useful information

When and how to protect wood


When using wood outdoors, the base preservative must penetrate deep enough into the wood.

How to protect wood ?

When designing a wooden product or building regardless of its intended use, it is important to ensure that wood is adequately structurally protected from precipitation and that water drains away as quickly as possible after each rainfall event. Areas that never or hardly dry out (e.g., between joints of wooden elements) are the greatest risk for fungal infestation of a wooden structure and consequently rotting of wood.

Only healthy and dry wood should be used for the manufacture or installation of wooden products and building construction, whether under a roof or outdoors. For high quality wood protection, effective wood preservatives (such as Silvanol or Silvanolin®) or thermal treated wood using the special Silvapro® process can be used, whereas for wood used in the most demanding conditions (constant contact with soil, water or seawater), a vacuum pressure impregnation process shall be used to achieve adequate wood protection. This is also recommended for the protection of all non-resistant and low-resistant woods.

When protecting wood with liquid wood preservatives, the manufacturer’s instructions and recommendations should be followed, as only a sufficiently large amount of preservative and a sufficient penetration depth will provide effective protection against wood pests. Wood preservatives should not be diluted as this will reduce their effectiveness.

Special attention should also be paid to the application of decorative finishes on wood, as they should also allow the wood to dry after heavy precipitation. Thick coatings do not allow the wood to dry and often crack in critical areas where water enters, and the wood cannot dry quickly enough due to the coating. Most of the time, such wood has a high humidity suitable for the development of pests, which in such cases will destroy the wood much sooner than they would have done it even without any protection.

Vacuum-pressure wood impregfnation chamber.

Wood preservative soaking basin.

It is therefore essential to protect the wood in a stepwise manner: in the first step, the non-resistant wood species should be appropriately treated to make it more resistant to pests, and in the second step, a decorative surface coating can optionally be applied, which shall be thinly layered and vapour-permeable to allow the wood to dry out after heavy precipitation.

The first step is therefore to apply a wood preservative suitable for the intended use of the wood element (wood under a roof, wood exposed to precipitation) and to adjust the amount of preservative to the intensity of the exposure (façade, terrace, pegs in the ground).

For less exposed outdoor wood (e.g., roof trusses, eaves, wooden cladding under roofs), it is usually sufficient to protect the wood by repeatedly coating it with a wood preservative. For more exposed wood (e.g., façade cladding, garden fences, etc.), it is recommended to soak the wood in wood preservative in special made reservoirs (e.g., wooden foil coated or stainless steel), which results in a higher penetration of the wood preservative within 8 to 24 hours, and also improves the quality of the wood preservative performance, as all key parts that are normally less well protected by coating are fully coated and protected in this way. However, if the wood is constantly exposed to running water, soil or even seawater, the wood protection process should be carried out by a professional. In these cases, only special equipment with vacuum-pressure impregnation can provide sufficiently effective protection against wood pests.

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Wood impregnation equipment for better penetration of preservatives (basin, vacuum-pressure impregnation chamber).

The basic wood pest protection treatments are divided into:

  • Preventive – when protection is applied for the first time, with the aim of preventing the colonisation of wood pests.
  • Restorative – when protection is repeated to restore or increase the effectiveness of the preventive protection.
  • Curative – when protection is applied for the first time or repeated with the aim of eliminating wood pests that have already colonised the wood.

Silvaprodukt offers effective wood preservatives (Silvanolin®, Silvanol), wood impregnation services (by dipping and vacuum pressure chamber) and thermal modification services using the Silvapro® process.

You can also consult our experts on post-treatment and curative wood protection, as well as on the restoration of apartments and other buildings infested (affected) by wood-decay fungi or other wood pests.

When to protect wood?

Outdoor wood is particularly threatened by living (biotic) decomposers, the most dangerous of which are wood-decaying fungi. In favourable conditions (humidity above 20%, moderate temperatures), they can decompose wood in just a few months. Slovenia has one of the most unfavourable climates in Europe in this respect, with conditions that are most of the time very suitable for wood decay.

The non-resistant wood varieties (beech, spruce, fir, pine, larch, etc.), which are the most abundant in our forests, can only be effectively protected from decay outdoors by means and procedures for the fundamental, primary protection of wood.

What is the basis for deciding whether and what protection is necessary and appropriate for a wooden product/object?

The following table will help you decide whether or not your product/object needs to be protected:

First, we need to determine to what extent the wood in our product/object will be exposed to moisture and wood pests (using the European standard EN 335: 2013: Durability of wood and wood-based products – Use classes: definitions, application to solid wood and wood-based products):
Class UC 1: the wood will be used indoors and its humidity will be below 20% at all times.
Class UC 2: the wood will be used indoors and its humidity will be occasionally above 20%.
Class UC 3: the wood will be used outdoors and its humidity will be occasionally or frequently above 20%.
Class UC 4: the wood will be used outdoors, in constant contact with the ground, soil or running water.
Class UC 5: the wood will be used outdoors, in constant contact with saline seawater.

2.Next, we need to determine the natural durability for the type of wood we are going to use (as fungi are the main organisms that threaten wood outdoors, we usually specify a fungal resistance class (using the European standard EN 350: 2017: Durability of wood and wood-based products – Testing and classification of the durability to biological agents of wood and wood-based materials):

Class 1: very durable wood types: e.g., Cumaru, Ipe, Robinia (Black locust) (1-2)
Class 2: durable wood types: e.g., chestnut, oak, American cedar
Class 3: moderately durable wood types: e.g., walnut, larch (3-4), red pine (3-4), meranti
Class 4: slightly durable wood types: e.g., spruce, fir, pear, apple, limba
Class 5: not durable wood types: e.g., beech, hornbeam, birch, ash, maple, alder

* In case of wooden load-bearing structures (e.g., bridges, footbridges, etc.) and parts that are difficult to replace, we recommend the higher value, which is indicated in brackets.

When used outdoors, not durable and slightly durable wood types should be protected with a suitable primary preservative (e.g., Silvanolin®) or a proper wood treatment procedure (e.g. Thermal modification Silvapro®). Protection is also highly recommended for moderately durable wood types (e.g., larch), as the climate of most parts of Slovenia and Europoe in general (high rainfall, high humidity, moderate temperatures) allows for rapid development of wood pests and consequently the rapid deterioration of wood products and buildings.

3. Taking into account the chosen wood type and the intended use (exposure of the wood to biotic and abiotic factors), the following table shows the required minimum application rates of Silvanolin® wood preservative according to the class of use of the wood in order to ensure adequate protection of the wood:

Recommended application rates of Silvanolin® S1 for each use class:

Use class according to

EN 335: 2013

Required minimum wet absorption of Silvanolin® S1 (2500 ppm) *Application methodRequired impregnation depth for spruce wood according to

EN 351-1:2007

Penetration class required according to

EN 351-1:2007

UC 1Wood permanently in dry conditions (e.g., roofing).200 g/m2
up to 1 mmNP1
UC 2Wood under roof, occasionally wetted (e.g., pillar of a hayrack).250 g/m2Coatingup to 3 mmNP2
UC 3.1Wood outdoors, allowing rapid drainage of water (e.g., façade).80 (150) kg/m3Soaking for 24 hours3 mmNP2
UC 3.2Wood outdoors, prolonged water retention (e.g., terrace).250 (400) kg/m3Vacuum-pressure impregnation6 mmNP3
UC 4Wood in permanent contact with soil or running water.450 (600) kg/m3Vacuum-pressure impregnation8 mm, full bleachNP6

Introduction video