Wood Preservation and Impregnation: Use Classes, Treatments and Durability in Europe

Why preservation is a sawmill topic

Most sawmills never treat a board with a biocide, yet almost every sawmill sells into a market where the buyer eventually asks, "which use class is this for?" Fence rails, decking, cladding, posts and garden structures sit in the gap between what untreated spruce or pine delivers and what the customer expects outdoors. Design, preservation and modification are the three tools that close that gap. A sawmill that understands them can decide whether to stay a producer of untreated timber, buy contract impregnation, or invest in its own line.

This article is a synthesis for sawmill operators and buyers. Statements that come from a source I opened are tied to that source, with region and date where given. My own interpretation is labelled as such, and unverified points are listed at the end. One limit up front: the European standards themselves (EN 335, EN 350, EN 351-1 and others) are paywalled and I did not read them. What is said about standards comes from public catalogue entries, certificates, industry pages and open papers.

The use class concept: EN 335

According to the public catalogue entry for EN 335:2013, the standard applies to solid wood and wood-based products, defines five use classes as service situations and identifies the biological agents relevant to each. It states that "A use class is not a performance class": it describes how aggressive the environment is, not how long a product will last. The entry gives CEN approval on 5 February 2013 and says the standard superseded EN 335-1:2006, EN 335-2:2006 and EN 335-3:1995 (source: iTeh catalogue entry, European standard). Older documents still say "hazard classes"; I read this as the same graded-exposure idea under an earlier name (interpretation).

A German-market knowledge page citing DIN EN 335 describes the classes as follows.

Source: Baukobox knowledge page on DIN EN 335 (Germany); no edition date given. The page says the classes differ in hazard from fungi, beetles, termites and marine organisms but does not assign organisms to classes.

The UK Wood Protection Association (WPA) buyers' guide, referring to its January 2021 Code of Practice and BS 8417, gives typical products: Use Class 2 for tiling battens, framing and roof timbers; Use Class 3(u), exterior above ground and uncoated, for deck boards, fence rails, cladding and fascias; Use Class 4 for fence posts, agricultural timbers, retaining walls and playground equipment, where "Use Class 4 levels of protection MUST be achieved". It warns against substituting indoor-treated wood outdoors (source: WPA, UK). The "3(u)" label is a UK practice and should not be assumed elsewhere.

Svenskt Trä, the Swedish wood industry's information site, adds that the rot risk above ground "ranges from almost negligible to nearly as high as ground contact" and is hard to judge, and that the first defence is design that keeps wood from staying damp; chemical protection comes only when that is not possible (source: Svenskt Trä, Sweden). My interpretation: whether a decking board is class 3.1 or 3.2 is often decided by drainage, ventilation and end-grain exposure, which the sawmill does not control. A design sheet delivered with the timber can prevent more claims than extra preservative.

Natural durability: what untreated wood can do

Swedish Wood states that sapwood "generally has no resistance to biological attack", while heartwood resistance ranges from poor to very good and shows mainly when the wood is exposed to moisture. Heartwood must contain no sapwood to meet the durability tables (source: Swedish Wood, Sweden, undated). It reports the SS-EN 350 five-step scale for heartwood in ground contact, from 1 very durable to 5 not durable.

Source: Swedish Wood, "Durability and resistance". I did not read EN 350. A research paper below states that EN 350-2 classes Scots pine heartwood as class 3; I have not reconciled this with 3-4.

The same page notes that spruce heartwood absorbs moisture more slowly than pine sapwood, so its above-ground durability is probably better than its class suggests, which suits cladding. Practical reading (interpretation): none of the main domestic softwoods is a safe choice for long-term ground contact without treatment or modification, while untreated heartwood in a well-designed class 3.1 detail can be adequate.

Preservative types in broad terms

The FAO Wood Preservation Manual (1986, global) groups preservatives into tar oils (creosote), organic-solvent types, non-fixed water-soluble salts (borax, boric acid, copper sulphate) and fixed water-soluble types such as copper-chrome-arsenic (CCA). It predates the EU Biocidal Products Regulation and lists substances that are no longer allowed in Europe, so use it for principles only. It notes that non-fixed borates and copper sulphate are effective but leach, suiting only above-ground use.

In the sources I opened, the current families appear as follows.

• Copper-based water-borne systems: a Swedish NTR certificate covers pine treated with a preservative whose active ingredients are copper and HDO (Wolmanit CX-8 WB); a Turkish paper used an alkaline copper-quaternary system with benzalkonium chloride and cyproconazole; a Ghent study compared CCA, copper-azole, copper-amine and copper-quat.
• Organic biocides: the UK Vacsol guide describes a water-based formulation of organic fungicides and insecticides for low-pressure treatment.
• Borates: boric acid, boric oxide, disodium tetraborate and disodium octaborate tetrahydrate are on the Danish PT8 list below.
• Tar oils: creosote is on the same list but heavily restricted (see below).

The EU Biocidal Products Regulation

Wood preservatives are biocidal products under Regulation (EU) No 528/2012 (BPR), product type 8 (PT8). EUR-Lex and the ECHA legislation page blocked automated access, so I did not read the regulation text. The points below come from three pages I could open.

The European Parliament Legislative Observatory summary of amending Regulation (EU) No 334/2014 says a product can be authorised only if its active substances are listed in Annex I or approved for the relevant product types; that after an authorisation is cancelled, amended or not renewed a grace period of up to 180 days applies to placing on the market plus up to 180 days for using stocks; and that treated-article amendments apply from 1 September 2013, with a phase-out where no approval application was submitted by 1 September 2016 (source: EP Legislative Observatory, EU).

An ECHA presentation on treated articles (date not verified) says every active substance in a treated article must be approved for the product type and use; labelling is required if a biocidal claim is made or the approval conditions require it; labels must state that the article incorporates biocides, describe the property and name all active substances and nanomaterials, in the national language (source: ECHA, EU).

The Danish Environmental Protection Agency lists PT8 active substances in Annex I "as a guide only". Extract below; expiry dates are as listed and renewals were not checked, so this is not current approval status.

Source: Danish Environmental Protection Agency (Denmark), page metadata dated 12 August 2025.

My interpretation, not legal advice: a sawmill selling only untreated timber is outside these rules for that product; one reselling treated timber needs the preservative, its authorisation and the accompanying label; one operating a treatment plant is a user of biocidal products and must use only authorised products, as authorised, and label goods correctly. Ask suppliers for the product authorisation and permitted use classes, not only the active substance name.

A draft Commission Regulation hosted by the Austrian Federal Economic Chamber (WKO) would amend entry 31 of REACH Annex XVII on creosote. Per the draft, entry 31 already bans placing creosote on the market and using it to treat wood; the draft would prohibit reuse of creosote-treated wood by the general public, allow professional reuse only under strict conditions in the same Member State, keep derogations for railway sleepers and utility poles, and require records for ten years after disposal. Its adoption and date are unverified.

Treatment processes

The FAO manual describes the full-cell process (preliminary vacuum, filling, pressure, final vacuum) as normal for water-borne solutions, and empty-cell processes (Rueping, Lowry) with no initial vacuum, giving deep penetration with limited retention. It gives usual pressures of 800 to 1400 kPa and stresses that wood must be dry, recommending below 30 percent moisture content (source: FAO, 1986).

The Holmen Braviken certificate shows the result in a quality-assured setting: European redwood (Scots pine) treated with a copper and HDO preservative, approved for Nordic classes A and AB, requiring "penetration class NP5 according to SS-EN 351-1 (full sapwood penetration)". RISE issued it on 4 May 2018, valid until 4 May 2023; its current status is unknown and it serves here only as an example (source: RISE certificate, Sweden).

The Arxada Vacsol guide (UK, July 2022) describes double vacuum in four steps: air evacuated from the cells; vessel flooded under vacuum and the vacuum released so preservative enters at atmospheric pressure; a second vacuum removes excess; venting pushes surface preservative back in before drip drying. It states that low-pressure treated timber cannot be used for Use Class 3.2 and 4, and suits Use Class 2 and Use Class 3.1 with a maintained coating, at a maximum pre-treatment moisture content of 28%. It gives no penetration depth. This is one manufacturer's document.

Svenskt Trä stresses that dipping and coating substances "don't penetrate deeper into the wood", whereas pressure treatment is the industrial method, mainly for pine. The WPA guide says freshly cut surfaces made on site need end-grain preservative and that cut ends must never be put in the ground. My interpretation: plan the drying line and the treatment plant together, and ask any contract treater what moisture content it requires.

Penetration and retention: EN 351-1

The Swedish Standards Institute entry shows SS-EN 351-1:2023, edition 3, replacing SS-EN 351-1:2007. It says the standard classifies preservative-treated wood by penetration, with guidance on retention classification and terminology, and is not a treatment specification. The entry does not list the classes (source: SIS catalogue, Sweden). Only "NP5, full sapwood penetration" is verified here, from the Holmen certificate; the definitions of the other classes and any 2023 changes could not be verified from open sources and are not reproduced.

My interpretation: penetration (where the preservative is) and retention (how much) are separate conditions, and a customer asking for "pressure-treated" timber without naming use class, preservative and penetration has specified very little.

Svenskt Trä describes the Nordic classes: NTR/AB for sawn and planed wood up to 38 mm thick, or up to 50 mm thick and 125 mm wide or less, and NTR/A for all other dimensions. They are based on European standards, mainly cover pressure-treated pine, and now include industrially treated spruce. EWPA says NTR A corresponds to use class 4 and NTR AB to use class 3.2, that 108 companies in 9 countries hold certificates, and that the system, begun in Denmark, Finland, Norway and Sweden in the 1960s, is open to all producers in Europe.

Treatability of the main species

I found no open table of treatability classes for all four species, so the picture is assembled from several sources.

Scots pine. Svenskt Trä says sapwood absorbs preservative well while heartwood takes it only superficially, so full sapwood penetration is needed. Behr, Larnøy and Bues sampled pine heartwood from 25 stands in 6 northern European countries and note that EN 350-2 classes it as "not treatable" although industry reports uneven results; permeability was generally low, with a few samples unexpectedly high (source: Behr et al., undated abstract).

Norway spruce. Svenskt Trä says only a limited amount of preservative penetrates, so treated spruce protects less than treated pine. A 2025 paper on Norway spruce (Ganguly et al., Wood Material Science and Engineering) found microwave pre-treatment raised retention and penetration, sapwood was always more treatable than heartwood, and heartwood improved most (24% gain at 1260 MJ/m3 per the abstract). I do not reproduce its table because the column headings were garbled.

European larch. Bakir (2022, BioResources 17(3), Turkey) treated larch sapwood with an alkaline copper-quaternary preservative under vacuum (40 min, 40 mbar). Control copper uptake was low: 2.016 kg/m3 at end grain and 5.146 kg/m3 longitudinally. Bioincising with the fungus Physisporinus vitreus raised these to 8.183 and 7.530 kg/m3, while mechanical and laser incising did little (about 2.3 kg/m3 end grain). For Caucasian spruce, laser incising raised end-grain uptake from 7.98 to 19.4 kg/m3. The author recommends laser or mechanical incising for spruce sapwood and bioincising for larch sapwood. These are laboratory results on small samples, not plant settings.

Douglas fir. Evidence is indirect: Bakir's introduction says pit aspiration and extractives make Douglas fir hard to treat and that incising is recommended. The USDA Wood Handbook chapter (Lebow, 2010) is cited here only for the general point, from its abstract, that heartwood resists treatment more than sapwood. Species-specific Douglas fir treatability is unverified.

Table is my compilation of the statements above, not EN 350 classes.

Quality control, marking and certification

In the Nordic system most treated wood carries an NTR label, controls are certified by a third party covering finished-product quality, chemicals, absorbed quantities, full penetration and environmental issues, and treated wood without NTR control is also sold (Svenskt Trä). The Holmen certificate shows the mechanics: an institute grants approval, the holder may mark products, the approval expires and can be withdrawn, and it is tied to a site and species (European redwood only).

EWPA lists other schemes: Switzerland's Lignum Gutezeichen; the UK WPA Benchmark; Germany's RAL scheme for impregnated timber elements; and the Dutch KOMO scheme, which requires a declaration that the preservative is appropriate in addition to legal approval. It says treated wood in Europe carries labels on the wood or at point of sale. The WPA guide advises specifying the use class in writing and asking the supplier to confirm it on the delivery note, invoice or treatment certificate.

A control list of my own, not a scheme requirement: record species, dimensions, sapwood share and moisture at entry; record preservative, batch and process parameters per charge; take penetration samples on a schedule; state use class and preservative on the delivery note; keep records for as long as warranties and national rules require.

Contract impregnation as a service

I found no open data on contract impregnation volumes or prices, so this section is interpretation built from the technical points above.

Contract treatment lets a sawmill add outdoor products without a pressure vessel, preservative store, bunded areas and permits. The interface needs agreement on: species, dimensions and moisture content (limits of 30% and 28% appear in the FAO and Arxada documents); identification of sapwood material for pine; incising or dimension limits for spruce and larch; fixation and drying time after treatment; certification, since marks are tied to a certified site and species; and liability, where the use class is the customer's requirement and treatment quality is the treater's. Advantages are low capital and quality-assured treatment; drawbacks are logistics, lead time and dependence on one supplier. A sawmill with large pine fencing volumes may justify its own plant; one with small mixed volumes usually will not.

Environmental and safety aspects

De Vetter and colleagues (Ghent University, Annals of Forest Science 65, 2008) tested use class 3 materials for fungal efficacy and leachate toxicity to Daphnia magna. Untreated wood leachates were non-toxic (below 2 toxic units). One-day EN 84 leachates of copper systems at about 10 kg/m3 in pine sapwood were quite toxic (above 16 TU), falling after 14 days to 4.9 TU for CCA, 4.5 for Cu-quat, 2.6 for Cu-azole and non-toxic for Cu-amine; under the milder OECD procedure most were non-toxic. CCA and Cu-azole fully protected the pine, Cu-amine at that retention did not. Thermally modified spruce gave 3.6 TU at day one and was protected against one test fungus but not another. The authors note only one test organism was used. The lesson is that efficacy and leachate behaviour do not always move together and that test method matters; it is a 2008 laboratory study and products have changed.

I did not open sources on occupational exposure limits. Generic points: operators need the safety data sheet and label, a controlled area for freshly treated wood, and a handling plan for fixation and drying. The Arxada guide says "Use wood preservatives safely" and that desired service life is not a guarantee. Svenskt Trä notes that metal fixings corrode faster in treated wood, so stainless or hot-dip galvanised fasteners are advised, and that scrap needs a planned route. My own observation: keep sawdust and offcuts of treated wood apart from untreated residues destined for pellets, board or garden use.

Treated wood waste

EWPA (industry body, undated, no figures) says sound portions of treated timber can be reused where adequately protected; chipping into particle products is generally not seen as an option; energy recovery is the preferred option in a number of member states; landfill or incineration without energy recovery is possible, with attitudes varying across the EU; some treatments can make the waste hazardous; and coatings and national law matter.

The Fraunhofer WKI project WoodTreat says large volumes of treated waste wood, especially from dismantled roofs, cannot currently be recycled and in Germany usually end in incineration under the Waste Wood Ordinance. It aims to produce pollutant-free secondary raw materials, build an online system to distinguish contaminated from less contaminated wood and propose rules allowing reuse. It is funded under Horizon Europe (Grant Agreement No. 101214288) and runs 1 September 2025 to 31 August 2029. Waste codes and hazardous thresholds were not verified. My interpretation: documented, detachable, clearly labelled treated timber is more likely to be reused or routed well later.

Non-biocidal alternatives: overview

Thermal modification. Svenskt Trä describes heating softwood or hardwood in oxygen-free conditions at 160 to 220 degrees C. The wood turns brown, then grey outdoors, absorbs less moisture and moves less, but is weaker and more brittle and should not be used in load-bearing structures. Two unofficial classes exist: S (Stability) for indoor use and D (Durability) for outdoor cladding, fencing, furniture and decking.

Acetylation and furfurylation. Mantanis (BioResources 12(2), 2017) reviews the two industrial chemical methods. Acetylation reacts wood with acetic anhydride, esterifying accessible hydroxyl groups; furfurylation impregnates furfuryl alcohol and catalysts and heats the wood to polymerise it. In 2017 acetylated wood (Accoya, Netherlands) was produced at about 40,000 m3 a year and furfurylated wood (Kebony, Norway) at about 22,000 m3. Per the review both are rated class 1 under EN 350-2, acetylated wood having superior dimensional stability and furfurylated wood being harder and darker. These are 2017 figures, not current capacity. Svenskt Trä says both are done mainly abroad.

Modified wood is not a drop-in replacement for a pressure-treated post: the Ghent study found a modified product unprotected against one fungus and a monomer-treated furfurylated product with toxic leachate. For most sawmills the realistic options are untreated durable heartwood with good design, bought-in treated or modified material, or an own pressure plant where volume justifies it (interpretation).

A decision path for sawmills

1. Define the customer's use class; ask about ground contact, wetting, drying and coating.
2. Check what the species achieves alone; for classes 3.2 and 4, spruce, pine, larch and Douglas fir normally need treatment or modification.
3. Match process to class: double vacuum only for classes 2 and 3.1 per the Arxada guide; class 4 needs deep-penetration pressure treatment with an authorised preservative.
4. Specify penetration and retention through the standard or scheme, with third-party control (NTR A or AB in the Nordic countries).
5. Verify BPR authorisation and the labelling of treated goods.
6. Document, label and plan end-of-life routes.

Sources used

• iTeh Standards catalogue, EN 335:2013 (scope, dates, supersession, "A use class is not a performance class") - https://iteh.es/catalog/standards/cen/e5d368b1-2232-47e2-8349-ee85cb6c895b/en-335-2013
• Baukobox knowledge page, use classes according to DIN EN 335 (Germany) - https://baukobox.de/en/knowledge/321-use-class-wood-according-to-din-en-335
• European Parliament Legislative Observatory, summary of Regulation (EU) No 334/2014 amending the BPR - https://oeil.europarl.europa.eu/oeil/en/procedure-document-summary/pdf?id=1346062
• ECHA presentation on treated articles under the BPR - https://www.echa.europa.eu/documents/10162/2174242/11_bshd_bernsel_treated_articles_en.pdf/6c9b1bf2-2be8-4340-9cbb-cd0888d9ed85
• Danish Environmental Protection Agency, PT8 wood preservatives active substances - https://eng.mst.dk/chemicals/biocides/application-in-accordance-with-the-bpr/when-to-apply/check-the-status-of-active-substances/active-substances-included-in-annex-i/wood-preservatives-pt-8
• Draft Commission Regulation amending REACH Annex XVII entry 31 (creosote), hosted by WKO - https://www.wko.at/oe/news/0262-su-260211-annex-xvii-creosote-act-comreg.pdf
• Wood Protection Association, buyers' guide to preservative treated timber (UK) - https://www.metsagroup.com/globalassets/metsa-wood/attachments/uk-docs/wpa-buyers-guide-to-preservative-treated-timber-6.pdf
• FAO Forestry Paper M-34, Wood Preservation Manual (1986) - https://www.fao.org/docrep/015/an800e/an800e00.pdf
• SIS (Sweden) catalogue entry for SS-EN 351-1:2023 - https://sis.se/en/produkter/chemical-technology/products-of-the-chemical-industry/woodprotecting-chemicals/ss-en-351-12023
• Svenskt Trä, Wood protection (Sweden) - https://integration.svenskttra.se/en/wood-facts/about-wood/wood-protection
• Swedish Wood, Durability and resistance - https://www.swedishwood.com/wood-facts/about-wood/from-log-to-plank/durability-and-resistance/
• EWPA, national quality regimes and industry standards - https://ewpa.org/specify-design-treated-wood/national-quality-regimes-and-industry-standards
• EWPA, re-use, recycling and disposal - https://ewpa.org/treated-wood-is-sustainable/re-use-recycling-and-disposal
• EWPA, specify and design treated wood - https://ewpa.org/specify-design-treated-wood
• RISE NTR certificate, Holmen Timber AB, Braviken sawmill (2018) - https://int.holmen.com/globalassets/alla/certificates/wood-products/ntr-certificate---holmen-timber---braviken-sawmill--en.pdf
• Arxada, Vacsol Specifiers Guide (UK, July 2022) - https://www.arxada.com/content/dam/arxada/CMC/wood-protection/wood-protection-emea/brochures/vacsol/Vacsol%20Specifiers%20Guide_EN_Jul22.pdf.coredownload.pdf
• Behr, Larnøy and Bues, treatability variation of Scots pine heartwood from Northern Europe - https://wsenetwork.org/?p=3076
• Bakir (2022), BioResources 17(3), incising and permeability of refractory spruce and larch - https://bioresources.cnr.ncsu.edu/wp-content/uploads/2022/07/BioRes_17_3_5021_Bakir_Effect_Incising_Permeability_Refractory_Wood_20207.pdf
• Ganguly et al. (2025), Wood Material Science and Engineering, microwave treatment of Norway spruce - https://jukuri.luke.fi/server/api/core/bitstreams/d2583e12-6e63-43a4-bdc7-5934dcc0a106/content
• USDA Forest Service Treesearch, Lebow (2010), Wood Handbook chapter 15 abstract - https://research.fs.usda.gov/treesearch/37417
• De Vetter et al. (2008), Annals of Forest Science 65, efficacy and ecotoxicology of treated and modified wood - https://annforsci.biomedcentral.com/counter/pdf/10.1051/forest:2008030.pdf
• Mantanis (2017), BioResources 12(2), acetylation and furfurylation review - https://bioresources.cnr.ncsu.edu/resources/chemical-modification-of-wood-by-acetylation-or-furfurylation-a-review-of-the-present-scaled-up-technologies
• Fraunhofer WKI, WoodTreat project - https://www.wki.fraunhofer.de/en/research-projects/2025/woodtreat_recycling-contaminated-waste-wood-for-a-circular-bioeconomy.html

Limits of this article

• No European standard (EN 335, EN 350, EN 351-1, EN 599-1) was read; they are paywalled. Information on them comes from catalogue entries, certificates, industry pages and papers. The full definitions of penetration classes NP1 to NP6 and the 2023 changes in EN 351-1 are not reproduced.
• The BPR (Regulation (EU) No 528/2012) text on EUR-Lex and the ECHA legislation page could not be opened. Legal statements rest on the Parliament summary of the amending regulation, an ECHA presentation without a verifiable date, and a national authority's list. This is not legal advice.
• The Danish PT8 list gives original inclusion expiry dates; renewals were not checked, so current approval status of copper, borates, azoles and other substances is unverified.
• The creosote REACH text is a draft; its adoption and application date are unverified.
• Treatability of Douglas fir and larch heartwood, formal EN 350 treatability classes, and the exact requirements for treating spruce under NTR are not verified. Larch and Douglas fir results rely on one laboratory paper on small sapwood samples and its introduction.
• Pine durability class is reported as 3 in one source and 3-4 in another; not reconciled.
• The FAO manual (1986) and the Ghent study (2008) are dated; substances and products have changed since.
• Waste codes, hazardous-waste thresholds, occupational exposure limits and national building rules were not verified.
• Contract impregnation economics, volumes and prices could not be sourced; that section is interpretation.
• Acetylation and furfurylation production figures are from 2017.
• Industry bodies (EWPA, WPA, Arxada) represent the preservation industry; their framing is not neutral.
• Regional scope: statements are mainly Nordic, UK, German and EU-level; other member states' practice may differ.