Wood rot seems to be a growing epidemic in homes. Some of the rot can be traced to poor maintenance by homeowners, some to poor building practices by the trades and some to the lumber companies.
The dry facts
If you want to prevent wood from rotting, you need to keep it dry. Wood rot is caused by various fungi that are simply consuming the wood. The fungi need water to live. You keep wood dry by keeping it painted and sealed. It's a great idea to keep it above horizontal masonry or concrete surfaces by at least 2 inches.
Water, water, nowhere
Before you attempt to repair wood rot, you need to stop the water problem. It's imperative you hunt down and eliminate the water source that's causing the rot. If the wood has been placed too close to the soil, a sidewalk, a patio, a roof or somesuch, you may have to re-engineer the situation so the wood doesn't get wet -- or, if it does, it dries rapidly.
The air apparent
Once you have the water situation solved, the wood needs to dry completely. This can take days, weeks or months, depending on the season. Blowing air across the wood will accelerate the process. Be very careful about using a heat gun or other artificial heat source to dry the wood. Wood that's rotted and dry ignites very readily and can burn fiercely.
Some people have had luck revitalizing rotted wood with chemical products that soak into the dry wood fibers. These viscous liquids adhere readily to the rotted wood. Drilling holes into the rotted wood can enhance deep penetration of the liquids.
These liquids, once dry, add considerable strength to the rotted wood fibers. Any gaps, holes or voids can be filled with paste epoxies that adhere very well to the wood. The dry epoxy can be sanded and painted. You don't have to be a master carpenter to work with these DIY-friendly epoxy products.
As with any repair product, you have to read all the instructions on the product label. What you'll discover with most, or all, of the chemical and epoxy repair products is that the wood needs to be dry. You want this anyway, so the good wood doesn't rot further.
Preventing wood rot is not as hard as one might think, but it requires a mixture of common sense and best practices. Buy lumber that has built-in rot resistance, if possible. Redwood and cedar are exterior wood species with natural chemicals that stave off wood rot except in the worst conditions.
Beware of hybridized lumber that's now grown by lumber companies. This lumber has vast quantities of spring wood in it. Spring wood is the lighter-colored band of wood when you look at the end of a piece of lumber. It's softer and readily absorbs water.
Look who's caulking
Keeping wood painted and sealed is the most basic form of maintenance you can do. It's not the silver bullet, but it can help. Caulk cracks that allow water to penetrate into wood crevices.
If you want to give untreated lumber a chance to fight fungi, you also can treat it with borate chemicals. These borate powders readily dissolve in water. If you then soak the dry lumber with the borate solution so it seeps into the wood fibers, the borate chemicals stay behind after the water evaporates.
Fungi dislike the borate chemicals and will not eat the wood. The only problem is that the borates are water soluble. This means you can't leave the wood unprotected from repeated wet/dry episodes. If that happens, eventually the borate chemicals are leached from the wood, and the fungi move in and begin to feast.
After the wood is treated with borate and you let it dry, you can paint, stain or seal the lumber successfully.