Regardless of the care taken in building a structure, wood ages when exposed outdoors. The sun's ultraviolet rays begin to degrade the surface of unprotected wood within weeks of outdoor exposure. The sun's ultraviolet rays will change the color of the wood. Cedar and Redwood will tend to get lighter after initial installation as the colored extractives and lignin are removed. Light wood, such as pine and fir, tend to get darker.
Rain washes the degraded wood materials from the surface.
Erosion is more rapid in the less dense earlywood than in the latewood, which leads to an uneven surface. In many cases, the wood erosion will also provide an uneven and eventual darkening of the wood's surface as mildew develops. In some climates, such as along the seashore, wood tends to weather to a silvery gray. This change in color is followed by a loosening of wood fibers and gradual erosion of the wood surface. The sun's ultraviolet rays also provide problems for clear natural finishes.