Recommended screw fastener length

Question

Is there an industry standard for recommended screw lengths to fasten to pieces of material together? For instance, if I need to fasten a piece of 3/4" plywood to a vertical brick wall, how long should the Tapcons be? What about screwing 1 1/2" pieces of wood together vs. something like, say, a decking board?

What makes sense to me is that the screw should extend the same distance into the receiving material as the thickness of the item you are attaching. So, for example, the screw length for a 3/4" piece of plywood should be 1 1/2". However, everyone seems to suggest using 3" deck screws to fasten decking boards down. I'm sure there are exceptions such as decks, since decks are exposed to weather and wear and tear but is there a "standard" or general rule of thumb to go by? I always struggle with choosing which length of screw to use.

Answer 1

I will to try to give a few numbers you may find useful based on what I have found over my years of connecting one thing together or the other. On Tapcons, Philcons or Con-certs or just good old concrete screws, 1" embedment into is the typical depth I see referred to. The screws are sold at those lengths with that in mind. 1 3/4" for 1X material. I checked the website and it is mentioned that the depth can be as deep as 1 3/4" bury into masonry. In my experience, in hard concrete, or brick, a screw will break after 1 1/4" into a hard surface. If you are setting it in the joints of the brick or perhaps cinder block or a soft brick like older handmade brick that are not real dark, 1 3/4" into the material is pretty easy to do. There are 2 different diameters to chose from too. That can be as simple as the judgment call of "this thing is really heavy and I don't want it to move so I will use the bigger diameter one" or, "This will not be too tough to set, so the small one will do." If the item you are going to secure is going to vibrate, I strongly advise against using concrete screws, unless glue can be used in some way to help keep it together. Wedge fasteners may be a better choice for that, or better still an anchor bolt with expansive concrete set into a drilled, cleaned hole will hold the most reliable if strength and resistance to vibration is what you are looking for.

When fastening wood, once the piece to be fastened is gone through, generally I find 1" into the second, or the piece you are fastening to is enough to hold. Of course, there are variations on this, depending on the circumstances. On deck boards, IMHO, 3" screws for a 5/4 deck board is way over kill, 2 1/2" is plenty for that. For 2X deck boards, 3" is a good fit. I have also seen 3/4" or less, work well in thin material that you may not want to go all the way through, but there may be need for many screws needed in a given area to have a strong hold. When screwing into oak or other hardwoods, 3/4" will hold very well. When fastening a piece to the end of another, as in making a corner for a box, I heard a long time ago the screws needed to be 3 times longer than the piece you are fastening, because end grain does not hold a screw as well as going into the wood perpendicular to the grain. The part about end grain not holding a screw well is true, but I find the screw does not need to be that long at all. A 4 1/2" screw to hold 2X together?? A bury of 2" works pretty good in most cases, in a joint that does not have much stress applied to it. If the screw can be slightly angled, instead of going straight into the end grain, what it can hold will be greatly increased. Whenever possible/practical, it is best in any case to use pilot holes in the piece to be fastened, to reduce or eliminate splitting. There will be cases where the pilot needs to be in the other piece as well to prevent splitting. Oak is a good example of needing properly sized pilot holes in each piece. I might add more to this when I get more time, or depending on the comments received, if clarification is needed.

Answer 2

Your idea that the thickness of the mounted item should dictate the corresponding depth of screw penetration into the mounted to item is not a correct analysis. In actuality it is far more complicated than that if you wanted to actually analyze it to the finest detail. You have to consider the application of what you are trying to support. Will the screws primarily be use in sheer or in direct pull type load? What is the thread holding capability of the material taking the screws? How thick is the "mounted to material"? Are there concerns of penetrating the "mounted to" material too far? Will the application be one where you worry about the screw becoming loose by turning itself out?

All of these and more will play into screw selection. Many times the factors will cause you to have to re-consider the diameter of screw used so that a bigger screw gives better thread holding characteristics. If there is any rule of thumb to be had it would be to maybe consider using the largest diameter fastener with length as long as possible for any given application. Obviously this has to be tempered with common sense. You would not use 4" long Tapcon's if you were fastening into hollow concrete blocks where the screw was not engaging the web of the blocks. You would not use 8" long lag bolts to mount a TV bracket to a wall stud that was 3.5" wide and covered with 1/2" drywall on both sides. You would probably be advised against using 3/4" diameter lag bolts going into the 1.5" width of a stud.

Answer 3

For specialty fasteners, such as Tapcons, consult the manufacturer's literature. It will contain full engineering data regarding applications, allowable loads, and installation requirements.

For standard wood screws, the National Design Standard for Wood Construction contains the requirements. These are incorporated by reference into standard model codes. However, manufacturer's may also provide sound guidance given the infrequency with which NDS appears on bookshelves.

Answer 4

Sorry, I know this is an old question, but I came across the answers and they seem to evade giving a direct answer to the question. While I agree other parameters become involved and thus, the answer isn't quite that simple, a simple answer could have been given followed by caveats and addendums, yet wasn't.

Answer: Effectively, you should go at least half way into the securing material.

caveats and addendums: If the screw goes THROUGH the securing material, then the threads of the screw in most cases are not going to bite into the material and thus the screw isn't holding itself in place and will likely strip. In other words, the threads, and not the shank of a screw (if any), are what secure the screw into the securing material. OR, if the screw is too long and doesn't have any shank, then you are not using the minimum most effective screw; it will take too long to bottom out, the length of the screw which pierced the far side isn't doing anything, and thus is a waste of materials and effort.

If the screw only goes in one quarter of the way into the material, then the screw has not purchased enough threads into the hole to securely fasten in place. "it is only hanging on by a (couple of) thread(s)."

However, NOW the questions are, how thick does the securing material have to be, how much sheer weight or pull is the screw going to endure, what type of material is the screw being secured into, and what type of screw is being used ("the right tool for the job"), etc. The last being "is the screw all thread or does it have a shank?"

In the example(s) given, let's say the brick is a solid masonry brick; 3-1/2" deep x 2-1/4" high x 8" long brick. Going through 3/4" plywood, plus at least half the distance into the material, or a minimum of 1-3/4", then the screw should be 2-1/2" long.

However, if you are talking about a cinder block or any other brick type that has voids to reduce weight, then remember you don't want the screw piercing these voids for reasons I gave above. Also, you don't want a screw with a shank (if there is any) any longer than 3/4" (the thickness of the plywood).