Clarifying the Correction on Wood Movement

Some people were left confused by the correction to “The Anarchist’s Workbench” I posted yesterday. And I don’t blame you – the ideas of right and wrong, correct and incorrect were being juggled furiously in the entry.

So let’s start over.

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Construction lumber is usually sold in a wetter state than furniture wood. I’ve bought about four metric tons of it in my time, and it seems to come in about 14 percent to 18 percent moisture content on average. That means that as the wood dries to your shop’s equilibrium moisture content, the board is going to move a bit on you.

When wood loses moisture, the bark side of the board (the outside of the tree) tends to cup. The heart side of the board (the inside of the tree) tends to bow out. See my drawing above.

If you need to laminate the construction lumber face to face – such as gluing up four layers to make a big workbench leg – you should use the above fact to your advantage.

I glue up my legs so the heart side of one board faces the bark side of its neighbor. That way as the boards dry, they will all move in unison and keep the joints closed. Like this:

If you flip one of the boards, so the heart side faces the heart side of its neighbor, the edges of the lamination tend to open up as the boards dry. Like this:

If you glue the bark side to the bark side of its neighbor, the interior of the lamination tends to open up.

I’ve seen this happen. I’ve never seen the joint fall apart because of it, but it ain’t pretty to look at.

I hope you will forgive me for yesterday’s confusion. In the coming days I’ll correct the free pdf.

— Christopher Schwarz


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