I've used screws before without any problem. Although if you were planning to put a lot of weight on the deck (say, a hot tub) I would be a bit more concerned about that.

Aside from that, I can't think of any other places where screws would be a problem.

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Can you expand on screws not having the necessary shear strength? Are you talking about brittleness? –  Ates Goral Aug 3 '10 at 20:39
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@Ates - Not bittleness really, but shear strength (for example, given a scissors, how much force is required to cut them). The holes that you put the screws through rest on the screws. Given enough weight, the joist hanger can act like a knife and cut into the screws, eventually shearing the heads right off the screws. –  Eric Petroelje Aug 3 '10 at 20:57
    
Simpson does make screws designed specifically for joist hangers. –  Mike Powell Aug 4 '10 at 2:34
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The major problem with ALL fasteners is just grabbing what's on hand and hoping it will do. TECO nails are hardened to be strong in shear and not bend. 16 penny framing nails have the same diameter but are soft, so they pull out slightly and start bending over. Now the head bears the weight and starts pulling through the hole in the bracket and being soft also deforms yet more. A structural screw equivalent of a TECO has the same hardness, but won't pull out of the beam. –  Fiasco Labs Feb 13 '13 at 5:47
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Nails are used in almost all framing and structural applications. Most code books are designed with nails in mind and will have specific minimum nailing requirements and patterns for different applications.

Structural screws are coming more and more on the market every day, but because most code books don't include them you will need an engineer's approval of their use to pass inspection in many cases. If you want to use structural screws without paying the big $$ for an engineer you should talk with your municipal build dept. first to see if they will allow it.

Your normal wood screws are not structural. Standard screws are brittle. If you take a normal screw and drive it part way in and whack it with a hammer, it will snap. If you do this with a nail, the nail will bend. Which would you rather have holding your deck up? Something that bends but stays intact or something that can snap?

I mostly use screws only for temporarily holding things in place while I nail stuff up and for the decking surface.

However, on one deck I built last year I used a ton of structural screws. I have a couple of times had the head snap off of a structural screw when driving them in, and three of them snap when a small machine hit some lumber. While undoubtedly considerably stronger than normal screws, this leads me to still question their shear resistance.

In the end: nails, hex bolts and lag bolts are still better in most applications. Structural screws are just easier than bolting and cooler than nailing. I do love using them, but they don't (and shouldn't) completely replace traditional fasteners.

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If you're using structural wood screws, they won't be brittle like that. This is where most people get into trouble, they don't bother paying attention to the type of screw they're using. –  Fiasco Labs Feb 13 '13 at 6:00
    
That is why I tried to make it clear in the opening sentence in that paragraph that normal wood screws are not the same as a structural screw. However, I'll change the whole paragraph to make it more clear. –  fungku Feb 13 '13 at 7:56
    
cool video in your initial version (it was correct to edit it out) –  Steven Feb 14 '13 at 3:36
    
thanks. I was iffy when i posted it, then after thinking about it figured it didn't really add anything useful. –  fungku Feb 14 '13 at 3:45
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Thanks for the clarification. Another plus for the lowly nail. In framing, they're long, ductile and given proper placement during construction, quite strong at binding the structure together. In earthquake country, it allows a stick-built house to be just flexible enough to remain standing with fasteners that don't suddenly fracture because they won't bend and pull out a slight amount. –  Fiasco Labs Feb 14 '13 at 4:39
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Screws would tend to be slightly thicker than nails (because of the threads) so you would probably want to watch out for splitting of the wood and might want to require drilling pilot holes if you switch to screws. For the most part though, screws would work better than nails in the long term (would not pop up over time) but would be harder to use (pilot holes, snapping, etc.) during the installation for your backyard deck.

BUT if you have some sort of project that is supplying the nails/hardware then I would be cautious about swapping out one sort of hardware for another. The pre-supplied product could have been specially chosen for the job and so you would not want to go and swap it out because you think it is better to have screws. One such example would be some wooden playsets where all of the hardware is supplied... make sure you use what they give you (and what the engineers said will work).

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Plus it's possible in cases like those that after market hardware could void your warranty. –  Wayne Werner Aug 4 '10 at 18:22
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that fact that a nail will pop rather then break can at times be a good reason to use a nail –  Walker Aug 17 '10 at 10:46
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Screws can be easily removed. I really believe this is reason #1 to use screws. I think of everything as a work in progress. I never know whether I will need or want to change work I've done. The future is uncertain, thus you should always choose to screw when possible. Also, when screwing or nailing, I often screw up. With screws, you simply remove them and fix the problem. Removing nails may lead to damaging your project.Screws are much easier to control. Whether using a hammer or nail gun, exact placement of a nail is almost impossible. Screws don't really suffer this limitation.

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One thing that no one has mentioned: screws WILL NOT pull down a deck board nearly as tightly as a nail. If you have a twisted board or a board with a crown, a screw is pretty much useless.

Screwing deck boards also creates large holes for water to soak into and rot much faster. Using a 3 inch galvanized nail and nailing it flush will pull the boards tighter versus sinking through the wood. The decking will last longer (same concept as wood siding) and last but not least, come on guys, a nail looks better.

Oh, and screws are not easy to remove!!!

Local framer with all kinds of experience. :)

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If you are using the right kind of screws, this is not true; the thinner shaft of the screw above the threads will allow the screw to spin in the board, and pull the board very tight. –  Eric Gunnerson Oct 19 '12 at 3:32
    
Stand on boards you are screwing down, and clamp boards you are screwing together in any other orientation. I often work alone so I use clamps extensively for many reasons, but one thing they do especially well is hold boards tightly together for screwing or nailing. –  fungku Feb 14 '13 at 7:07
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A general rule of thumb is use screws when you need pull strength and nails when you need shear strength. For example use a screw to hold down a decking board, but a nail to hold the joist up. Screws hold thing tighter then nails, but screws are brittle when hit from the side giving nails the advantage. This is not a hard fast rule, but covers 90% of of situations.

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Starborn makes great decking screw lines (Headcote & Deckfast) and that have tremendous boring and holding power. Not only that, but they sell them in expoxy coatings (headcote are 305 stainless core) that can be matched to just about any natural wood or laminate decking material on the market. They also make self-feeding guns and a plugging system to conceal the screw afterward. My deck was built with them and it looks as if the entire deck was assembled in a factory and bolted to my house.. amazing. And sturdy as a rock.

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Also Deckmate screws with torx bit heads, I pretty much swear by them when flathead screws are in question. Philips head flathead screws are a POS IMO. –  amphibient Feb 13 '13 at 5:25
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From an engineers point of view nails have greater shear strength and screws work better in tensile mode. Assuming the base frame for a deck is well built perhaps even including joist hangers for extra strength then the decking fixing may be addressed. For an elegant but time consuming finish, assuming the boards are hardwood and appx 30mm thick.Using either a dowel guage or ask your local workshop to turn the scraps of decking to 25mm/same size as the flatbit cutter you have. Conversely use dowel cutter on scraps to produce the number of plugs required.

  1. Using a depth setting rod on a drill pre-drill appx. 10mm into the boards with the flatbit.
  2. Drill a pilot hole all the way through to the joist of a size to match the first third of the screw shank diameter.
  3. Drill a second pilot only through the board to the diameter the screw shank where the thread ends (Fixing 30mm boards requires 100mm screws for best fixing)
  4. Mix a waterproof glue such as CASCAMITE and use a syringe to place a few drops in each hole, screw and fix. A 'yankee' type screwdriver is far better than most power drivers to get maximum downforce and a tighter fixing. With practice it is very fast too.
  5. Leave overnight. Cut the plugs to appx. 15mm length. If a plug cutter was used all the better. Smear a small qty of waterproof glue in each hole, align the grain of the wood plug and hammer in. Leave overnight. Using a belt sander with a medium grit, say 180 grit, run over the deck in the grain direction and level the wood plugs, dampen the surface and repeat with a fine grit an hour later.

Allow to dry, apply the finish of choice....my personal preference is tung oil but it needs applying by being rubbed in and allowed to dry overnight at least five times for a glowing non-slip finish....avoid polyurethane varnish at all costs. Quicker than oiling are the many deck finishes on the market.

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good answer, formatting this with indented numbers would improve readability. –  HerrBag Sep 23 '13 at 19:04
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I dispute nails having greater shear strength is even a factor in most wood connections. Typically the controlling factor in such connections is the area of wood bearing on the fastener. All other factors being equal, the stronger shear connection will be the fastener with the greatest diameter. –  bcworkz Sep 24 '13 at 2:08
    
This answer offers nothing useful relative to the OP's Q about screws versus nails. It is overwhelmingly a HowTo about plugging holes. While it does offer one sentence (the first) that addresses screws versus nails, that sentence is merely an assertion without substantiation. Down voting. –  mike Sep 24 '13 at 16:00
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