The crumbling, dry edges of a board indicate that it's infested with a fungal infection commonly known as dry rot. Any wood exposed to moisture is at risk of infection by fungi. To help prevent dry rot in wood, paint it with a preservative, such as a borate solution.
Protect Yourself First
Before mixing any homemade wood preservative, put on shoes, long sleeves and pants, safety goggles and a dust mask. In addition, keep children and pets away from the mixing and application areas, and off the treated wood. Borax and boric acid can severely damage your eyes and lungs, and they cause serious skin irritation.
Homemade Borate Solution
Make a homemade borate preservative by mixing 1½ cups of powdered borax with 1 cup of boric acid. Add the borate mixture to 3 quarts of warm water in a 5-gallon bucket. Mix it thoroughly, until the powder completely dissolves, and then add warm water to make 1 gallon of a 10 percent solution. Then immediately paint the borate solution onto the bare wood, allow it to dry and paint a second coat over the first.
Often Mentioned but Not a Good Idea
While a homemade borate and propylene glycol solution is often mentioned as a wood preservative, the instructions call for boiling the mixture until it reaches 260 degrees Fahrenheit. While borax and boric acid are not flammable, the flash point of propylene glycol is 224.6 degrees Fahrenheit; it may catch fire or explode. The risk of heating propylene glycol is too great to recommend its use as a wood preservative.