Preservative treatment of structural timber is one of the few operations that, once the roof slope is closed, is practically impossible to repeat. Nobody will reach the top and side faces of the rafters after that, and it is precisely in the carpentry joints — where the beam meets the wall plate, the collar beam and the purlin — that moisture collects. The preservative goes on once, and it has to be done properly.
We have been supplying structural timber from our sawmill in Studzionka near Pszczyna since 1990, and the question that comes up most often is: “is this C24 already protected?” It is not. Below we explain why, what preservative treatment really protects against, which method to choose, and at what point in the build to carry it out so that it makes sense.
What preservative treatment of structural timber protects against
Three threats. Each one works differently.
Wood-decay fungi. The most dangerous is dry rot (Serpula lacrymans). It causes brown rot: it consumes cellulose and hemicelluloses, leaving brown lignin behind, so the beam breaks up into the characteristic cubes. It can destroy more than half the mass of the timber within a few months. That is no longer a defect, it is the risk of structural collapse. To develop it needs a timber moisture content above 20%, a temperature between 18 and 27°C and access to air — exactly the conditions found in a poorly sealed attic in summer.
Wood-boring insects. The house longhorn beetle (Hylotrupes bajulus) attacks above all the sapwood of softwood in new buildings and buildings up to fifteen or twenty years old. It usually leaves timber more than a hundred years old alone. That is bad news for a newly erected roof structure. The larvae bore oval galleries, and the adult beetles leave oval flight holes 6–10 mm across. The flight season runs from June to August, with an optimum around 30°C — precisely the period when the roof structure most often stands open on site. The common furniture beetle or woodworm (Anobium punctatum) leaves tunnels 1–2 mm across and can breed in timber for many generations.
Fire. Solid timber planed on all four sides is classified as D-s2,d0 to PN-EN 13501-1. It is combustible. That is a separate subject, which we return to below.
There is also a strength dimension that is rarely mentioned. As moisture content rises towards the fibre saturation point, around 30%, timber loses about 40% of its bending strength and roughly half of its compressive strength. Above that threshold the values no longer fall, but the timber enters the range in which fungi develop. Protection against moisture is therefore not only a matter of durability but of load-bearing capacity.
Is C24 timber treated at the factory?
No. And this is the most common misconception in the whole subject.
C24 is a strength class defined by PN-EN 338 and confirmed by strength grading to PN-EN 14081-1. It describes load-bearing capacity: bending 24 MPa, modulus of elasticity 11 GPa, mean density 420 kg/m³. It says nothing about protection against fungi, insects or fire.
Kiln-dried timber is sometimes presented as “not requiring treatment”, because it is dried at temperatures of up to 75°C, which kills the organisms present in the material. That is only half true. Kiln drying is a one-off sterilisation: it eliminates whatever is already in the timber. It confers no resistance for the future. It gives no protection against reinfestation by the house longhorn beetle during construction, against wetting during wet trades, against condensation under the roof covering, or against fire.
Nor does the natural durability of the species used for Polish roof structures help — pine and spruce, described in more detail in the article on timber for roof structures. To PN-EN 350 the heartwood of Scots pine has durability class DC 3–4 (moderately durable to slightly durable), spruce heartwood DC 4 (slightly durable), while the sapwood of every species is classified as DC 5, that is, not durable. That is the technical reason why a pine or spruce roof structure needs protection.
The practical conclusion: preservative treatment is a separate service and a separate line in the order. If it is to be carried out, it has to be stated explicitly.
Use classes — the starting point for choosing a preservative
Before choosing a product, establish the conditions the timber will work in. That is what this standard is for: PN-EN 335.
| Class | Conditions | Examples |
|---|---|---|
| 1 | Interior, dry, moisture content ≤20% | furniture, floors, interior elements |
| 2 | Covered, no precipitation, occasional wetting | roof structure in a closed roof, timber-frame construction |
| 3.1 | Exterior, above ground, limited wetting | eaves, projecting rafter ends, cladding boards |
| 3.2 | Exterior, prolonged wetting, standing water | balustrades, pergolas, decking elements |
| 4 | Contact with ground or fresh water | posts set in the ground, sole plates |
| 5 | Permanent immersion in salt water | marine piling and structures |
A roof structure in a closed, covered roof falls into use class 2: the timber is covered, but it can periodically exceed 20% moisture content — from condensation under the roof covering, construction moisture from walls and plaster, leaks, or flooding during the build.
Here is the catch that most guides miss: elements projecting beyond the outline of the roof already work in use class 3.1. Rafter ends at the eaves, wall plates on a projecting slope, canopy and arcade structures. A product declared for use class 1–2 will be washed out there within a few seasons. When buying a preservative, check the technical data sheet for the use class the manufacturer declares it effective for.
Types of preservatives for structural timber
| Type | Advantages | Disadvantages | Where |
|---|---|---|---|
| Salt-based (water-borne), borates and copper compounds | good penetration, inexpensive, odourless, 10–15 years of protection | raise the moisture content of the timber during application, partly leachable | enclosed roof structures (use class 2), factory application |
| Solvent-borne | deeper penetration, resistant to leaching, do not raise moisture content | flammable, strong odour, require ventilation and personal protective equipment | eaves, use class 3.1 elements |
| Oil-based | water repellency, reduce cracking | surface action only, 5–8 years of service life | exposed elements, roof structures over terraces |
| Fire-retardant | raise the reaction-to-fire class to B-s1,d0 or B-s2,d0 | usually interior use only, strict requirements on application rate | buildings subject to the NRO non-fire-spreading requirement |
| Dual-function (biocidal and fire-retardant) | one application, one set of paperwork | usually interior use only | roof structures in buildings subject to the NRO requirement |
Arsenic-based products (CCA) were withdrawn from general circulation in the European Union on 30 June 2004, and chromium-based products, CCB among them, left the market in the following years as part of the review of active substances under the biocidal products directive. Today a simpler rule applies: the product must hold a current authorisation as a biocidal product. If someone offers you a “tried and tested old product”, ask for the number of that authorisation.
Ordering timber for a roof structure? We cut C24 structural pine and spruce to the dimensions given in your design. If the timber is to be protected before it leaves us, we agree that at the ordering stage, together with the method and the scope. See our offer roof structures or send us your roof plan.
Application methods — how much actually gets into the timber
| Method | Penetration depth | Application rate / retention | Use classes |
|---|---|---|---|
| Brush, roller, spray | 1–3 mm | 150–200 ml/m² (salt-based), 100–150 ml/m² (oil-based), 2–3 coats | 1–2 |
| Dipping / immersion | 3–10 mm | depends on time and concentration of the solution | 2–3 |
| Vacuum-pressure impregnation | 20–40 mm, saturates the sapwood zone | 14–80 kg/m³ of dry active substance, depending on the class | 2–4 |
Two points that save misunderstandings when pricing.
First: application rate in l/m² and retention in kg/m³ are two different quantities. The first applies to surface methods and is calculated from the surface area of the elements, the second to deep-impregnation methods and is calculated from the volume of the timber. Confusing them produces quotes that differ by an order of magnitude.
Second: in pressure treatment of pine and spruce it is the sapwood that becomes saturated, while the heartwood remains untreated. That is not a flaw in the process but a property of the species — pine sapwood falls into treatability class 1 (easy to treat), the heartwood into class 3–4. Spruce is generally harder to treat than pine.
For a typical single-family house what counts is not the volume of the timber but its surface area: a roof structure containing 6–8 m³ of timber has around 150–250 m² of it. With two coats at 150–200 ml/m² that comes to roughly 50–100 litres of ready-to-use solution, plus 10–15% in reserve. Salt-based products are sold as a concentrate, so convert that volume to concentrate using the ratio given in the technical data sheet.
When to treat: moisture content, temperature and the sequence of works
Moisture content of the timber. The practical optimum is 15–18%. For brushing and spraying some manufacturers require as little as ≤12–15%, dipping allows 20–25%, and pressure treatment works at 25–30%. The rule is simple: the more superficial the method, the drier the timber has to be. Wet fibres will not absorb the product — it stays on the surface and washes off with the first rain.
Temperature. Optimally 5–20°C, permissibly from +5 to +35°C, with air humidity up to 85%. Work is not carried out in rain, fog or snow. One thing matters here: the temperature applies to the timber itself as well. A cold beam in warm air chills the product and blocks penetration.
Sequence relative to cutting — this is the heart of the matter. Timber is cut to size, drilled and worked before treatment, not after it. The reason is purely geometric: brushing penetrates 1–3 mm and pressure treatment saturates the sapwood zone, so the core of the section stays bare. Every cut, mortise or hole made after treatment exposes the unprotected interior of the beam, and does so exactly where moisture collects in the carpentry joint.
If cutting on site is unavoidable — and with a traditionally framed roof structure it usually is — every fresh surface has to be re-treated with the same product, generously, ideally twice, before the element is installed. A tin of preservative and a brush should stand at the carpentry station throughout the installation.
Waiting time before the roof is closed. The interval between coats is 2–6 hours, full drying takes 12–48 hours, and the product needs at least 48–72 hours to fix before installation, longer after a generous salt-based application. A salt-based preservative is a water solution: brushing raises the moisture content of the surface layers of the timber by a few percentage points, and dipping or deep impregnation by considerably more. Closing such a roof structure under a membrane the next day means trapping water inside the structure.
The sequence that works: taking delivery and sorting the timber, moisture check with a moisture meter, cutting and carpentry work, treatment, waiting time, installation with on-the-spot treatment of every cut, and closing the roof quickly with a membrane.
How many elements have to be treated depends on the type of structure — a purlin-and-brace roof has noticeably more of them than a rafter roof. We compared the systems in the article on types of roof structures.
Fire-retardant treatment and NRO — when it is mandatory
Untreated timber (D-s2,d0) does not meet the non-fire-spreading requirement. NRO under Annex 3 to the Polish technical requirements means at least class B with a mandatory d0, that is, no flaming droplets. This is achieved with a fire-retardant preservative that raises the timber to class B-s1,d0 or B-s2,d0, depending on the product.
When this actually applies to the client:
- A single-family house of up to three storeys above ground is exempt from fire resistance class requirements altogether — § 213(1)(a) of the Polish technical requirements says so explicitly. The roof structure then carries no fire resistance requirement at all, and fire-retardant treatment is an option rather than an obligation.
- Public buildings, residential institutions and blocks of flats fall into fire resistance classes in which a requirement for the roof structure appears at all: R 30 in classes A and B, R 15 in class C. In class D, which covers most low-rise multi-family buildings, the roof structure still carries no fire resistance requirement.
The distinction that trips people up most easily: a fire-retardant preservative changes the reaction to fire — and so allows the NRO requirement to be met. It does not provide R 30 fire resistance. Fire resistance is achieved through an adequate cross-section of the element, with an allowance for charring, or by encasing it in type F plasterboard. Citing preservative treatment as a way of achieving R 30 is a design error.
A separate matter: the classification of the roof covering. NRO for the covering means class BROOF(t1) — it is determined by the arrangement of the covering layers, not by treatment of the roof structure. If the covering spreads fire, the required distance between buildings increases by 50%, and where this applies to both buildings, by 100% (§ 271(2) of the technical requirements); at the boundary of an undeveloped plot, half the distance given in § 271 applies (§ 272(1)). This is not a prohibition, only a consequence for the siting of the building.
Classification is always settled by the design and by a fire protection expert, not by the client and not by the timber supplier.
If a fire-retardant preservative is in play, keep its minimum application rate in mind — of the order of 200 g of dry salt per square metre of timber surface, across all coats combined. Below that threshold the class is simply not achieved, and brush application can deliver only about 60% of that dose even with five coats. That is why dipping, not brushing, is used wherever NRO is a real requirement. Without a documented application rate the timber is painted, but there is no NRO.
The most common mistakes
- Treating wet timber. The product stays on the surface.
- Brushing the roof structure after installation, from below only. The tops and sides of the rafters are left bare — and that is where water collects.
- Failing to treat cuts made on site.
- One coat instead of two or three.
- Mixing products of different types. A water-borne product over an oil coating has neither adhesion nor penetration.
- Choosing a product without checking the use class — a use class 2 preservative on eaves that work in class 3.1.
- Closing the roof immediately after salt-based treatment, without allowing 48 hours for evaporation.
- Assuming that C24 or kiln-dried timber is already protected.
Which documents to ask for
A preservative is a biocidal product, not “wood paint”. An honest supplier will provide, without hesitation:
- the safety data sheet (SDS) — hazard classification, H and P statements, composition;
- the authorisation number for the biocidal product;
- the technical data sheet (TDS) with the declared use class to PN-EN 335, the application rate and the application conditions;
- a PZH hygiene certificate for indoor use;
- for fire-retardant products — an ITB National Technical Assessment and a fire classification confirming B-s2,d0;
- a record of the treatment carried out: date, product, batch number, method, application rate, timber moisture content. That one document is worth keeping in the site file.
Frequently asked questions
Is C24 timber bought at a sawmill already treated?
No. C24 is a strength class to PN-EN 338, confirmed by grading to PN-EN 14081-1 — it concerns load-bearing capacity, not biological protection. Even kiln-dried timber is merely sterilised: drying at temperatures up to 75°C eliminates the organisms present in the material, but gives no protection against reinfestation or against condensation under the roof covering. Treatment has to be ordered separately, and you should ask for a document confirming that it was carried out.
When should a roof structure be treated — before or after installation?
Before. The product penetrates 1–3 mm when brushed and saturates the sapwood zone under pressure treatment, so every cut made after treatment exposes the bare interior of the beam. Once the roof is built, access is only from below, and the top and side faces of the rafters, together with the contact surfaces against the wall plate and the collar beam, are left unprotected. Cuts made on site are treated on the spot with the same product before the element is installed.
How many coats of preservative are needed and how much product should be bought?
Two to three coats as standard, at an application rate of 150–200 ml/m² per coat for salt-based products and 100–150 ml/m² for oil-based ones. Calculate from surface area, not from volume: a roof structure containing 6–8 m³ of timber has around 150–250 m² of surface, which with two coats comes to roughly 50–100 litres of ready-to-use solution plus 10–15% in reserve. The interval between coats is 2–6 hours, and before the roof is closed the timber should wait at least 48–72 hours.
Which preservative for the eaves and projecting rafter ends?
A different one from the rest of the roof structure. Elements projecting beyond the outline of the roof work in use class 3.1 to PN-EN 335 — they are exposed to precipitation and leaching. What is needed is a product with declared effectiveness for class 3, usually solvent-borne or oil-based. External coatings are renewed roughly every 5 years, whereas deep salt-based protection in a closed roof lasts 10–15 years.
How much does preservative treatment of structural timber cost?
Two pricing models are in use across the country. Per m² of timber surface for surface treatment — market surveys indicate a range of the order of PLN 22–43/m² including material and labour. Per m³ of timber for deep pressure treatment — here the figures vary widely and always depend on the use class, the product and the size of the charge. The product alone for the roof structure of a typical single-family house costs in the region of PLN 200–800. These are general market figures, not an offer — the actual quotation is given by the contractor after taking measurements.
Preservative treatment of structural timber costs a fraction of what the roof structure itself costs, and it decides whether that structure lasts long enough for your grandchildren. If you need C24 timber cut to size, send us your design. We have been cutting timber near Pszczyna since 1990, and we know where fungus shows up first in a finished roof.
