Vacuum-pressure impregnation is carried out in pressure vessels, so-called autoclaves, where dried wood material with a moisture content of less than 22 percent is placed in a sealed horizontal pressure vessel. The air is sucked out of the vessel and from the wood's cellular structure, after which the fire retardant is added under constant vacuum. The process then moves on to the solution being pressed into the wood under high pressure, whereafter the vessel is closed with an after-vacuum that removes any excess. The process is programmed after either absorption rates or a time frame that best fits the manufacturer's routines.

The extra amount of added fire retardant is registered into a computer that records how much is added and absorbed by the wood. The uptake usually varies around 150-350 litres per cubic meter of wood / plywood, but can sometimes be as high as 600 litres per cubic meter, depending on the type of wood. The fire impregnation gives the wood its own fireproofing properties that encompass all sides of the material as well as the cellular structure of the wood.

Fire impregnation comes in different makes and types and can vary in its fireproofing and durability properties. This is partly due to the fact that fire retardants sometimes contain raw materials with completely different properties; this means that the different types of agents may vary significantly in terms of resistance, smoke and fire properties, hydroscopicity, or corrosion. It is therefore important to choose the right kind for the intended purpose in order to ensure, for example, that all legal requirements are met; the fire class of the product does not necessarily mean that the product is suitable for use. There are other properties to check for when choosing a fire retardant, such as long-term durability in exterior or moist environments.