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I have heard some people say that one doesn't have to drill a clearance hole if the screws are partially threaded when joining two boards. The argument being that the shank just won't grip into the first board at all and there won't be a risk of bridging.

But I have noticed on my projects that the shank is often much too short to clear the first board.

So my question is, if the shank doesn't fully clear the first board, and the upper thread is still gripping into the first board, do I need a clearance hole in the first board to ensure the two boards remain tightly together?

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  • Please edit your question to tell us what type of material, dimensions, and screw fasteners you are referring to. We can guess wood screws and real wood, but we ought to be sure before providing a best answer.
    – jdv
    Oct 21 '20 at 12:33
  • Also, your YouTube link isn't referenced in the text of your question. In general, it is best to make sure everything related to the question is present in the text, as ex.ernal links will be unreachable in an internet moment. Take the tour to see what I mean.
    – jdv
    Oct 21 '20 at 12:36
  • Previous related Q&A: woodworking.stackexchange.com/q/4640/5572 woodworking.stackexchange.com/q/5943/5572 woodworking.stackexchange.com/q/2708/5572 (An argument could be made that at least two of these address your question, making this one a duplicate.)
    – jdv
    Oct 21 '20 at 12:40
  • You've sort of answered your own question here, "But I have noticed on my projects that the shank is often much too short to clear the first board." If the threads can grip the board through which the screw passes, you need (or to be more accurate, should ideally use) a clearance hole.
    – Graphus
    Oct 22 '20 at 9:18
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Wood screws work best as clamping devices. That is, no part of the screw fastener should engage with the top piece, leaving only the head of the screw to clamp the top piece to the bottom piece.

This implies that, ideally, you drill two sizes of holes: pilot holes for the threads and relief holes for the shank.

It follows that for those cases where the threadless shank can't clear the top piece ideally the pilot hole is wide enough such that none of the threads engage except in the piece we are clamping to.

In practice what we often get is nearly none of the thread engaging in the top piece and nearly most of the threads engaging nicely into the bottom piece. This is usually just fine for rough construction (just back out the screw, tighten up the clutch, and drive it in harder) but will never give you the tight clamping you want in woodworking.

The glue-and-screw combination is one of the strongest, convenient, and forgiving joinery we've ever created, but only if you install the fasteners the way they were designed to be installed.

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  • +1 for mentioning screws as clamping devices (in another weird coincidence I read them referred to as such in something I read earlier in the week). But -1 for the last sentence. You don't want that at all, while you can certainly get away with it the idea is that none of the threading engages with the through piece; even a tiny amount of hold there is enough to cause bridging.
    – Graphus
    May 21 at 8:23
  • Agreed on the last sentence, but I'm just reflecting on what many do. I'll fix it to make that clear.
    – jdv
    May 21 at 13:06

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