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Are there any guidelines or rules of thumb for how many screws I should use when screwing two pieces of wood together?

For example if I have a 2cm x 10cm plank I want to screw onto a 10cm x 10cm post. Then how many screws should I use and is there an optimal pattern the screws should be placed in?

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3 Answers 3

up vote 5 down vote accepted

I don't know if there's something for screws in wood, as wood's a rather strange material.

For bolts into steel, however, there's the AISC Manual (no prices listed, but expect it to be in the $300+ range); there used to be a separate book on joints, so you could calculate the strength based on the size of the fastener, bolt pattern, etc. There was also a section for calculating the strength of different weld patterns.

Now, the rules for the optimal strengthy are going to be similar -- further apart will support a greater moment, so when the contact patch is a square, you want the first two to be in opposing corners. If using four, fill all of the corners. However, because it's wood, you have a chance of spliting, so you don't want to go too close to the edges. (if you have to; pre-drill).

The other thing to remember is that with screws and bolts, the fasteners shouldn't be taking the full load -- they're pulling the structural material together, so that the load's transfered as friction ... this means if you see a gap between the two pieces, you need more fasteners. If you have a really large contact patch, drop another screw in the middle.

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It's going to really depend on the load the joint is expected to carry.

For your example two screws should be enough if there's no or only a light load, but if the plank was expected to bear a significant weight then a third screw might be advisable. You'd also need to use longer and thicker screws too.

As for a pattern, again it depends on the usage. Screws in line tend to look neater, but you should should probably screw across the diagonal or use a shallow "V" shape for increased stability.

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If there's considerable load, metal tie plates are available to do the job and spread the screw pattern in a manner that doesn't split the wood. Consult for example, the Simpson Strong-tie catalog for ideas. http://www.strongtie.com/

This will give you an idea of the joint configuration and what's necessary to keep it together under earthquake and strong wind loading.

If you expect gravity to hold it together, often a lot fewer screws are used.

The American Wood Council is another resource for timber construction recommendations http://www.awc.org/

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