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I'm trying to figure out when you should recess a lag bolt in lumber, vs. just using a lag screw? Does it depend on the type of wood being screwed into? Hardwood? Softwood? Plywood?

I know you can't just drive a lag screw into fiberboard, but standard lumber can hold a screw, so why are recessed bolts used sometimes?

  • You mean like a carriage bolt type head that is meant to be pulled into the wood to keep the head from spinning? – CrossRoads Jan 9 at 17:07
  • So that they don’t protrude above the surface. – Solar Mike Jan 9 at 17:08
  • @CrossRoadsm meaning they use a bolt with a nut, instead of a lag screw. – J. M. Becker Jan 9 at 17:33
  • In common vernacular, lag bolt and lag screw refer to the same thing (the big screw things with a hex head). Just "bolt" would be understood to take a nut. – JPhi1618 Jan 9 at 17:34
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Simply, a bolt is not relying on the holding strength of the threads in wood fiber. It is a metal-to-metal sandwich that is holding boards together from both sides. A bolt connection will normally be stronger than a lag screw, but that increased strength is often not needed, and access to both sides of the joint is not always possible.

If there is any question about the holding strength of the wood, a bolt is a better choice. If the wood isn't very dense, or if it could get wet and soft (like an outdoor swingset), or if you need the maximum amount of strength then choose a bolt.

If the wood is solid and you don't have access to both sides, a lag screw is a proven solution.

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  • “Metal- to -metal sandwich”? I don’t understand. Isn’t this about bolting wood together? – Lee Sam Jan 9 at 18:20
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    @LeeSam, I'm talking about the "sandwich" of the metal bolt head and the metal nut on the other side. This is in contrast with a screw that has metal on one side (the head) and just the holding strength of the threads in the wood on the other side. – JPhi1618 Jan 9 at 18:22

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