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I am trying to attach pieces of 2-by wood to the frame tubes of a conex shipping container. The steel is about 1/8" thick. I probably need screws at least 1/4" diameter to get the strength I need (100+ lbs shear).

Found this fairly comprehensive article about all the different types of sheet-metal screws:

http://www.fastenermart.com/understanding-tapping-screws.html

... and they say you need "thread cutting" (aka. "Type F") instead of "thread forming" for "thicker and harder" materials.

My question is: would 1/8" thick steel (I'm assume it's mild steel in a ubiquitous thing like a shipping container, and it wasn't that hard to drill through) be consider a "thicker" material, or will an ordinary #14 sheet-metal screw suffice ?

An ancillary question: If ordinary sheet-metal screw is unsuitable as I expect, will a "self drilling" type such as:

https://www.fmwfasteners.com/collections/hex-washer-head-self-drilling-screws-zinc-plated/products/14-14-x-2-1-2-hex-washer-head-self-drilling-screw-zinc-plated

... work ? I would likely drill a pilot hole anyhow, and use these as thread-cutting type. The self-drilling seems easier to find than thread-cutting.

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  • I use 14-1 self tappers all the time on shipping containers to mount equipment on the walls including transformers, service panels and disconnects. The only problem with self tappers is the tip protruding on the outside. To solve the protruding tip "safety" problem I use a thin "razor" cut off disk for metal and cut flush with the metal. Using self tappers in this way will support a very heavy load even if the containers are moved.
    – Ed Beal
    Commented Mar 7, 2017 at 3:07
  • Sounds like you're attaching to the sheet metal (14ga ?) of the container walls ? I'm trying to avoid penetrating the "envelope", instead attaching wood to the square-section tubes that run along the top edge. Then I can nail shelves etc to that and to the wooden floor. So I'm asking about those top rails, which appear to be made of 1/8" steel. Commented Mar 7, 2017 at 5:03
  • The nomemclature is confusing me. You mention "self tappers" but that's not actually a term that appears in my link above (which is by far the most detailed thing I've read about these screws, but still doesn't really answer my question). Sounds like you're talking about what they call "thread cutting" or "self drilling". Commented Mar 7, 2017 at 5:07
  • Seems to me like I need a "thread cutting" screw. They seem to have finer threads (1/4-20 is typical), whereas the "self drilling" type seems to have the much coarser threads of regular thread-forming screws. So I tend think the latter wouldn't provide enough threads in metal that's only 1/8" thick, yet the metal is too thick to be warped into a thread (which is what I figure "thread forming" must mean. Commented Mar 7, 2017 at 5:34
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    I would also suggest, if you don't plan to remove them fairly soon, is a thread lock compound also. No matter what size thread you go with, it will lock it to the container. Blue is the lowest strength, if you plan to someday remove them, use Red if you don't plan to possibly ever remove them.
    – Jeff Cates
    Commented Mar 7, 2017 at 21:49

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I'd consider 1/8" in the "thicker" category.

Thinner would be like steel ductwork, thick enough to receive & hold screws, but thin enough to use an ordinary sheet metal which just has a pointy threaded tip (looks similar to a wood screw).

I have recently drilled wood into steel on a few different projects which was about 1/8" thick and used screws like this: enter image description here

Not pushing any particular brand but this type did work well for my application. The key element of the screw is the tip which is similar to a drill bit. The flutes of that drilling tip have to be at least as long as the metal is deep.

A tip is to make sure your screwdriver bit fits the screw head really well. Since it takes some time for the screw to bite (compared to screwing into wood) you need a good solid connection to your driver. If the wood you're attaching is thick enough that's not likely to be a major problem, however.

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  • This looks a good bit like the "type F", which is what I ended up using, and it worked quite well. I did pre-drill though. Commented Feb 6, 2020 at 2:48
  • @RustyShackleford sometimes I've had luck without drilling and other times I've had to. I think if the materials being joined are likely to move or have some stress between them then snapping of the screw is more likely and drilling is needed. Drilling also means you can use the screw on thicker metal than it was meant to drill itself through. Commented Feb 6, 2020 at 13:06

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