The only sure way to avoid timber decay is to use good quality, well-seasoned timber, to ensure that the building design provides protection or ventilation of all timbers, and regularly inspect any vulnerable timbers and adjacent materials. It is vital that any timber that will get wet is able to dry out; rot-producing fungi will only attack timber that remains damp over a sustained period.
If the use of a preservative is thought to be absolutely necessary, then the best option is some sort of boron-based treatment. Another option could be heat treatment, which is being pioneered in Finland.
To read more about all these options, see our information sheet on Timber Treatment.