On the other hand, if it is the lowest relative humidity season in your area (such as January or February in the Toronto area), you can assume your lumber is at its smallest year-round size. So, all 3⁄16" of annual wood movement will be expansion after the piece is completed. In that case, the panels only need to go a small distance into the dados to begin with and the dados need to be at least 3⁄32" deeper per side so that the panel can expand fully without bottoming out. Again, pinning the panel at its centre point is very useful or else the panel might slide one way, pulling out of the dado on the opposite side.
Whatever your calculations, it isn't a bad idea to overestimate anticipated wood movement if there is no structural or design reason that you can't. In other words, if you think the dados need to be 3⁄32" deep, why not make them 1/4" deep instead? It's far more than you'll ever need, but what is the downside to going deeper in this situation? What if this piece is put away in a storage facility for a year while you take a job in Australia and the non-humidity-controlled storage locker hits phenomenal humidity conditions in the summer? Or if you move from Regina to Vancouver?
In my workshop, I control the relative humidity all year to the approximate midpoint of annual relative humidity conditions in a typical home in my area of the province. So, whatever amount I calculate for annual wood movement, I assume half of the movement will be contraction and half expansion; then I design accordingly.
A shift in focus
You may find it difficult to get excited about wood movement. But, the first time you watch a hundred hours of your painstaking work get ruined by wood that warps, twists and cracks, you’ll develop an interest.
You don't need to be a scientist to work around wood movement, but you do have to understand the basics and think before you design. And once you start thinking in terms of seasonal movement, it will become second nature.
Read more in Workshop Essentials and Resources
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