Timber can be treated so that it ignites less readily and spreads flame more slowly. It cannot be made non combustible, and the language used to sell treatments frequently implies otherwise. What treatment changes is the material's reaction to fire classification, which governs where it may be used as a surface or lining.
The two routes
Pressure impregnation forces a retardant chemical into the cell structure in a vessel, giving a treatment that runs through the section and survives cutting and weathering better. Surface coatings, including intumescent varnishes and paints, are applied to the finished timber and work at the surface; they are easier to apply and are vulnerable to wear, cleaning, overcoating and any cut that exposes untreated wood.
What classification it can reach
Reaction to fire is classified by a European system that grades materials by their contribution to a fire. Untreated timber sits in the middle of that range. Properly treated timber can reach a higher class, which is what allows it to be used as a wall or ceiling lining in locations where untreated timber is not permitted. The class achieved depends on the species, the thickness, the treatment and the substrate behind it.
What treatment does not change
The timber is still combustible. It will still burn in a developed fire. Structural fire resistance of a timber member comes from its size and its charring rate, not from a retardant, and treating a joist does not give it a fire resistance period. Confusing reaction to fire with fire resistance is the most consequential error in this area.
Evidence and durability
Treated timber should arrive with documentation naming the treatment, the class achieved and the test evidence, and for external or humid applications a durability classification, because some retardants leach. Site cutting exposes untreated material in surface treated timber, so the specification has to say whether cut ends need re-treatment.
Questions people ask about fireproofing wood
Does fire retardant wood become non combustible?
No. It reaches a better reaction to fire class and remains a combustible material.
Can existing timber be treated in place?
Only with a surface coating, which is a different and generally lower performing route than impregnation, and the evidence has to cover application to that substrate.
Is treated timber acceptable for external walls?
That depends on the building, its height and the requirements that apply to external wall materials, which are considerably more restrictive than internal lining requirements.