As discussed in Build Green: Wood Can Last for Centuries, water is the main culprit in wood decay. Because of this, buildings should be designed to minimize wetting of wood or to maximize how quickly wood dries when wetted by rain.
As an example, cliff dwelling ruins of the ancient Anasazi people at Kiet Siel (“broken house” in Navajo) in northern Arizona date back to the 13th century. Dwellings in this semi-arid region are sheltered from occasional precipitation by cliffs overhead. Wood beams and superstructures have remained sound for centuries because they have always been too dry to decay.
Dry wood will last indefinitely. It may come as a surprise then that wood can also be too wet to decay. Just like all living organisms, fungi require oxygen to live. When wood is submerged in water, air is driven out of all the cells, and decay fungi cannot grow. As an example, the remains of 34 Byzantine ships dating from between the 7th and 11th centuries were uncovered below sea level in Istanbul, Turkey. The wood remained intact because there wasn’t enough oxygen to permit wood-decay fungi growth (see below). Wood too wet to decay is not likely to be an issue for the homeowner, but these historical examples illustrate the point that wood-decay fungi need the right mix of air, moisture, temperature, and food source material in order to thrive.