Outdoor cabinet wood movement

[Roger Feeley]

I’m building an outdoor cabinet with a stainless steel top for my daughter. While it will live outdoors, it will be protected from rain and snow unless there are really high winds. It’s about 24” deep and 5’ wide with two pairs if doors in front.

she wanted western red cedar for the cabin and I stupidly agreed. Never again. That stuff is soft and likes to chip.

To help handle wood movement, I’m not making solid panels. I’m using 3” tongue and groove slats with bits of foam in each groove. My plan is that expansion will be absorbed by all of the slats.

I’m concerned about extreme movement. I’m using a reference from workshoppages.com (http://www.workshoppages.com/WS/Articles/Wood-Movement-Charts.pdf). It gives a maximum moisture content of 9%. Is that high enough. Here in northern VA, we get some really humid weather. Put another way, how high can wood moisture content go without exposure to liquid water?

[Jamie Buxton]

Interior wood gets exposed to wide humidity swings mostly because of heating homes in the winter. Exterior wood usually sees a much smaller humidity swing. Here is a page which shows the average exterior atmospheric humidity for the DC area. https://weather-and-climate.com/average-monthly-Humidity-perc,falls-church-virginia-us,United-States-of-America . As you can see, the relative humidity is quite constant.

[Robert Engel]

Soaking wet construction lumber can be 20-25%.

The extremes of humidity probably aren't going to be more than 40% winter to summer. I would anticipate at most 1/2" of movement across 24".

You could also use battens on the back side with elongated screw holes. Rather than foam, you could look at types of roof caulking that are extremely flexible.

[Stewart Lang]

I built a planter for my shop out of purpleheart. Dovetailed the sides into the legs. 18" high sides. At most it probably has moved 1/2", but it's also exposed to rain, constant moisture from the soil, high humidity in the summer and low humidity in winter (FL). I wouldn't expect your 24" panel panels to move more than 3/8" being protected like they are.

[Jamie Buxton]

Here's a publication from the US Forest Products Lab -- https://www.esf.edu/wus/documents/EMCofWoodFPL268.pdf. Table 2 shows the month-by-month EMC of wood which is outside at various locations across the country. For Norfolk Virginia, the EMC ranges over the year from 11.5% in April to 13.4% in September. That is, the EMC swing is 1.9%. The hygroscopic expansion rates of western red cedar are very similar to redwood, which is shown in the workshoppages.com page cited in the original post. Entering the 1.9% EMC swing into that workshoppages says that a 12" flatsawn board will swing .05" -- that is, a tad less than a sixteenth inch. A quartersawn 12" board will swing .02" -- less than a thirtysecond.

[andy bessette]

You can't go wrong if you use boat building techniques for outdoor cabinets.

[Roger Feeley]

Thanks to all. I feel a bit better.