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I am looking at a full lap joint between two pieces of softwood, held together by two screws. I may need to screw something else to it as well, which got me wondering whether there is a limit to the density of screws in such a joint before adding more screws makes the joint weaker (because of removing material for the pilot holes, and the screws crushing the wood fibres, etc). Clearly filling every available square millimetre with screws is going to make the thing fall apart, but up to a point adding fastners will add strength.

Are there any guidelines for this e.g. in terms of screws should be x times the root diameter apart?

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    If in doubt, use glue and temporary clamps instead of some or all screws. Glue is stronger than screws and screws add no strength to a well made and well glued lap joint. – RedGrittyBrick May 30 '17 at 10:03
  • i cannot give you an answer, but i can suggest a pattern for drilling the holes - which is the 60° pattern iperf.org/perforating/knowledge-center/perf-handbook/… . Everybody seems to agree that the more wholes, the less resistant the material will become. I might suggest googling "strength of perforated material/plate" to find out the relation between resistance of material and the density of holes – Newton fan 01 May 30 '17 at 15:39
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Western Woods Products Association and West Coast Lumbermen’s Association have produced tables and charts based on “stress” for wood fasteners in shear and withdrawal in various species of wood.

The values are based on the diameter of the fastener and if it is in “stress” parallel to grain or perpendicular to grain.

So, for wood to wood connections (the spacing is closer for metal side plates) it is:

For PARALLEL to grain loading: 1) center to center along the grain is 4 Times it’s diameter, 2) row spacing is 3 Times across the grain (parallel), 3) edge distance is 1.5 Times , and 4) end distance (in tension) is 7 Times diameter and when in compression it can be 4 Times diameter...and when installed with less, it’s certain to split the wood.

For PERPENDICULAR to grain: 1) center to center across grain is 5 Times diameter for fasteners with a slenderness ratio of 2 and 6 Times diameter for fasteners with a slenderness ratios between 2 and 5, 2) row spacing is 4 Times diameter, and 3) edge distance is 4 Times for stressed side and is unimportant for opposite side.

There’s a formula for calculating STAGGERED spacing (and they can be closer together when staggered) but it’s too hard to calculate, so just know that they can be closer if staggered.

So, to answer your question, yes it does matter and there’s a way to determine the value if you’re Einstein.

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To know this answer with confidence, you will need to recreate the lap joint several times in test pieces to be used in destructive testing.

Experiment with different quantities of screws until you are satisfied with the quantity and strength.

Then repeat that test several times with the same positioning, and load each one until it fails to get an average failure load. Divide that by a safety factor you are comfortable with and use that as your max load.

Alternately, start by deciding what max load you want to design for. Multiply that by a safety factor you're comfortable with and test your lap joint replicas to that limit. Start with just two screws, then add screws, one by one; re-testing between each until failure.

Repeat the test multiple times with the number that fails to be certain.

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