What will winter do to your wood?

A comprehensive guide to calculating seasonal wood movement

By Hendrik Varju

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Movement in length, which is known as longitudinal movement, is considered zero for furniture design purposes. While it isn’t actually zero, longitudinal movement is incredibly small for the board lengths commonly used in furniture.

Why does all this matter? It can mean the difference between heirloom quality-furniture and a project that will fail in the coming months. There are four simple mistakes beginners make:

Error #1:

If you edge-glue a 2″-thick quartersawn board to a 2″-thick flatsawn board when making a solid-wood panel, such as a tabletop, the quartersawn board will move more in thickness than the flatsawn one. While glue is flexible enough that the joint won’t fail, a step will develop over time at the joint line, making it look like something is wrong.

 

Error #2

If you join an 8″-wide flatsawn rail to a vertical post, you have something called unacceptable cross-grain construction. The rail needs to expand and contract across its width but is joined to the post where it has zero longitudinal movement. Failure occurs when the rail wants to move but the post restrains it. The rail will crack during a period of low relative humidity when it tries to contract.

 

 

 

 

Error #3

If you attach a solid-wood tabletop to a table base so that the top can’t expand and contract freely as relative humidity changes, the table will fail. The tabletop can expand enough in high humidity to blow apart the leg-to-apron joinery, or contract sufficiently in low humidity to crack down the middle. This principle also applies to frame-and-panel doors; a solid-wood panel must be allowed to float freely within the frame. Do not add glue to the joints.

 

Error #4

If you underestimate the amount an inset drawer front might expand vertically in high humidity, you may find that the drawer wedges itself into the case and can’t be opened easily. I think we all have at least one piece of furniture like that at home.

 

 

 

 

Illustrations by Stephen Hutchings



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