We know the power of the elements is astounding, but we may not practically realize just how destructive they can be. The sun, rain, high temperatures, low temperatures, and humidity all play a role in gradually wearing a given material down. Obviously, by keeping an object indoors, you can eliminate the rain and greatly reduce the damaging effects of the others as well.
This is why a structure or DIY project of any kind must be treated accordingly if they are to be permanently exposed outdoors. Wood in particular, depending on type, can be very weak to repeated attacks by the sun and rain. But even before it begins actually rotting and decomposing, it will change shape as it absorbs and desorbs moisture – often considerably.
This change-shape occurs due to a localized expansion or contraction of the wood, manifesting itself in various forms of wood-warping. It follows that if the entire piece would expand or contract in a uniform fashion, the board would only change in dimensions but wouldn’t skew or warp. However, this particular scenario will never play out in the real world with boards of any substantial volume or length.
But depending on wood type and the direction of grain, the level of warping can be considerably different. Certain types of wood are simply more resistant to moisture and insects, and quarter-sawn boards – due to the direction of the grain – are more resistant to warping. You put these two qualities together and you get yourself some fairly high-grade boards with which to build outdoors.
An old Table Exhibiting some pretty Major Shape-change
Here’s a first-hand example of what can happen to a piece of furniture that is left out in the sun and rain with absolutely no regard to its well-being. Granted, the table top isn’t a solid piece of wood but a composite of smaller wooden segments, making it significantly weaker to the elements. However, although its time has clearly passed, I’m sure there was once a day when this table was a stunning piece of furniture in its own right.
On dry, sunny days, the table top curls up into the shape of a smile when viewed cross-sectionally. Why? Because the direct sunlight literally sucks the moisture out of the top surface of the table top causing it to contract. This contraction is what pulls the wood into the nice – but impractical – shape of a smile. The opposite occurs on a rainy day. The same top surface will expand, causing it to return to its original flat shape.