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Today's Urban Design Observation: Falling Apart at the Seams

By Rain Noe - November 30, 2017

in Urbanism

It's a shame that everyone from name-brand architects to homeowners to neighborhood beautification organizations often miss this important fact: Wood, if not properly maintained, is actually a very poor materials choice for outdoor products and facades.

Outdoor-dwelling wooden objects need to have a protective coating that must regularly be re-applied. And the object must be designed with seasonal wood movement in mind. When these things are not heeded, this happens:

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These planters are on Lafayette Street. I remember when they first put them in, and I want to say they looked spiffy for about six months. But in the photo above you can see the vertical boards are pinned in on both sides by carriage bolts, meaning that when they expand and contract there is no place for them to go, and they crack.

Some of the finials have snapped off, allowing rainwater to infiltrate the joints.

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Someone on the neighborhood improvement committee has been desperately trying to save these by attaching bracket after bracket. This person also did not account for expansion/contraction and has probably contributed to the splitting over time.

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Evan Jones

6 years ago

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I’ve seen you mention planning for wood movement a bunch of times, Rain. I’d love to see a series (like the great ones you ran last year about wood species and sawmill cut patterns) about how to make wood projects stand up to temperature and humidity changes. I’d be grateful for the education

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- Fraser Pyke
  
  Evan Jones
  
  6 years ago
  
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  I second Evan.
  I work with plastic and composites but am always happy to learn. I would be interested to learn about the basic rules or techniques for accommodating wood movement.
  
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- Rain Noe
  
  Evan Jones
  
  6 years ago
  
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  Guys, I'm thrilled you asked; I didn't think anyone cared about that stuff. Will work something up, and will also keep an UDO eye open for more examples of wood use/misuse.
  
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Ken K

6 years ago

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Quickie tutorial until Rain gets a real article out: Think of wood as a bundle of straws (the lengthwise grain) held together. The straws won't change their length much at all — you can count on length being very stable. But the wood will expand and contract "sideways" in all directions with changes in humidity. The expansion/contraction differs quite a lot for different species, but figure about 1/8" per 6" of width or thickness as a very, very crude rule of thumb. There are online calculators for really accurate planning. Putting finish on the wood will not change the amount of expansion/contraction, although it will slow it down a bit.

In practice, this is why solid-wood drawer bottoms in older furniture always had their grain run side-to-side and the back of the drawer bottom was always loose and stuck out the back a bit. It's why table tops have weird movement-friendly attachments to their aprons, and why the "breadboard ends" of breadboards are attached only in the center. It's why door panels have their rails and stiles running lengthwise (stable length) around the center panel(s), and why the center panel is floating loose and "too small" in its grooves and is never glued all the way around.

And it's why furniture that combines metal and wood usually fails. It looks really good for the portfolio photo shoot, then fails after a few seasonal humidity changes. The rare exceptions are made by folks who understand wood expansion/contraction.

For a really good, readable, in-depth overview (is that contradictory?): http://www.thisiscarpentry.com/2010/09/03/moisture-content-wood-movement/

As a side note, since Evan mentioned it, temperature changes per se have almost zero effect on wood. That old cliche, "It's not the heat, it's the humidity" really applies to wood. What *does* have an important effect is UV exposure, which will change the color of virtually all woods. UV is why purpleheart and all the other gloriously naturally colored woods turn a dull brown after a while, even indoors, unless you hide them in the back of a dark closet. There are some allegedly UV-resistant finishes that don't really do the trick, and apparently some nano-particle transparent UV blocking additives are in the works, but I'm not up on the latest; maybe this has been solved since I last checked.

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