Dealing with rot in wood
will rot spread? Like say you take away the moisture that was cuasing the rot, but one rotten board is still there, will it effect others?
Fungi and bacteria cause the physical disintegration of wood, what we call "rot". The microscopic critters are literally eating the wood....as do termites, but these are smaller, perhaps only the size of a single wood fiber.
Fungi need four things to flourish and prosper: Air, Moisture, Warmth and Food.
Take away any one or more of those and rot is slowed or stopped.
Air...there will be always a very little amount of air, with its oxygen, diffusing through anything. Removing air to the greatest extent possible will slow things, but you can't seal the wood inside a glass or metal shell in a vacuum, so there will always be a little air.
Warmth...this is difficult to do anything about, as it depends on external weather and sun. Near freezing, we don't have much rot, and in very hot dryer climates we don't see much rot. There's not a lot of rot in Phoenix, Arizona or Taos, New Mexico.
Moisture...here we can do something. Providing ventilation helps the excess moisture in the wood to evaporate into the air. Dryer wood is less hospitable to fungal growth. Two pieces of wood in contact...here the fungi easily spread from one piece to the other, as there is little ventilation between those two pieces and the little critters can crawl or grow from one right onto and into the other.
Think of fungi as animals without skins, so there is no external membrane to keep the moisture inside their bodies. That's why they need a very humid atmosphere..about 85% relative humidity or above, and that's what you find inside a damp piece of wood. The wood is made of cellulose, that has the ability to "hold" chemically, enough water to provide the 15-40% moisture content inside the wood that makes fungi happy, and then they rapidly grow, eat the wood, and leave their spores, or eggs, everywhere.
When you dry out the wood, or, even better, dry it and take away its ability to hold that excess moisture, then the wood can't get damp enough to support fungal growth, and that's why the penetrating epoxy products work to restore the wood and keep the rot from starting up again. That kind of epoxy glues onto parts of the cellulose that would ordinarily hold the excess water, so the wood can't take up as much water and so it's a dryer environment and fungi and bacteria don't like dry wood.
Part of this kind of restoration process is stopping the source of water intrusion with good waterproofing design, proper drainage and flashing, and so forth. Excess water got in there to make an environment where rot could start, so there was something physically wrong in the first place. You need to find that and fix it.
Food...here also we can do something. There are poisons that can be put in the wood, that last for some long or short time, and kill some or most of the little critters that want to eat it. Pressure-treated wood nowadays has some copper compounds, which unfortunately corrode steel fasteners. That was the Law of Unintended Consequences, when CCA was taken off the market for use in Pressure-Treated Wood. There are borate compounds such as Tim-Bor (di-sodium octaborate), that are good, at fairly high levels, but are water-soluble and can leach away. Again, you need to protect the area from water intrusion. Besides the pesticides/preservatives/poisons, there are the epoxy products that say they just make the wood taste bad, so the little critters don't eat it. That seems to work; I was talking to a mold-abatement guy in Florida a few days ago who said he uses it and has never seen mold growing on it; maybe right up to it, but never on it.
So, if you take away their food one way or another, basically by telling them they should not or may not eat the wood, then you have successfully defended your wood against what are technically called Wood-Destroying Organisms.
Now, the spores of these things are in the air, everywhere, and any insect or animal carries them on their feet, and rainwater plucks them from the air and carries them everywhere. Paint and primers and restoration of the wood beforehand, all these things can help wood to last longer, but there is absolutely no substitute for keeping the water from getting inside the structure in the first place, and stopping it from getting in when you find it is happening.