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I'm trying to understand some basic principles here so I can make better design decisions for the deck I'm building.

Suppose I want to fasten a 4x4 post to a joist/piece of blocking/beam/bottom side of stringer. Assume that the screw penetrates both members completely, and penetrates through the face grain of both members. Would it matter whether the screw goes from the post to the other member or vice versa? If so, why?

I have a feeling there may be a difference depending on the shank and the threaded portion of the screw, but not sure how to think about it correctly. My understanding is that the threaded part is what provides withdrawal resistance. But in both cases, the same amount of threaded material is in contact with wood.

If I had to guess at an answer, I'd say that screwing the member into the 4x4 is better: Leverage on the 4x4 will create a withdrawal force that tries to suck the screw out of the other member. If the screw goes through the member into the post, that means that the head of the screw would need to be pulled through the member in order to be sucked out. Whereas if the 4x4 was screwed into the other member, it would be easier to suck the screw out of the other member since it's only the shaft of the screw needs to be pulled out. Is this the right way to think about it?

I've illustrated this question below. Blue is the 4x4, red is the joist/blocking/beam/stringer, and green is the screw. On left, screw goes from joist/beam/etc, to 4x4 and on right, screw goes from 4x4 to joist/beam/etc. (in the case where the red member represents a stringer, the red surface shown is the end grain of the bottom step - i.e. surface to which the first riser would be attached to).

Would the answer to this question depend on whether the red member is a joist or a beam or a piece of blocking or a stringer?

enter image description here

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    Trick question. It has to be through bolts, so.... no? Only place screws go is to hold blasters or the decking itself; everything else is a triple galvanized nail or a SS bolt. ... iirc, a post needs two, minimum 5/8" through bolts. chicago.gov/dam/city/depts/bldgs/general/EZPERMIT/…
    – Mazura
    Commented Jan 15, 2023 at 4:24
  • well I'd be using GRK RSS, or perhaps strong drive SDWS timber screws. I should have clarified that I'd be using structural screws.
    – spacediver
    Commented Jan 15, 2023 at 4:30
  • Not in Chicago. W/e kind of screws they are, even if they are basically lag bolts, still aren't through bolts. Correction: lag screws are ok to secure a guard rail to a post, but not the post itself. - That being said.... Your pic doesn't show what happens in reality: you don't use a 6" screw to attach a 2x4 to a 4x4; it'll stick out. You use a three or a four, and then it's obvious which way you have to screw it in. Seriously, look at the link; everything is bolt, otherwise people die.
    – Mazura
    Commented Jan 15, 2023 at 4:41
  • Thanks, the document has some good diagrams, may look for some inspiration. I figured structural screws (with the appropriate blocking) would be ok given the information here: finehomebuilding.com/project-guides/decks/….
    – spacediver
    Commented Jan 15, 2023 at 4:46
  • We had porch collapses a few decades ago so we're super critical of it, but if you follow that admittedly overblown guide you can't go wrong. But notice even in the picture in your link; that post at the end has through bolts going both directions through both edge beam and ledger as well as the actual post from below. It's also like 3' off the ground so w/e.
    – Mazura
    Commented Jan 15, 2023 at 4:52

2 Answers 2

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Ideally, you want the unthreaded section of the screw to match the thickness of the piece the screw is being driven from, as in your left illustration. That way, the threads are all in the other piece, so the two can be drawn together as the screw is tightened.

In your right-hand illustration, you can see that if the two pieces start with a slight gap, tightening the screw will not draw them together; since there are threads in both pieces any looseness will be maintained. If you have to drive the screw in this direction, one workaround is to predrill a slightly larger hole in the blue piece, so the threads don't engage and the screw is pulling only on red.

In many cases, of course, this won't matter very much, especially if you clamp the two pieces together before driving the screw. But you get the best strength if the screw pulls the pieces tight against each other.

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  • Thanks very much. I remember hearing about how partially threaded screws allow for tighter fastening, and had forgotten to consider this here. On that note, would this tighter fastening still occur if there was a even a tiny portion of the piece-the-screw-is-being-driven-into that had threads? For example, in the left side of my illustration, if the red member was 1.5 inches thick, and the threads extended into the red member by 1/8 of an inch (i.e. the unthreaded part was 1 3/8") , would that completely negate the effect?
    – spacediver
    Commented Jan 15, 2023 at 4:11
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    Tiny portion.. Generally in that case there's enough force from the other threads that this bit rips out and the right thing happens. But that depends on the materials and the fasteners.
    – keshlam
    Commented Jan 15, 2023 at 14:41
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You always want the grabbing part of the screw or lag bolt to go into the wood member that has the greatest thickness. In your illustrations this is the scenario in the left hand image.

You want the hole through the thinner member to be a clearance hole so that there is no chance of the threads of the screw or lag bolt to bind up on the thinner member.

The hole into the thicker member wants to be a "pilot hole" that is ideally the minor diameter of the screw threads. This is less of a consideration if the screw has a self cutting tip on it but even these can benefit from a pilot hole that may even be a bit smaller diameter than the threads on the screw.

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  • That's interesting, thanks. So with a clearance hole through the thinner member, the only part of the thinner member that the screw is in contact with is the head, and that head produces a compressive force that sucks the two pieces together, if I'm understanding correctly..
    – spacediver
    Commented Jan 15, 2023 at 4:59
  • Well you may be way over thinking this whole thing.......but yes if the clearance hole was large with regard to the shank of the screw/lag bolt AND it was perfectly centered in the hole then what you say could be true. However normally you make the clearance hole a close fit to the screw/bolt shank and/or the major diameter of the threads so one side of the hole almost certainly fits against one side of the shank. This is even more true if the thin member being attached is loaded from above.
    – Michael Karas ♦
    Commented Jan 15, 2023 at 5:15
  • Thanks for clarification.
    – spacediver
    Commented Jan 15, 2023 at 5:28

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