As we have discussed, water is a B**** in capital letters and Nature will always win. Decomposition is natural, beneficial, and absolutely essential to the cycle of life- but it is rarely a pretty process. Plants, fungi, bacteria, and a host of other organisms combine to break down and digest natural material, and while those natural materials are alive their cells are constantly making repairs. When death stops the repairs, decomposition thriftily recycles everything. Remember the Laws of Thermodynamics? Nature is a closed system and entropy is always increasing.
On a macro scale, that thrifty recycling looks like slime, muck, and… rot. This is why Egyptian and Andean mummies can remain intact for thousands of years, and poor Yorick lasted “some eight year or nine year” in the wet soil of Denmark. Water is essential to the process of decomposition- for multiple reasons.
Reason #1: Water makes things mushy.
Reason #2: Water expands when it freezes, cracking harder materials apart.
Reason #3: Water fuels the growth of mosses, lichens, fungi, and other plantlets whose roots sink deep into materials and help break them up. (This is how dirt gets made)
Reason #4: Water is attractive to more mobile life forms like termites, beetles, ants, bees, etc etc
Rot is a particular kind of decomposition and one that always requires water. Rot most often affects things that can become saturated with water, excluding more impervious materials such as stone. The most common permeable building material is wood.
Above is the most common kind of rot found in wooden buildings- plain old wet rot. Wet conditions encourage the growth of microorganisms, which munch away at the structure of the wood. Wood is primarily composed of cellulose, a tough carbon-based polysaccharide that when dry is very, very hard. (This is the theory behind bioplastic) When it’s wet and mushy, it is both easier to break down and to digest.
This kind of rot is most commonly found in perennially wet areas of a building such as gutters, roof crickets, and column bases. The biogrowth, if advanced, can present as a green-black slime and is made up of a mixture of microbial flora, fauna, and some types of fungi like molds. Nasty, icky, and wet- these require a lot of water.
Often builders attempt to’design out’ these problem areas using sacrificial and less pervious materials like metal flashing. Another common solution is… paint! See Preservation 101: Paint for why. If you keep your buildings painted, the paint either coats the surface (like latex paint or limewash) to keep water off the wood or itself saturates the wood so there is no room for water to intrude (oil paint).
Above is dry rot, a highly misleading name for a type of fungal colonization in wood. The fungus species that cause dry rot, like the microorganisms mentioned above, like to eat cellulose. Remember, polysaccharide basically translates as “many-sugars”. You like sugar, fungi like sugar, everybody likes sugar. Dry rot fungi thoroughly eat the cellulose and leave only the fragile lignin structures, which crumble into powder under pressure.
Why is it called dry rot? Because the fungi bodies are very very good at absorbing water through the air, from ambient humidity. There need not be standing water in a basement or a leak in an attic roof- there just needs to be a persistently damp climate.
SEVERE termite damage
Closeup of termite borings
A type of water-related damage similar to rot is insect damage- water attracts them, but it isn’t a true rot. However, the principle is the same. Termites eat cellulose, and they are delighted when it’s wet and mushy because it’s easier to digest. Termite borings are easy to spot because they are long and tubular like the termites themselves. Termites excrete a sticky fluid (sticky requires water, hence why termites like wet wood) that they use to build nest structures. Sometimes the borings are not obvious, especially if they are not well advanced, but the sticky deposits usually are visible on the outside of the timbers. Termites do the same thing that the microorganisms and fungi mentioned above do- eat cellulose- but on a bigger scale.
So again, please, for the love of the building gods*, keep your wooden structures dry, well-flashed, roofed, and painted!
*side note- can anyone explain to me why the patron saint of architects is St. Thomas the Apostle? I feel like “Doubting Thomas” is a poor role model, I’d rather not be in doubt that the building will stand!