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I have a project where I will be screwing some 11" strips of 3/8th flat steel (this one right here https://www.lowes.com/pd/Steelworks-4-ft-x-2-in-Hot-Rolled-Weldable-Steel-Slotted-Metal-Flat-Bar/3024806) into a 4x4, like so enter image description here

The Image is a top view (looking down on it, the 4x4 will be stretched out horizontally, the red is the screws location.).

SIDE VIEW BELOW enter image description here

What kind of screws do I use? I would like them to lay flush against the steel flats too if possible. Heavy duty aswell that can handle strain of weight on top.

For instance this is essentially going to be acting like a bench. However it should be able to comfortably be able to hold 700lbs without the screws stripping out or breaking.

The force of the weight will be will be evenly distributed for the most part. the weight will be pushing downwards, and at times slanted as image below will depict. enter image description here enter image description here

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  • Welcome to WSE. Is your sketch a top view or a side elevation view? I am trying to understand how the 700# load is applied to the steel plates (across the 2" width or down on the 3/8 edge face). A little more information regarding how this assembly is used would be helpful in answering.
    – Ashlar ♦
    Commented Jun 9, 2019 at 19:21
  • its a top view looking down, the weight will be applied to the 2" width of the steel.
    – JoseDeCommas
    Commented Jun 9, 2019 at 19:53
  • From what direction will the load be applied? Will it be a static or dynamic load? High shock? Details about the application and orientation of the work piece are important.
    – LeeG
    Commented Jun 10, 2019 at 2:33
  • you right I added new images to help depict what the weight load will look like and as far as will it be static or dynamic it will be static for the most part, and not high shock, the most outside of static this will receive is maybe someone (adult size 200lbs) walking over it. the weight will be slowly added.
    – JoseDeCommas
    Commented Jun 10, 2019 at 4:49

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I would like them to lay flush against the steel flats

This will be the role of countersinks you make in the steel rather than determining what screw to use. Note however that if you decided to use coach screws (which are essentially a type of bolt) they can't be made flush because the head is too large and doesn't have a countersink.

What kind of screws do I use?

I'm not sure if you were hoping for a specific recommendation (screw X made by company Y) here or general guidance but apart from the first being off-topic for SE I don't think you need to overthink this as individual screws are very strong against shear forces, and their resistance to withdrawal even from softwoods can be nearly unbelievably high*. Paired or combined in higher numbers (see below) they distribute the force applied so each one is required to do less work anyway.

All you really need is general guidance which can be neatly summarised by: don't use skinny, short screws. Which you probably wouldn't have done anyway :-)

Use beefier screws, with coarser threads, that are as long as the job can allow and you should easily exceed the strength you need here. Do remember that the screws should go into pre-drilled pilot holes in the 4x4s.

As I read your diagrams it appears the steel strips will be inset into the surface of the 4x4s, if this is the case it will add hugely to the robustness of what you're building.

If in doubt, use more screws
I presume there's nothing forcing you to use only two screws per steel strip, by adding only one extra you reduce the load on each screw by a noticeable amount.

If still in doubt, use glue as well
There are a handful of adhesives you could use here, including some of the newer construction/marine adhesives (urethanes & silicones) and epoxy.

It may not be the strongest option any more but epoxy allows for easier reversibility than some other choices should you ever want to take this apart and epoxies are available everywhere. Some installation tips follow.

Heavily abrade the steel and if you're not insetting the steel into the wood as I surmised above freshly sand the surface of the 4x4 where the two overlap. Apply your epoxy generously, screw the steel into place, wipe up your squeeze-out and then just leave the thing to rest undisturbed for the glue to cure.

*Just a half-inch screw can take 100 pounds or more to tear free from softwood! For a 1" screw this about doubles, but with a 2" screw it can go up by a factor of five or six. Source: The Holding Power of Wood Screws, Bureau of Standards, 1926.

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  • Yeah thanks for your advice, and you are correct I will be insetting the steel strips into the 4x4. The screws I decided to go with (amazon.com/gp/product/B004Q0433U) also will be doing 4 per steel strips instead of the initial 2 per steel strip. Also quick question I know you said the longer the better so with that in mind, the screw I chose are 2 7/8" in length they sell a 3 1/2" one but I chose the 2 7/8" version since i figured the 3 1/2" one are too long and will protrude out the other end, am I correct in choosing the 2 7/8" version?
    – JoseDeCommas
    Commented Jun 10, 2019 at 11:24
  • Those screw heads will be hard to sink nicely flush with the top of the steel. (Unless you have access to a Bridgeport-type milling machine or the right sized endmill in a drill press.) That said, they aren't very high profile heads, so maybe you can just leave them proud. 2-7/8" is a good length.
    – Aloysius Defenestrate
    Commented Jun 10, 2019 at 12:18
  • If you want a flat top, go with something like these (amazon.com/dp/B000I651U0/?tag=stackoverfl08-20). You could then use a standard 82 degree countersink to make them flush.
    – LeeG
    Commented Jun 10, 2019 at 14:39
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    Yes the longer one could well have been just a bit too long to be safe to use and anyway a 2 7/8" screw will be more than sufficient as you're already exceeding the needed strength..... withdrawal force for just one old-school 2" screw can be nearly your entire expected load O_O Re. the screw type a flathead screw won't be fully flush of course but they sit pretty low so shouldn't be much of a snag hazard, however if that is a priority it would be better to use a countersink-head screw and countersink the drilled holes in the steel. Doing this can get the heads below flush if desired.
    – Graphus
    Commented Jun 10, 2019 at 20:06
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    @Graphus Thanks for all your helpful post, I have decided to go with the flatheads one I linked as they are low profile and it wont be a issue. I learned alot through this question and just wanted to say thanks to all who participated.
    – JoseDeCommas
    Commented Jun 11, 2019 at 6:18

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