Why timber needs protection
Timber is a combustible material. In a fire, the main hazard it poses is not loss of strength, as with steel, but its contribution to the fire: it ignites, spreads flame over its surface and adds to the fire load. The primary aim of timber fire protection is therefore to reduce combustibility — to make the material ignite less readily, spread flame more slowly and give off less smoke.
The second aim is to increase the fire resistance of load-bearing timber structures. When solid timber burns, it forms a charred layer on the surface which in itself protects the inner layers, so large sections can carry load for quite a long time. For slender members, or where requirements are high, board lining or special coatings are used.
Biological protection is a separate matter: timber rafters, floors and frames suffer from rot, mould and insects. This is why fire- and bio-protective treatments, which do both jobs at once, are widely used on the CIS market.
Types of fire protection products for timber
Impregnations (fire retardants and fire- and bio-protective treatments)
Aqueous solutions of salt-based and other fire retardants that penetrate the surface layer of the timber. When heated, they decompose, releasing non-combustible gases and promoting the formation of a char layer. Impregnations are inexpensive, barely change the appearance of the timber and are widely used for roof structures, battens and frames. Their limitations are leaching in damp conditions and the need for re-treatment after the period stated by the manufacturer.
Clear fire protection varnishes and systems
Clear intumescent coatings for interiors where the texture and colour of the wood must be preserved: wall and ceiling finishes, timber elements in public buildings. They usually form a system consisting of a fire protection coat and a protective clear topcoat.
Opaque intumescent paints
Pigmented coatings that work on the same principle as paints for steel: when heated, they form an insulating char “blanket”. They are used where a painted surface is acceptable.
How performance is assessed
In CIS countries, the performance of fire protection products for timber is traditionally expressed as a fire protection efficiency group: Group I is the higher, Group II the lower. The group is determined by testing at a specified application rate, so the same product can achieve different groups depending on the quantity applied per square metre. European documents specify a reaction-to-fire class to EN 13501-1 instead. The design or the expert review states what result is required — and that is the starting point.
How to choose
- Required result. Fire protection efficiency group, fire hazard class of the material or fire resistance rating of the structure — as specified in the design.
- Where the timber will be in service. Inside a heated building, in an unheated roof space, under a canopy, outdoors. Many products are intended for interior use only; exterior use requires grades approved for it and a protective topcoat.
- Appearance. Concealed structures — impregnation; exposed interior timber — clear varnish; surfaces to be painted — opaque paint.
- Species and moisture content. Dense species and wet timber absorb impregnation less readily. The manufacturer specifies the moisture content required before treatment.
- Compatibility with finishes. If paint or varnish is to be applied over the impregnation, make sure the manufacturer permits this and that the finish will not impair the fire protection.
- Biological protection. For rafters and structures at risk of getting wet, it makes sense to choose a fire- and bio-protective treatment.
Common mistakes
- Treating wet, dirty or previously painted timber.
- Too little product applied — the declared group is not achieved.
- Using products “for interior use” outdoors or in places that regularly get wet.
- No re-treatment once the period stated by the manufacturer has expired.
- Treating only the visible faces, leaving out end grain and joints.
- No quality control — many impregnations are tinted so that the evenness of application can be checked visually.
Materials from leading manufacturers
- Pyroplast Wood T — Sherwin-Williams (formerly Sika) — clear water-based fire protection system for timber in interiors.
- Pyroplast Wood P — Sherwin-Williams (formerly Sika) — white opaque water-based coating for timber and wood-based panels in interiors.
Frequently asked questions
Can timber be made non-combustible?
No. Fire protection reduces combustibility and slows flame spread, but it does not turn wood into a non-combustible material.
How often does the treatment need to be repeated?
The manufacturer states how long the fire protection properties last, separately for interior and exterior conditions. The treatment is repeated when this period expires, and also after wetting or mechanical damage.
Can timber be painted after fire protection impregnation?
In most cases, yes — but only with products permitted by the impregnation manufacturer, and only once the impregnation has dried completely. Some coatings reduce the fire protection effect.
What is the difference between Group I and Group II?
Group I means higher fire protection efficiency as established by testing. Achieving it usually requires more product per unit area than Group II.