I'm cutting some shelf supports, and am curious which grain direction is optimal for holding a screw + load.
(Of course this is negligible in the given context, but I'd like to know for reference)
I'm a visual thinker so, here:
A:
B:
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My intuition is that A is better for screws, and B is better if it were wood glue. Ready to be wrong.– dprenMar 7, 2022 at 4:28
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This is probably more suited for DIY.SE, though in this case grain direction doesn't matter. That screw will pull out or snap before the grain of the wood is stressed to any degree. Splitting isn't a concern because you will be drilling pilot holes.– user5572Mar 7, 2022 at 13:23
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As covered in the Answer, technically support strips in this configuration aren't holding the screws, their shafts will instead pass through clearance holes (deliberately so that the thread won't engage in the wood, only with the plastic plug or other wall fastener).– GraphusMar 7, 2022 at 13:31
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3Those are wonderful visual aids! I love the paper stand-in for the wall and how you've even drawn the screw embedded in the wall. Fantastic! I wish all questions were this clearly illustrated. +100 if I could!– FreeManMar 7, 2022 at 15:38
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Couldn't agree more with @FreeMan. I didn't get the answer I was looking for down below but this question and the visuals is absolutely bang on. It's like you're inside my head. Well done!– Joshua PinterJul 2, 2022 at 15:32
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3 Answers
Vertical (quartersawn) grain is the strongest in terms of flexion loads, and consequently one would assume would be slightly stronger against compression loads. The wood is also less likely to split across a grain line vs along one, but I think that's dependant on species. An example here is guitar braces. They use straight grained spruce with vertical grain lines for the optimal strength/stiffness to weight ratio.
Slightly more importantly is wood shift. Wood moves the most parallel to a grain line as humidity changes. One would assume you will have enough compression in the (soft) wood from the screw to tolerate any changes in the wood, however in an application using bolts and hardwood with minimal compression, the change in size of the wood can cause an otherwise torqued-to-spec bolt to come loose. An anecdotal example: I have to tighten the wooden holds on my climbing wall in the winter, because the wood shrinks just enough that the holds will spin under the heavy load of dynamic climbing.
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Volfram K's answer is totally correct, but I chose this because it illustrates some situations were this would be a concern.– dprenJan 30 at 17:27
In this example, the wood grain direction is not your primary concern. In any situation, the load on the shelves will be too small to stress the supporting wood.
This is because the screws and the fixing in the wall are under the most strain, not the support piece (in this example, the piece of wood).
When the shelf is overloaded, the screws will bend or tear from the wall plug, there will be little damage to the supporting piece of wood. It is more important to pay attention to how the shelves are attached to the wall if they will hold much weight.
In other contexts, grain orientation is indeed important, and the wood will be stronger when the grain is parallel to the exerted force.
This is more a question of growth rings than grain. The grain runs lengthwise in the beam, so it is parallel to the wall in either case, as it should be. The growth rings affect the strength of the wood less than the main grain direction, but there is still some difference.
It is unlikely that the load would be high enough to split the wood. Usually wood constructions are limited by the amount of sag (bending due to force) rather than the break strength.
However, seasonal moisture variations may eventually split the wood along the growth ring. The screw direction A would hold the split wood together, while the screw direction B would pull the split wider.
Similarly if the screw is driven too close to the end in direction B, it splits the wood easier than in direction A.
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1If this is done right, the screw won't be "driven" into the wood. It will be clamped to the wall by the shoulder of the screw, and the shank will pass through a pilot hole larger than the threads of the screw.– user5572Mar 7, 2022 at 19:49
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Growth rings are still folded into the general subject of wood grain, since they are an aspect of the grain..... hence, end grain.– GraphusMar 7, 2022 at 20:09
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@jdv The image shows a screw with tapered countersunk head, they do always apply some amount of splitting force when tightened against the wood.– jpaMar 8, 2022 at 6:31
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@Graphus The end grain is at the end of the main grain direction. The side that is at the end of the growth ring direction is often called edge grain.– jpaMar 8, 2022 at 6:32
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1@jpa, I'm going to disagree that the shoulder forces are the same as internal splitting forces. The physics are obviously different and experience indicates the same thing. I've installed metres of cheap furring strips and pilot holes make the only difference that matters. The same will hold for this application. If the shoulder splits the wood here, then the wood used is only good for kindling.– user5572Mar 8, 2022 at 13:05