There are two commonly available inexpensive materials that will kill rot in wood and prevent its recurrence. First, there are borates (borax-boric acid mixtures) which have an established record in preventing rot in new wood and in killing rot organisms and wood-destroying insects in infested wood.

Second, there is Glycol, most readily available as auto antifreeze-coolant. Glycol is toxic to the whole spectrum of organisms from staphylococcus bacteria to mammals.

Ethylene glycol and propylene glycol are clear liquids used in antifreeze and deicing solutions. Exposure to large amounts of Ethylene Glycol can damage the kidneys, heart, and nervous system. Propylene Glycol is generally regarded as safe for use in food. Antifreeze that is sold as "Earth Friendly", "Environmentally Safe", etc are generally based on Propylene...Make Sure You Buy The Propylene Glycol, NOT the Ethylene!!!
Source of Information : Agency for Toxic Substances and Disease Registry (ATSDR). 1997. Toxicological profile for ethylene glycol and propylene glycol. Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service.

Both borate solutions and glycol penetrate dry and wet wood well because they are water-soluble; in fact, penetration by glycol is especially helped by its extreme hygroscopicity -- its strong attraction for water. For both, the fact that they are water-soluble means they are not permanent solutions to rot in wood that is continually exposed to water-below the waterline and in ground-where they will eventually be extracted-dissolved out.
I have had good results by adding a water soluable "Sealer" to my solutions which make them more water resistant.

There are two types of borate products commercially available for treating wood, Tim-bor®).. A powder you mix with water and spay apply to all wood including studs, crawl spaces, subfloor, rafters and exterior sheathing. It provides a termite and rot resistant envelope treatment that can last for 30 years or more.

Treating an existing problem is best done with Bora-Care®).. It is a liquid concentrate that is mixed with water and sprayed on the affected wood and all of the wood surrounding that area.
A solution of Tim-bor can also be used if the wood has a moisture content of 20% or greater.

Their equivalents and more concentrated solutions can be easily prepared from borax, boric acid, and antifreeze at much lower cost.

Glycol by itself has one big advantage over solutions of borates in water. Glycol penetrates rapidly through all paint, varnish, and oil finishes (except epoxy and polyurethanes) without lifting or damaging those finishes in any way. You can treat all of the wood without removing any finish. Once bare wood has been treated with glycol or the borate solutions and become dry to the touch it can be finished or glued. If a borate solution leaves white residues on the surface, it will have to be washed off with water and the surface allowed to dry.

Glycol's toxicity to humans is low enough that it has to be deliberately ingested (about a half cup for a 150 lb. human); many millions of gallons are used annually with few precautions and without incident. It should not be left where children or pets can get at it, as smaller doses would harm them. The lethal dose of borates is smaller than of glycol, but the bitter taste makes accidental consumption less likely.

BORATE WOOD PRESERVATIVES:
COMMERCIAL AND HOME-BREWED

Bora-Care®: 40% solution of sodium octaborate in ethylene glycol; 27% borate content; $90/gal. for the concentrate.

Home-Brew Water Solution of Borates:
Based on U.S. Navy spec. of 60% borax-40% boric acid (this ratio gives the maximum solubility of borates in water);

#1. This is equiv. to Tim-Bor®... 6 parts of borax and 4 parts of boric acid.
To prepare one gallon of a 10% solution, start with an oversize container (larger than 1 gallon ) add 1 lb. of powder to appx 3 qts of water agitating until the powder has dissolved, then add additional water to end up with 1 gallon of mix. To prepare a 15% solution, add 1.5 lbs. of powder, then add the remainder of the water and mix as previously. Approximately 1 gallon of solution will be needed to treat 200 square feet of wood surface area. (Note: solutions should be used immediately and not stored.) .

EXAMPLE: Prepare 5 gallons of 10% solution:
Add four (4) gallons of clear, warm water to a six-gallon bucket.
Add five (5) lbs. of powder while gently stirring.
Add enough water to bring the final volume to 5 gallons, and continue to stir until all of the powder has dissolved.
Agitate the solution briefly at the beginning of each spray job, or after the solution has been standing for an extended period.
Do not spray or spill onto soil or foliage.

Apply two applications of a 10% solution to wood surfaces by brush or spray. Apply one application of a 15% solution to wood surfaces by brush or spray. Applications may be made to wood structures including decks, fences, steps, sheds, barns and other out-buildings.

#2: This is equivalent to Bora-Care®
Prepare the concentrate:
Mix 1 Gallon glycol antifreeze, 4 1/2 pounds borax, 3 1/2 pounds boric acid.
Mix the ingredients and heat till boiling gently. Boil off water until a candy thermometer shows 260°F. This removes most of the water of crystallization in the borax.
This solution is stable at 40°F and has a borate content of 26%. This is equivalent to Bora-Care® at about $90/gal. for the concentrate. The concentrate must be diluted with an equal volume of water before being applied.

Application: Add 1 gallon of water to every gallon of concentrate and stir thoroughly until solution is completely uniform. Always use diluted within 24 hours after mixing. If kept for longer periods of time, the active ingredient can drop out of the solution.

Note: is toxic to plants and shrubbery; if necessary, cover plants, root systems and surrounding soil with plastic to avoid contamination. Apply only to bare wood. Remove any finish or water repellent coating before applying . Wood surfaces should be free of dirt and other contaminates. Apply diluted by spray or brush to all exposed wood surfaces. It may occasionally be necessary to apply more than one coat of to attain the recommended application rate. This is especially true for larger, smooth surfaced wood members. Wood surfaces should be allowed to dry for at least 2 hours between applications. Do not apply in the rain or snow. If inclement weather is expected, protect exposed treated surface with a plastic tarp for at least 24 hours after treatment. One gallon of concentrate will treat up to 800 board feet of wood. Only diluted should be applied to any wood surface. Prior to application, check wood surfaces for an existing water repellent finish by spraying a small amount of water onto the surface of the wood or logs. If the water beads up or is not absorbed into the wood, a finish is present which must be removed before applying the diluted solution.