Choosing a route: a decision path, quality control and limits

A decision path for a sawmill

The portal's preservation article suggests six steps. Use them every time a customer asks for outdoor timber.

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

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). Large pine fencing volumes may justify an own plant; small mixed volumes usually will not.

Regulation, as concepts only

In the EU, wood preservatives are biocidal products under Regulation (EU) No 528/2012, product type 8. The concepts a mill needs, without legal detail:

• A product can be used only if its active substances are approved for the product type and the product is authorised. Approvals have fixed terms and change, so a list in a course goes out of date.
• Treated articles can require a label naming the biocide, in the national language.
• A mill selling only untreated timber is outside these rules for that product. A mill reselling treated timber needs the preservative's authorisation and label. A mill running a treatment plant must use only authorised products, as authorised. Ask suppliers for the authorisation and permitted use classes, not only the active substance name. This is not legal advice.

Current approval status of named substances was not verified in the source article. Check current rules.

Quality control and marking

Certification schemes exist in several countries, such as the Nordic NTR label with third-party control. A certificate is tied to a site and species, can expire and can be withdrawn.

A control list of your 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 as long as warranties and national rules require.

Safety, fasteners and waste

• Operators need the safety data sheet and label and a handling plan for fixation and drying.
• Metal fixings corrode faster in treated wood, so stainless or hot-dip galvanised fasteners are advised (and stainless for thermally modified wood).
• Keep treated offcuts apart from untreated residues.
• Treated waste is difficult. An industry body says energy recovery is the preferred option in several member states; large volumes of treated waste wood cannot currently be recycled. Documented, labelled, detachable treated timber is more likely to be reused or routed well later.

Engineered wood context

Engineered wood products such as glulam and CLT are built from dried, graded lamellas in service class 1 or 2 (dry or sheltered). The sources give a CLT target moisture of 12 %, with up to 16 % at delivery in one, and lamella thickness of about 40-45 mm for glulam. These outlets buy specified, dry, graded input, so durability comes from dry service and design rather than preservative. Ask the plant for its specification.

Worked example: a quote

A customer wants decking (use class 3.2 detail, trapped water likely) and a retaining wall (class 4). A sensible reply: for decking, treated pine to the correct class, thermally modified material or a more durable species, plus a drainage detail sheet. For the wall, pressure-treated pine with full sapwood penetration to class 4 requirements. Write the use class on the invoice.

Limits of this course

This course was drafted with AI from the portal's fact-checked articles and does not add facts beyond them. It is not a specification. Durability class values differ between sources, standard texts (EN 335, EN 350, EN 351-1) were not read, approval and legal status of preservatives were not verified, and several figures are regional or dated (2017 modification output, 1986 FAO manual, 2008 laboratory study). Always check current standards, regulations and supplier data before specifying, treating or selling.

Key points

• Start from use class, then check the species, then choose the process, then specify, verify and document.
• Concepts of approval, authorisation and labelling matter more than any list of substances, which goes out of date.
• Keep records per charge and state use class on delivery documents.
• This course is an orientation: check standards and regulations.