Borax for Wood Preservation

Hi everyone,

I'm making a new yurt and was thinking of using borax to protect the wood from mould, rot, insects etc.

There seems to be mixed opinion about the efficacy of borax in wood treatment, with people trying to sell borax or deterring customers biasing the perspective!

In general, my brief research has led me to believe that borax is a pretty effective, long lasting wood treatment if given an immersion treatment before construction, providing the wood is protected from prolonged immersion in water (which washes out the borax).

There was also a claim on a permies post: 'Best oil to treat lumber with' from Bryant Redhawk that Boric acid breaks down the lignin in the wood, thus reducing its strength. I don't have an opinion on that yet since I can't find much evidence pointing in either direction, so would love to understand more.

Would anyone be able to share their knowledge / experience with using Borax on wood?

My plan would be to soak wood in 2 cups of borax : 12L water, as recommended by Bryant.

Thanks all!

Charlie :)

I can't comment on the big question, but I have used it in a limited way.

I had an antique wood cabin and  carpenter ants were determined to set up shop in the corners. I saturated the joints with dissolved borax multiple times. The ants left it alone after that.

I did some experiments with borax/boric acid treatment as a fire retardant, for which it does indeed work quite effectively. After the big fires in 2020, I sprayed down my covered wooden deck with it. I am not sure about the preservative effect, but I will say that it seems unlikely that a surface treatment would cause any problems with the structural soundness of the wood. I doubt it soaked in very deep, and from what I can tell, the wood is no different now than prior to treatment. My solution was nearly saturated with borax, I want to say I did 9 oz borax and 4.5oz of boric acid per gallon.

I followed a recipe for a DIY version of "Timbor" for a firewood rack. Essentially 65% water, 20% Borax, and 15% boric acid. It is less than a year old so I can't say if it's working for sure.

The real struggle was slowly dissolving the borax and boric acid in the water, which had to be heated (I used an outdoor burner.) Poured it in a pump sprayer and the sides were crystallizing badly by the time I was done. So I'm not sure how much stayed in suspension vs how much just crystallized and stuck to the side walls of the tank.

Here's where I got my info: https://sawmillcreek.org/showthread.php?233066-Home-made-bora-care-amp-timbor

I feel like it was good enough for the wood rack, but when I do a more structural project (build a bridge in the woods), I'll probably just buy Timbor to apply. That way I know it's right and will hopefully guarantee along life to the wood.

Hi everyone,

Thanks so much for your feedback, your own experiences, it's always helpful.

I've done quite a bit more research into this and am amazed how much debate there seems to be on forums and things when there's actually been really good studies into borax on wood - you just have to find them!

The results that I can see is that borax is a pretty effective wood preservative and fire-retardant as long as it is not constantly exposed to the leaching of water.

Based on my findings it seems that borates either have no effect, or actually improve the structural strength of wood. Good protection from insects and fungi, although perhaps not moulds. Low toxicity to humans. A slight drawback that it can blunt blades a little faster than untreated wood!

Firstly wood strength! :

This study experimented on the effect of borax on bamboo, and I assume the results could also be applied to wood:

https://www.researchgate.net/publication/347109744_The_Effect_of_Borax_Solution_as_Preservative_to_the_Mechanical_Properties_of_Bamboo

This excellent study from 2020 concluded that:

"This  study indicated  that  30% to  50%  borax in  the preservative  solution is  sufficient  to provide  significant
increase in strength for  compressive strength,  tensile strength, and  bending strength  of bamboo  specimen."

(Great study - I'm just slightly puzzled about how one could achieve a 50% borax solution in cold conditions, am I misunderstanding something?!)


More research on borates (including boric acid) for timber preservation from Australia in 2004, contains lots of findings from many other studies, encompassing a lot of helpful information:

https://www.researchgate.net/publication/328600265_Treatment_Methods_for_the_Protection_of_Hardwood_Sapwood_from_Lyctine_Borers

"Boron has a number of advantages as a wood preservative... Borates are relatively inexpensive, colourless, non-flammable, and can be applied by a variety of methods, ranging from cheap dip tanks to sophisticated VPI plants...

Boron also has sound health, safety and environmental credentials (not withstanding the directive from
Sweden mentioned above), as it has low toxicity to mammals, and low environmental impact
(Currie, 1997). "

"Borates are generally effective against sapstain fungi. However, at the levels normally used
commercially, borates do not protect timber from surface moulds. " (Amburgey, 1990)

"Boron treatment has no significant effect on the strength of timber" (Anon., 1994).

"Boron treated timber will blunt saw blades more quickly than untreated timber" (Davis and Norton, 1995).

"Provided the treated timber is dried to a moisture content below 18%, borate treatment should have no adverse effect on painting" (Anon.., 1994).  

"Borate treated wood can affect phenol formaldehyde glues, but most other glues seem to be
compatible." (K. Lyngcoln, pers. comm., 1996)

Cold water soaking:

"It usually involves soaking in 3 to 4.5% borax for 12-14 days (Johnstone and Humphreys, 1972). The length of time needed to treat timber by soaking depends on timber species and thickness. As a general rule for 4.5% borax solutions at 18C, timber of 2.54 cm thickness requires seven days soaking, while timber of
3.81 cm thickness requires 14 days soaking" (Davis and Norton, 1995).


There's loads more information in both of those studies quoted about the preservation powers of borax. As long as the wood is not being exposed to water constantly, I'm very encouraged that borax is a sound, environmentally friendly method of treating wood.