Four agents, four standards
Treatments are usually sold as a single promise. Testing does not work that way: each degradation mechanism has its own method, and a system certified under one says nothing about the others.
- UV
Photodegradation of lignin at the surface, causing greying, fibre raising and erosion.
Measured by EN 927-6
- Decay / rot
Brown, white and soft rot fungi, active above roughly 20 % moisture content.
Measured by EN 113 after EN 84 leaching
- Termites
Subterranean and drywood termites; relevant in southern Europe and warmer climates.
Measured by EN 117
- Fire
Reaction to fire — flame spread and heat release of the treated build-up.
Measured by EN 13501-1
Category comparison
Ratings describe the category as tested under the cited standards, not any individual product. They are relative, not a service-life prediction.
| Category | UV | Decay | Termites | Fire | Maintenance | Colour range |
|---|---|---|---|---|---|---|
| Untreated timber | None | Varies | Varies | None | Inspection only; no re-application | Natural tone weathering to silver-grey, unevenly where exposure differs |
| Film-forming coatings | High | Moderate | Limited | None | 6–12 years, shorter on south and west elevations | Full colour range, including greys, browns and black |
| Penetrating oils and pigmented stains | Moderate | Limited | None | None | 2–5 years on exposed elevations | Translucent naturals through browns to near-black; grey pigments available |
| Silicate and silane surface treatments | Moderate | Moderate | Varies | Varies | Manufacturer-declared, typically inspection at 5–10 years with re-application as required | Silver-grey and warm-grey tones; brown and black available in pigmented versions |
| Preservative impregnation | Limited | High | High | None | None required for biological protection; coating maintenance if colour matters | Green or brown base tone; can be over-coated in any colour |
| Modified wood | Limited | High | Moderate | None | None for durability; coating only for colour control | Pale (acetylated) to deep brown (furfurylated, thermally modified); weathers to grey |
| Fire-retardant treatment | None | None | None | High | Per certification; exterior use requires a hygroscopicity-tested (EN 16755) system | Largely colourless; over-coating must be compatible with the certification |
“Varies” means the property depends on the specific formulation or species and must be read from that system’s own test report.
What each category actually does
Untreated timber
No applied protection. Performance comes entirely from the species' natural durability (EN 350) and from detailing.
Evidence. Best-evidenced option: EN 350 classes and long-running EN 252 field stakes give decades of comparative data.
Specify. Weathering is a surface phenomenon. Grey does not mean decayed, but uneven greying is a common reason for early, purely aesthetic replacement.
EN 350 · EN 335 · EN 252 (field)
Film-forming coatings
A continuous paint or opaque stain film blocks liquid water and UV at the surface.
Evidence. EN 927-6 gives repeatable comparative durability data. Correlation between accelerated and natural exposure is good for colour and gloss, weaker for cracking.
Specify. Film failure is the risk: once cracked, water enters and cannot leave. Requires full preparation at every maintenance cycle.
EN 927-1/-2 (performance) · EN 927-6 (artificial weathering) · EN 927-3 (natural)
Penetrating oils and pigmented stains
Oil reduces capillary uptake; pigment absorbs UV before it reaches the lignin.
Evidence. UV protection scales with pigment load — clear oils give little. Decay claims for oils alone are weakly evidenced.
Specify. Fails gradually rather than catastrophically, which makes maintenance predictable but frequent.
EN 927-1 · EN 927-6
Silicate and silane surface treatments
Waterborne silicon compounds deposit in the outer wood cells and cure to a mineral phase. The surface is mineralised rather than filmed, so the treatment does not peel.
Evidence. Reaction-to-fire performance of silicate coatings over fire-impregnated substrates is the best-documented property: third-party classification reports exist (see the evidence register below), including cone-calorimeter testing after EN 927-6 artificial ageing. Independent long-term field data for decay is thinner than for preservatives or modified wood, and service-life figures quoted in years remain manufacturer declarations rather than standardised predictions.
Specify. Ask for the specific EN test report per property — decay, termite and fire performance are separate tests and a system certified for one is not automatically certified for the others.
EN 84 + EN 113 (decay, after leaching) · EN 117 (termites) · EN 927-6 (weathering) · EN 13501-1 (reaction to fire)
Systems in this category include Organowood, SiOO:X. Listed as factual examples only — WoodIntel does not recommend or endorse any supplier.
Preservative impregnation
Biocidal actives are pressure-driven into the sapwood to a specified penetration and retention.
Evidence. Strongest biological evidence base of any option — decades of EN 252 field data and use-class-linked specification.
Specify. Specify the use class, not the product. A UC 3 treatment does not serve a UC 4 detail. Heartwood does not take up preservative.
EN 351-1 (penetration/retention) · EN 335 use classes · EN 599-1 · EN 117
Modified wood
Acetylation, furfurylation or thermal modification chemically or thermally alters the cell wall so it holds far less water.
Evidence. Well documented, with ETAs and 20+ years of field exposure for the leading processes. Thermal modification improves durability far less than acetylation and reduces strength.
Specify. Durability is through-section, not a surface layer, so cut ends stay protected.
EN 350 (DC 1 claims) · EN 113 · EN 84 · ETA documentation
Systems in this category include Accoya, Kebony, Thermowood. Listed as factual examples only — WoodIntel does not recommend or endorse any supplier.
Fire-retardant treatment
Impregnated or coated retardants change the pyrolysis reaction to reduce flame spread and heat release.
Evidence. Classification is strictly standardised, but the class applies to the tested build-up only — substrate, thickness and any topcoat are part of the certificate.
Specify. A fire-retardant treatment is not a durability treatment. The two are specified and tested independently.
EN 13501-1 (classification) · EN 13823 (SBI) · EN 16755 (durability of reaction to fire)
Systems in this category include Burnblock, Woodsafe. Listed as factual examples only — WoodIntel does not recommend or endorse any supplier.
Evidence register
Documents that state a measurable result for a named build-up, with the issuing body and the standard used. Inclusion means the document exists and can be checked — not that WoodIntel recommends the system it describes. Manufacturer-issued documents are labelled as declarations and are not treated as independent evidence.
Classification appendix — silicate coating over Burnblock-impregnated cladding
Third-party- Issuer
- DBI (Danish Institute of Fire and Security Technology)
- Standard
- EN 13501-1
- Reported result
- B-s1,d0 at 19–42 mm thickness and B-s2,d0 above 42 mm, for spruce, larch, pine, western red cedar, frake, ayous, ash and Douglas fir, with two primer layers (max 225 g/m² wet) and one topcoat (max 115 g/m² wet). Thermally modified species reach the same class at the declared coating weights; thermo poplar drops one class.
- What it does not establish
- The class applies to the tested build-up only: substrate species, thickness, impregnation and coating weights are all part of the classification. It says nothing about decay, UV or termite performance.
Refs PCA10812, PCA10648A, PFA12076A/B · Source document
Cone-calorimeter testing of aged, coated, impregnated spruce
Third-party- Issuer
- ZAG (Slovenian National Building and Civil Engineering Institute)
- Standard
- ISO 5660-1:2015 / Amd 1:2019, after EN 927-6 artificial ageing
- Reported result
- Six specimens at 50 kW/m² incident heat flux. THR₆₀₀ 8.3–20.4 MJ/m², total smoke production 1.6–2.3 m² at 600 s, mass loss 53.8–63.1 % at 1000 s. ConeTools prediction of the EN 13823 THR parameter gave 2.41–6.15 MJ against the 7.5 MJ limit for class B.
- What it does not establish
- Cone-calorimeter prediction cannot replace EN 13823 (SBI) and EN ISO 11925-2 testing; the report states the test was terminated early and specimens were undersized. Ageing was accelerated (EN 927-6), not natural exposure.
2024 · Source document
Extended-duration surface burning — 30-minute tunnel test
Third-party- Issuer
- Southwest Research Institute (SwRI), USA
- Standard
- ASTM E2768-11 (2018)
- Reported result
- Flame spread index 0 and smoke developed index 55 over the initial 10 minutes; no significant progressive combustion across the 30-minute exposure. Conditions of classification met.
- What it does not establish
- North American method. Not interchangeable with EN 13501-1 Euroclasses, and applies to the specific impregnated and coated spruce assembly tested.
Tested 2025 · Source document
Certificate of constancy of performance, solid wood panelling and cladding
Third-party- Issuer
- Finotrol Oy (notified body 2412)
- Standard
- EN 14915:2013, AVCP system 1, Regulation (EU) 305/2011
- Reported result
- Fire impregnation treatments certified for B-s1,d0, B-s2,d0 and B-s3,d0, with factory production control assessed. First issued 2021, updated 15 September 2023.
- What it does not establish
- Certifies constancy of production for the listed treatments and plants — it is not itself a test result, and covers reaction to fire only.
2023 · Source document
Environmental product declarations, silicate wood and surface protection
Third-party- Issuer
- The International EPD® System; verified by Andrew Norton (Renuables)
- Standard
- ISO 14025 and EN 15804:2012+A2:2019
- Reported result
- Seven declarations covering wood protection (pigmented and unpigmented), surface protection, end-grain sealer and wash products, with GWP and other indicators reported per functional unit across modules A–C. Registered 2023-03-31, valid to 2028-03-30.
- What it does not establish
- An EPD quantifies environmental impact. It carries no information about durability, service life or reaction to fire.
2023 · Source document
Product data sheet — declared service life of a silicate fire-protection system
Manufacturer declaration- Issuer
- SiOO:X Wood Protection (manufacturer)
- Standard
- None cited for the service-life figure
- Reported result
- States a life span of up to 15 years on fine-sawn or micro-structured surfaces, subject to correct application, installation, maintenance and local weather.
- What it does not establish
- No test standard, field trial or exposure dataset is cited for the 15-year figure, so WoodIntel records it as a declared input and does not use it as a service-life prediction.
Version 2.2, 2024-12-10 · Source document
Open gaps at time of review
- Decay: no public EN 113 result on EN 84-leached samples for silicate surface treatments.
- Termites: no public EN 117 report for silicate surface treatments.
- Exterior fire durability: no public EN 16755 (DRF) classification for the coated build-ups above.
- Weathering: EN 927-6 ageing is used as a pre-conditioning step in the cone-calorimeter work above, but no standalone EN 927-6 durability result is published.
Colour and appearance over time
Every exterior timber surface moves towards grey. The choice is whether that happens unevenly and unpredictably, or in a controlled way.
- Grey tones. Achieved either by letting the surface weather, or by specifying a pigmented system that pre-empts the shift so the elevation greys evenly.
- Brown tones. Available from pigmented oils, stains and silicate systems, and inherent in furfurylated and thermally modified wood.
- Black. Deep pigment loads absorb the most UV and typically hold colour longest, at the cost of higher surface temperatures and greater thermal movement.
Orientation dominates. South and west elevations in the same building can need twice the maintenance frequency of north and east.
Documents to request before specifying
- Decay: EN 113 result on leached samples (EN 84), stating the substrate species.
- Termites: EN 117 report, if the project sits in a termite region.
- Fire: EN 13501-1 classification for the exact build-up, and EN 16755 for exterior use.
- Weathering: EN 927-6 data, with the natural-exposure comparison if it exists.
- Service life: the basis for any figure quoted in years — field trial, model or warranty term. These are three very different things.
WoodIntel models service life from durability class, climate and exposure with explicit uncertainty; treatment effects are handled as a declared input, not an assumed benefit.