Green Fire Retardants for Structural Timber

Existing Retardants

Currently Fire Retardants can be classified into one of two main categories: impregnations or intumescent coatings.

The impregnation methods discussed here all have the ability to take structural timber from a Class 3/4 (10) to a Class 1 material (11).

Dry Interior Type
This type of impregnation is based on water-based solutions containing inorganic salts such as ammonium phosphate, ammonium sulphate and sodium borate (7). This type of treatment is designed for a relatively dry environment, as the salts have a tendency to absorb water vapour, giving rise to concerns for metal fixtures/fastenings which could rust and fail. The BRE noted that this type of application could result in a flexural strength decrease of up to 20%  in the section (12).

ATP Generic ( is available for purchase as a DI type impregnation, although interesting its quality as a flame retardant has not been independently verified (11).

Humidity-Resistant Type
This type of impregnation is more commonly used for structural elements which may be subject to high humidity but not fully exposed to the weather. They consist of aqueous solutions like the DI type but may contain mixed organic and inorganic salts. In fully exposed conditions leaching could result in the removal of salts rendering the retardant treatment ineffective.

Dricon (, a product recommended by the Wood Protection Association (11) is a commonly used HR type impregnation, as is FirePRO (
Leach Resistant Type
This type of retardant may be used in fully exposed conditions as the resins chemically react during the treatment process to become non-hygroscopic. This greatly reduces the effects of water and high humidity on the protected member.

Non-Com Exterior ( is a BS-certified LR impregnation.

Intumescent Paints
These substances are less common than impregnations partly due to the requirement for controlled technical application on site, the lack of which may completely undermine the retardant (7). It should be noted however that steel has been coated with intumescents off site (13) to allow more efficient quality control, and this could be an option for structural timber.

It is noted in Code Practice for Off-site Applied Thin Film Intumescent Coatings (13) that intumescent paints may be given a protective coat during the construction phase to prevent harmful weathering effects.

Intumescent paints are generally considered to be formed of four main parts:

  1. A catalyst (ie. Ammonium polyphosphate)
  2. A carbonific (starch) which will have a tendency to char
  3. A binding agent
  4. A “spumific” agent. This will evolve non-combustible gases such as water vapour or carbon dioxide which will be trapped beneath the expanded char

Although the exact formulations of the existing intumescent paints are trade secrets, they will all undergo roughly the same process.



Chuang et al showed the effectiveness of an intumescent paint on plywood, shown here (14).