Wood protection encompasses all measures that prevent the depreciation or destruction of wood , wood-based materials or wooden structures (e.g. log houses , roof structures , furniture, timber , garden wood , power poles, railway sleepers ) from damage by the weather , insects and fungi and thus a long service life to ensure.
The term “wood protection” should not only be limited to the use of wood preservatives and wood preservation processes. With the correct selection of woods and felling times, as well as observing the rules of structural wood protection , chemical agents can be completely dispensed with in most cases.
Wood protection is almost as old as the use of wood itself. The Bible says: Genesis, 6: 13-14 “Then God said to Noah ... make an ark out of cypress wood for you. Equip them with chambers and seal them inside and outside with pitch ! ”With the construction of wooden structures, an early understanding of different wood resistances and the influence of the type of construction began. In this respect, the first wood protection should be located in the early days of wood construction. The idea of ill-considered, primitive construction of prehistoric wooden structures is refuted by various findings in archeology . The history of wood preservatives goes back to the beginning of the use of wood as a building material. In the early history of timber construction, attempts were made to make this building material more durable. In addition to the purely constructive wood protection, various physical processes and active ingredients were also used very early on:
- Charring , charring (from approx. 5000 BC, in Japan: Yakisugi )
- Vegetable oils such as myrrh , frankincense , oil yeast ( sediment from long-chain oil molecules in olive oil vessels) etc. (from approx. 2900 BC in Egypt, Scheden 1860, p. 56). In ancient Greece at the time of Alexander the Great, bridge wood was soaked with olive oil.
- Leaching by water (see also wet storage or rafting ) and sap emptying ( Theophrast , 4th book, 2nd chapter, p. 132 according to Seidensticker 1886, p. 274)
- Wood distillation products: wood tar , wood vinegar , creosote (from approx. 1000 BC e.g. in Greece according to Hösli JP, 1982, pp. 29–36)
- bituminous coatings: tar , pitch (documented from approx. 3000 BC according to Strabo (16p739) quoted from Hirt 1821, p. 160). The Romans coated their ships with pitch to protect them from drilling mussels .
- Mineral salts (such as borax )
- Mercury and arsenic compounds (e.g. approx. 800 BC in China, Pliny , XXXVI, 3, 19 after Seidensticker, 1886, p. 1886)
- Silicification with water glass , from around the middle of the 19th century
In the second half of the 20th century, due to the advancing industrial development and research, wood preservatives based on organochlorine compounds were increasingly used.
Some types of wood form a sapwood , which is often much less resistant than the heartwood . The durability of the heartwood depends primarily on the ingredients, but also on the pore structure. Beech wood can be soaked well due to the pores running through it, but without chemical wood protection it is susceptible to moisture penetration and fungal attack. Weather-resistant types of wood are usually naturally dense and have few pores and contain oils and resins that prevent moisture penetration. The minerals or tannic acid it contains also make the wood unattractive to pests and fungi.
Regardless of the type of wood, dry installed wood that is permanently protected from moisture has an unlimited lifespan.
Various factors can damage the wood:
- Moisture is a prerequisite for insect infestation and fungal growth
- Temperature fluctuations can lead to the formation of condensation and moisture in the wood
- Insufficient air movement delays the drying of the moisture contained
UV radiation causes photochemical reactions on the wood surface. Decomposition products are washed out by rainwater. Free weathering alone leads to an erosion of about 0.01 to 0.1 mm annually. This value is so low that wood that can otherwise dry quickly on all sides can easily become as old as the building itself.
If structural measures can prevent the wood from becoming damp, no further weather protection is usually required. Insect infestation is only possible from a wood moisture content of over 16% and fungal growth from 20%.
In contrast to steel and concrete, wood does not corrode in a salty atmosphere and is relatively resistant to diluted acids and alkalis . On the contrary, they can help protect wood. In general, a resistance of the wood in the pH range of 3–10 can be assumed. The resistance depends on the type of wood.
The wood substance is changed primarily by organisms such as fungi , insects and bacteria . These have different requirements for their living space. The parameters wood moisture and temperature play a central role .
Goals of wood protection
- prevent natural wood degradation by protecting against moisture and sun rays (UV)
- Prevention and control of insect and fungal infestations
- Change in absorption behavior (swelling and shrinkage due to water absorption or release)
- Improvement of fire behavior
- Increase in mechanical resistance
Constructive wood protection
The most important protection consists in keeping the wood dry. The protection of the wood from rainwater and waterlogging, as well as from UV radiation and condensation , is called constructive wood protection . The standard for constructive wood protection measures in building construction (DIN 68 800-2) shows exemplary wooden structures. Chemical wood protection is only necessary for statically stressed components such as wooden masts , which cannot be adequately protected against moisture penetration.
Timber that is installed vertically in particular can achieve a very long service life even if it is not treated, if it is observed that
- they are completely outside the plinth area and the splash area of other horizontal surfaces such as window sills, canopies or balconies and
- their areas affected by rain are not shaded or covered by vegetation or other elements, so that they are normally dried quickly by the sun and wind.
In particular, wood connections and other close contact points into which rainwater runs and is held back by capillary forces should be avoided .
The following components can usually be constructed in such a way that rainwater runs off quickly and damp areas can be dried off quickly by tanning or purging the air:
- Wooden facades , either vertically installed with a small gap or with cover strips or horizontally installed with drip edges
- Roof boxes and eaves boards
- Cladding of the verge
- Railings and (balcony) fences
- Garden furniture and play equipment whose contact points are equipped with spacers to prevent capillary water accumulation
- other wooden structures that are protected in the base area by post shoes or base masonry from ground moisture and splash water and from above by sufficient roof overhang from rainwater.
Weathered, horizontal wooden surfaces are generally to be avoided. The upper and lower end faces of vertically mounted timbers should be bevelled to allow rainwater to run off more quickly. Since end grain sucks significantly more than surfaces lying parallel to the grain, it can make sense to additionally protect the upper ends with a sheet metal cover, with cover timbers installed at an angle or with impregnation.
Façade vegetation such as climbing plants or espalier fruit hampers the drying of wooden facades, but at the same time protects them from being soaked through by the rain. On the south and east facing away from the weather, it can therefore generally be assumed that the wood cladding remains sufficiently dry or dries off sufficiently quickly. The stress on the west and north sides from driving rain and the drying process depends on the local conditions and the type and density of vegetation. Self-climbers are usually directly on the facade, so it should be checked, especially on the west side, whether rainwater running off the main strands leads to local moisture damage on the wooden facade.
In areas with high humidity, condensation often forms on roof boxes or on the wooden cladding of well-insulated facades as the component surfaces cool down at night. This can lead to the growth of mold or moss on the surface and consequently to an increase in wood moisture.
So-called dimensionally stable wooden components such as doors or windows must be protected from rainwater by (exchangeable) weather strips, sheet metal cladding or impregnation. The moistening of untreated wooden elements leads to swelling and shrinking . This opens the corner connections and the water can run into the construction.
If it cannot be ensured that every part of the built-in wood can be permanently protected from moisture penetration, critical areas should be protected by impregnation. A so-called blue stain protection is often used for this today . For centuries, the treatment of wood with vegetable oils, which usually penetrate deeper into the wood, has proven itself as chemical wood preservatives.
Natural wood protection
Natural wood protection includes the natural wood structure and wood constituents, which protect certain types of wood more than others, directly or indirectly, from fungal and insect attack. In addition, the impregnation of the wood with vegetable oils and the treatment with natural resins can also be used for natural wood protection, especially since these often have properties similar to those of the wood's own ingredients.
Since the primary aim of treating the wood with oil is to prevent moisture from penetrating, this is a physical form of wood protection , which is dealt with below.
The charring of the wood surface for preservation is called yakisugi in Japan .
Natural durability (also referred to as resistance in older regulations ) is the resistance to wood-destroying fungi, insects and marine animals. The use of resistant types of wood according to the respective usage class (GK) prevents structural damage. For example, teak contains poisonous substances that counteract decomposition by predators. The wood of some coniferous plants (e.g. larch, western red cedar) also protects itself.
The natural durability of selected types of wood is classified in the DIN EN 350-2 standard. Some types of wood not included there are also listed in DIN 68800-1. Different classifications apply to the different groups of wood pests (insects, fungi, marine wood pests). The following table shows the classification of the natural durability against wood-destroying fungi:
|Durability classes according to DIN EN 350-2|
|Durability class 1||Durability class 2||Durability class 3||Durability class 4||Durability class 5|
|very durable||permanent||moderately permanent||little permanent||not permanent|
Brilliant Nut (Sucupira, Cutiuba)
Cumarú (Dipteryx spp.)
Western Red Cedar
Robinia (false acacia) *)
Ipé ( Lapacho , Tabebuia spp.)
Massaranduba (Manilkara spp.)
Merbau (Intsia spp.)
Kapur (Dryobalanops spp.)
Thermowood (e.g. ash, beech, pine)
Bangkirai (Yellow Balau, Shorea)
stalk or Traubeneiche
garapa (Molaris, Apuleia spp.)
Nootka cypress (yellow cedar)
*) In Robinia, the adult heartwood is classified as permanent after 1–2, but its juvenile heartwood (the first 7 to 15 annual rings) as less permanent after 3–4 (Dünisch, Koch and Dreiner 2007)
The assignment to the durability classes is carried out separately according to sapwood and core, whereby sapwood is generally classified as not permanent (predominantly resistance class 5). In most cases, however, the sapwood can be impregnated well. A proper pressure impregnation can generally increase the durability of the sapwood up to resistance class 1. Impregnation of the core is technically difficult or not possible, depending on the type of wood , due to the thinning or the potted closure .
According to DIN EN 350-1, the determination of the durability of wood species has so far been carried out through field tests with wood in contact with the ground (use class 4) as well as through laboratory tests. More recently, more differentiated studies have been carried out in which the timbers were tested to a greater extent under realistic installation situations that occur more frequently in buildings (in use classes 3 and 4). The durability determined in this way showed that the tested softwoods largely corresponded to DIN EN 350-2. According to the previous, preliminary test results, however, the durability of the hardwoods oak and robinia is less than specified in the standard. It suggests that z. B. Oak heartwood under realistic installation conditions can only be assessed as "not very durable".
Physical wood protection
Methods and means of physical wood protection should protect the wooden component from weather-related damage, i.e. from moisture absorption, sunlight and mechanical influences. They are also known as weather protection agents. Purely physically acting agents do not require an evaluation by the Federal Environment Agency in Germany .
Wood preservatives without chemical effects
Physical wood preservatives are primarily intended to reduce the amount of moisture entering the wood in order to rule out insect or fungal attack. If they contain pigments or other means of protection against UV radiation, they also delay the decomposition of certain ingredients caused by solar radiation and thus the graying of the wood. Finally, they can also improve the mechanical strength of the wood surface.
Impregnations penetrate the wood and protect it from moisture penetration by either closing the pores or making them hydrophobic .
Traditionally, oils and waxes are used to impregnate wood. These reduce and hydrophobize the pore space of the wood available for water absorption without completely preventing the evaporation and diffusion of the moisture already contained in the wood.
Non-drying oils such as paraffin oil and cold-pressed vegetable oils penetrate particularly deeply into the wood, distribute themselves in all directions and can be repainted in any amount at any time without having to worry about sticky or permanently shiny surfaces. With frequent exposure to driving rain, they wash off the surface and the wood takes on its natural appearance again.
The molecules of drying oils cross-link through polymerization to form longer chains, so that the oil is immobile and less easily washed out on contact with water. Drying oils can fix pigments on the wood surface and can therefore also be used as a glaze and to protect against UV radiation. The crosslinking of the oil can take years and is therefore often accelerated by heating the oil or adding drying agents.
Disadvantages of vegetable oils
- The wood surface darkens. Linseed oil can lead to yellowing. Certain oils such as walnut , olive and paraffin oil result in less dark surfaces.
- Oils usually have only a weak or no fungicidal effect. If dust settles on surfaces that have been fully treated with non-drying oil, this can promote the formation of superficial mold in the event of permanently increased air humidity or frequent condensation. The prerequisites for this are usually only present in commercially used damp rooms or in areas with frequent fog formation.
Film-forming wood preservatives
Most commercially available coatings for wood, such as thin or thick-film glazes and varnishes, prevent moisture from penetrating by sealing the surface of the wood.
Since the diffusion of water molecules into the wood from the inside or outside cannot usually be completely ruled out, coatings on the outside should only be applied after previous impregnation in the form of blue stain protection.
By heating the wood, the chemical conversion of some wood constituents is achieved in order to make the wood unattractive for fungi and insects and possibly to achieve a certain surface effect. The increase in durability is usually accompanied by a certain reduction in strength.
Chemical wood protection
Chemical wood preservatives usually contain biocides , which fight wood-destroying or wood-discolouring harmful organisms by biological or chemical means.
For the commercial use of chemical pesticides, an expert examination is necessary, which is documented by the "Expertise certificate for wood preservation in construction". This certificate of competence confirms the qualification required in DIN 68 800, Part 4 and states that the owner has the knowledge and skills according to the state of science and technology for the preparation, guidance, implementation and testing of non-hazardous and environmentally friendly wood protection measures to combat wood-destroying fungi and insects as well as other influences.
The wood is either treated on the surface or saturated with the wood preservative. This can be done by hand, e.g. B. with a brush or spray gun, or industrially, e.g. B. in pressure impregnation, happen. See wood preservation method .
New wood preservatives increasingly contain water as a solvent or transport agent in order to reduce problematic organic solvents. The aim is that treated wood emits fewer pollutants through discharge, leaching or outgassing. However, the protective effect is possibly less than with conventional agents, especially in combating wood protection.
The incineration of impregnated wood takes place in special plants that are approved according to the ordinance on the incineration and co-incineration of waste .
In Germany there have been two apprenticeships in the dual system since 2007 , which conclude with the journeyman's examination: the two-year apprenticeship as a specialist for wood and building protection work and the three-year apprenticeship as a wood and building protector . Experienced specialists have had the opportunity to take a master craftsman's examination in wood and building protection since 2012. Since 1992 there has been an extra-occupational opportunity at the European Institute for Postgraduate Education (EIPOS), an affiliated institute of the TU Dresden , to complete a one-year training course as an expert in wood protection.
- German Institute for Building Technology (DIBt) (Ed.) Directory of wood preservatives. List of wood preservatives with general building authority approval - List of wood preservatives with RAL quality mark - List of blue stain protection agents in accordance with VDL guidelines. 55th edition 2007, Erich Schmidt Verlag Berlin, ISBN 978-3-503-10029-3 .
- Johann Müller: Wood protection in building construction. Fraunhofer IRB Verlag, Stuttgart 2005, ISBN 3-8167-6647-1 .
- Dietger Grosser: Plant and animal construction and timber pests. Munich 1984.
- Hans Peter Sutter: Recognizing and combating wood pests on cultural assets. Paul Haupt Verlag, Bern Stuttgart Vienna 2002, ISBN 3-258-06443-1 .
- G. Becker: Studies on the nutritional physiology of the longhorn beetle larvae. 1941, Z. see Physiologie, 29/3, 315–388.
- W. Behrenz, G. Technau: Investigations on the immunization of wood by hot air treatment . 1956.
- DIN , DGfH (Ed.): Wood protection. Structurally - chemically - fighting. Explanations on DIN 68800 Part 2, 3, 4. Beuth Verlag, Berlin 1998, ISBN 3-410-13959-1 .
- Uwe Wild: Lexicon of wood protection. BAULINO Verlag, Waldshut 2009, ISBN 978-3-938537-07-7 .
- Technical information (PDF) on the subject of wood protection
- German wood protection association for external wood e. V. (DHV)
- Gütegemeinschaft Impregnierte Holzbauelemente e. V.
- General information on wood protection and the current wood protection standard DIN 68800
- ↑ documented by finds in the Fayum desert and the Wadi Qena , see also Sutter 1986 p. 120
- ↑ Dieter Lehmann: Two medical prescription books of the 15th century from the Upper Rhine. Part I: Text and Glossary. Horst Wellm, Pattensen / Han. 1985, now at Königshausen & Neumann, Würzburg (= Würzburg medical-historical research , 34), ISBN 3-921456-63-0 , p. 229 ( Öle-drusene : Öldrusen, Ölhefe).
- ↑ Information on equilibrium moisture content on Holzfragen.de
- ↑ DIN 68364: 1979-11 Characteristic values of wood species - strength, elasticity, resistance ( standard withdrawn )
- ↑ a b c d e f g h i Stephan Winninghoff, Thomas Bauer, Martina Lorenz, Johannes Diebel, Christoph Schelhorn, Clemens Fauth, Christine Andres: The construction site manual for gardening and landscaping 6th updated edition, Forum Verlag Herkert GmbH
- ↑ a b Information on durability in the brochure ( memento of the original from October 6, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. on terrace construction, Eurotec, accessed in October 2015.
- ↑ a b Information on the durability classes on the Holzfragen.de page , accessed in October 2015.
- ↑ Mechanical parameters of beech, ash and robinia wood for load-bearing components , p. 5, dissertation by Ulrich Hübner, Graz University of Technology
- ↑ Information on the durability / resistance of wood on the Fahlenkamp website, accessed in October 2015.
- ↑ AO Rapp, U. Augusta, C. Brischke and CR Welzbacher: Natural durability of important native wood species under typical building conditions. in: 25th wood protection conference of the DGfH on September 20 and 21, 2007 in Biberach / Riß, (conference proceedings).
- ↑ Certificate of expertise in wood protection in construction. German Wood and Building Protection Association , accessed on January 15, 2017 .