Charred timber cladding turns a fire-based finishing technique into a building product. Designers choose it for a dark, textured facade or interior surface, and the market language around it often adds claims about fire, rot and lifespan.
This guide separates what documents support from what they do not. Manufacturer figures are attributed to their exact document, peer-reviewed studies are cited from the papers or abstracts we opened, and where we found nothing we say so.
What is charred timber cladding and how is it made?
Yakisugi, shou sugi ban and burnt wood
The Japanese technique is called yakisugi, and in English it is widely sold as shou sugi ban. Nakamoto Forestry describes its product as yakisugi siding and paneling in Japanese cedar, with finishes that differ in whether the soot layer is brushed off and whether an oil is applied.[1][2] Other makers use different names (Pioneer Millworks calls its product Shou Sugi Ban Larch, Yakiwood calls its product burnt wood) and other species.[5][6] We did not find a standard that defines the product, so trade names and finishes are not a specification.
What the char layer is
A 2018 Materials paper describes surface charring as a one-sided thermal modification that can create a hydrophobic surface for exterior claddings, and reports increased aromaticity of the surface by spectroscopy (abstract read).[15] That is a laboratory description of the chemistry, not a performance guarantee for a commercial board.
How deep is the char?
Peer-reviewed studies give a range because the process is not fixed. In a Latvian study, five exposure times of 90–150 s on a radiant panel produced char depths of 0.6–2.6 mm, and the authors chose 1.5 mm after a visual evaluation.[10] Using the traditional chimney method, Ebner and colleagues measured average charred layers of 2.5 mm in spruce and 4.5 mm in silver fir; a later gas-burner variant gave a thinner layer in a shorter time.[12][13] For one-sided contact charring of beech, a mild process gave a surface charred layer of about 2 mm, and harsher regimes affected density up to 4–6 mm below the surface.[14] The Nakamoto, Pioneer and Yakiwood documents we read do not state a char depth, so the depth of a purchased board has to be requested.[1][5][6][7]
How does charred timber cladding perform outdoors?
Decay and durability
The USDA Forest Products Laboratory tested 28 combinations of species, char level and finish bought from one US manufacturer (cypress, hemlock, white oak, black walnut, acetylated and furfurylated wood) for flammability (cone calorimeter) and for decay (a soil block assay with one white-rot and one brown-rot fungus). The authors concluded that the process did not systematically improve the flammability or durability of the siding; cypress showed slight improvements, and the authors note they could not examine Japanese cedar (Cryptomeria japonica).[9] A 2017 Coventry University research master's thesis found that charring improved surface durability in most test areas over medium-term exposure, with long-term durability not tested.[17] The manufacturers' statements are far longer. Nakamoto's Suyaki page says an unbrushed soot layer can remain intact for 30–50 years without maintenance, and elsewhere on the same page calls yakisugi a "100-year siding material".[2] Yakiwood's website FAQ says cladding can last "80 years or more" with proper installation, while its EPD uses a reference service life of 20 years.[7][6] We found no independent long-term field monitoring behind any of these figures.
Weathering and the surface
In a two-year natural weathering test of surface-charred Norway spruce panels, infrared spectroscopy showed degradation of the thermally modified lignin component, which the authors read as poor stability in weathering (abstract read).[16] The manufacturers describe the same behaviour as part of the product. Nakamoto says soot residue, caking and erosion are unavoidable and cause blemishes, especially along board edges, and recommends keeping boards slip-sheeted until installation to avoid abrasion marks.[1] Pioneer says some flaking of the char layer should be expected, periodic additional finish helps maintain the look, and the surface can never be fully restored once weathering has begun.[5]
Movement
Charring a single face creates a moisture and density gradient. Boards in the traditional chimney study showed significant cupping toward the charred side, 3.2–6 mm in spruce and 2.2–4.5 mm in fir, with no difference between annual ring orientations.[12] In hot-plate charred spruce, longer holding time reduced sorption but increased cupping.[15] Pioneer's larch is back burned "to promote stability", and Nakamoto's Gendai sheet notes that dimensional shrinkage after manufacture is possible in a solid-sawn product.[5][1] These are laboratory boards and product notes, not field measurements.
Does charring make timber fire resistant?
Not by itself, according to the independent evidence we read. In the USDA study, significant changes in cone calorimeter results were roughly equally split between better and worse performance.[9] The Latvian paper reports a partial gain while the char stays in place: charring cut the peak heat release rate of pine 1.9 times in the cone calorimeter, and the authors conclude that improvement is possible only if the whole char layer remains.[10] Its single burning item tests to EN 13823 on charred spruce reduced FIGRA0.4 by 35 % but the average of 262.9 W/s stayed above the 250 W/s threshold for class C; brushing after charring more than doubled FIGRA0.4 and the product remained in Euroclass D; total smoke production over 600 s rose 2.5 to 3 times and the smoke index moved from s1 to s2; and three fire retardants applied after charring gave potential C-s2, d0 (one specimen each).[10] The same paper found that a linseed oil coat on brushed pine raised peak heat release rate by 32 % in the cone calorimeter.[10] A 2026 study ran SBI tests on nine larch and spruce cladding systems and concluded that the reaction of a cladding system depends on the whole system, including wood type, surface processing, covering and board orientation; the best result was a charred notched spruce system treated with a flame retardant (abstract read, classes not given in the abstract).[11] The 2017 Coventry thesis reports that its charred sample performed significantly worse than the uncharred sample in flame spread testing, a result the author notes did not match existing knowledge.[17]
Manufacturer statements use the US ASTM E84 scheme. The Gendai Alkyd Brown sheet states Class B, and the Suyaki unoiled page states Class A. Nakamoto's fire page adds that, for California wildland-urban interface compliance, the 60-minute extended burn must be achieved with an engineered wall assembly because no nominal 1x softwood cladding it knows of withstands it, and that certification documents are available from sales. The same page lists the CSFM 12-7A-1 (WUI) status as "Pending" for every product family while its FAQ says some products qualify under SFM Chapter 7A listings, and it lists a fire-retardant option as available for Gendai while the Gendai Alkyd Brown sheet says pressure-treated fire retardant is not available; we did not resolve these differences.[1][2][3] ASTM E84 is a US surface-burning test and is not a Euroclass. For context beyond charring, a 2025 study of thermally modified timber cladding found that fire-retardant treatment reached Euroclass B in vertical slatted configurations and that long-term efficacy under ageing and weathering remained unexplored, and the UK Building Safety Regulator's 2025 summary of related research says Euroclass B was reached when new and notes that classification does not require explicit consideration of durability.[18][19] Neither tested charred wood.
What do the two EPDs show?
We found two EPDs for charred wood products and opened both. They differ in programme, unit and method, so they are in separate rows below and are not comparable.
| Product | Manufacturer | Declared unit | GWP result | Programme, method and validity | Source |
|---|---|---|---|---|---|
| Burnt wood (burnt, brushed, oiled boards) | Yakiwood UAB | 1 m³ (average density 470 kg/m³) | A1–A3 GWP-total −659 kg CO₂e (fossil +117, biogenic −787, land use +10.7); modules C1–C4 and D also declared | International EPD System, PCR 2019:14 (printed as 1.3.1; the registry lists 1.3.4), EN 15804+A2, cradle to gate with C and D; published 2023-09-26, valid to 2028-09-25; third-party verified; reference service life 20 years | [6] |
| Yakisugi "Shou Sugi Ban" / Suyaki | Nakamoto Zourin Co., Ltd (Nakamoto Forestry) | 1 m² of 15 mm board, 6.1 kg/m², no paint | 3.4 kg CO₂e (IPCC 2013 GWP100a): raw material 0.89, production 0.73, distribution 1.8 | SuMPO Japan EPD Program, PCR PA-120000-BC-03, intermediate product; externally verified; PDF gives publication 3/28/2025 and expiry 3/23/2030 | [4] |
Two reading notes. The Yakiwood EPD's content table lists 446.5 kg wood, 23.5 kg wood charcoal and 8 kg water-based oil per m³, which is 478 kg against the 470 kg/m³ declared density, and its module D column contains magnitudes (for example 3.58E+07 kg CO₂e for biogenic GWP and 4.45E+03 for fossil GWP) that appear inconsistent with its module D GWP-total of −328 and GWP-GHG of −326, so we quote only the A1–A3 figures.[6] The negative total comes from counting biogenic carbon uptake; the same EPD assumes 90 % of the wood is collected and treated in an incineration plant at end of life, so we do not net it against emissions. The registry record (EPD-IES-0010564:002, dated 3 July 2026) still attaches the 2023 PDF that prints PCR version 1.3.1.[6] The Nakamoto Zourin EPD states a carbon storage calculation of 6.06 kg wood × 0.5 = 3.03 kg C (11.1 kg CO₂) and includes a note that this English file is a self-declared translation and only the original EPD is binding; the link to the verified original in that note is an unfilled placeholder. The SuMPO registry also lists a Japanese version, which we did not read.[4] The SuMPO registry listing gives a publication date of 2020/04/06, last updated 2025/03/28, valid to 2030/03/23.[4] Degmeda's website shows an EPD badge but links no declaration, so we cannot say what it covers.[8]
How do the product documents compare?
Only some properties are stated by more than one maker in a comparable way. The table lists what each document says, with the standard or condition. Fire classes are US ASTM E84 statements and are not Euroclasses.
| Product | Manufacturer | Property | Value | Standard or condition | Source |
|---|---|---|---|---|---|
| Gendai Alkyd Brown (Japanese cedar, brushed, two coats of alkyd oil) | Nakamoto Forestry | Fire resistance | Class B | ASTM E-84, as printed on the spec sheet; no values or report; the fire page lists the Gendai family as Class A/B by finish | [1] |
| Suyaki Unoiled (Japanese cedar) | Nakamoto Forestry | Fire resistance | Class A | ASTM E84, product page; its fire page lists the Suyaki family as Class A/B/C by finish | [2], [3] |
| Shou Sugi Ban Larch, Deep Char | Pioneer Millworks | Fire resistance | Not stated | Spec sheet gives Janka hardness 830 (ASTM D143), kiln-dried 12–15 % before milling | [5] |
| Burnt wood | Yakiwood UAB | Fire resistance | Not stated | Neither the EPD nor the website states a class | [6], [7] |
These products are different species and finishes, so the table shows who has documented what rather than which board performs better. No document we read states a thermal conductivity, a char depth, or an EN 350 durability class.
Fixings, installation and finishing
Nakamoto's Gendai sheet specifies two ring or screw-shank headed face nails or screws per furring strip within 1″ of each edge, rigid furring at a maximum of 16″ on centre, a 15–25 % overage, and coating of all field cuts and rips with touch-up oil; for interior use it lists tube adhesive and pin nails. It advises a minimal amount of touch-up oil, wiped off immediately to avoid glossy spots.[1] The Suyaki page says boards are kiln dried to 9–13 % moisture content, which may fluctuate in transport, and asks for two weeks of on-site acclimation; it says coatings are technically unnecessary for the unoiled product, that its interior polyurethane finish is not suitable for exterior use or prolonged moisture, and that a heavy polyurethane coat used outdoors could potentially limit lifespan to five years (manufacturer statements).[2] Pioneer asks for at least 15 % overage and notes that extreme climate conditions can cause movement and performance issues.[5] The documents we read do not state a fixing material such as stainless steel.
When to specify it, and when not
It suits
- Appearance-led facades and interiors where a dark, textured surface is the goal and the client accepts that it changes. Both Nakamoto and Pioneer say the surface weathers and that soot erosion or flaking is expected.[1][5]
- Ventilated cladding with access for touch-up, because the manufacturers describe periodic oil or finish as the way to maintain the look, and Nakamoto specifies fixing to rigid furring.[1][5]
- Projects that can buy a documented product: the two EPDs above, an ASTM E84 statement and a stated finish each give something to put in a specification, although the EPDs cover the product at the factory gate, not the installed cladding.[4][6]
It may not suit
- Where the char is expected to supply fire performance. The USDA study found no systematic gain across the species it tested, the Latvian SBI tests left charred spruce short of Euroclass C despite a 35 % cut in fire growth, and brushing (SBI) or an oil coat on brushed pine (cone calorimeter) made results worse.[9][10] Specify the tested product or system with its classification report for your jurisdiction, and check that the tested finish is the one you will install.
- Where long service life must be demonstrated. Independent evidence is a laboratory decay test with no systematic improvement, a two-year weathering test showing poor stability of the modified surface, and a thesis with medium-term results only.[9][16][17] The manufacturers' 30–50 years, "100-year", 80 years and 20-year figures are not reconciled by any document we read.
- Where a stable, uniform surface is needed. Soot erosion, edge blemishes, flaking and cupping toward the charred face are all described in the sources, and the surface cannot be restored once weathering begins.[1][5][12]
- Where soil contact, immersion or a durability class is required. We found no use-class or EN 350 durability statement for any charred product.
Against named alternatives
Thermally modified wood is the nearest alternative. That profile reports that the ThermoWood handbook pairs EN 350 durability classes with EN 335 use classes and recommends a modification class per use class, which is documentation the charred products we read lack. The same profile reports strength loss and no automatic fire class for thermally modified timber, so neither choice replaces fire testing for the build-up. The comparison rests on those documents, not on a head-to-head test.
Climate-specific guidance is not given in the sources we read, other than Pioneer's general note on extreme climates. The sources also give no data on emissions, PAH content or machining dust for charred cladding.
Suppliers whose documents we read
Suppliers selected by our editors and checked against company documents and certificate records where available.
| Supplier | Products | Country | Certifications |
|---|---|---|---|
| Nakamoto Forestry (Nakamoto Zourin Co., Ltd) | Suyaki, Gendai and other yakisugi finishes in Japanese cedar | Japan (Hiroshima, Tokushima); North American distribution from Portland, Oregon | SuMPO EPD JR-BC-20001E-A; PEFC chain of custody stated on its pages (certificate not opened); ASTM E84 classes stated without reports.[1][2][3][4] |
| Pioneer Millworks | Shou Sugi Ban Larch, Deep Char (western larch) | United States (McMinnville, Oregon) | "PEFC-Certified Sources" in the spec sheet; FSC claim stated as N/A; no EPD or fire class on the sheet.[5] |
| Yakiwood UAB | Burnt wood: burnt, brushed and oiled boards | Lithuania | International EPD System S-P-10564 (registry status "Valid" on 14 October 2026); FSC chain-of-custody code given in the EPD (certificate not opened).[6][7] |
| Degmeda UAB | Charred spruce, larch, radiata pine, Accoya (homepage) | Lithuania | Homepage shows an EPD badge and lists FSC, CE and ISO claims; no document linked, so none verified.[8] |
Registry checks were made on 14 October 2026; confirm that documents are still valid before specifying. We could not find a datasheet, fire report or EPD for Delta Millworks or Cherry Valley, so they are not listed. Listing policy: editorial selection, not paid.
Frequently asked questions about charred timber cladding
What is charred timber cladding?
Timber boards whose surface has been burned to a char layer, then usually brushed, oiled or sealed. The traditional Japanese name is yakisugi, widely sold as shou sugi ban. Research describes it as a one-sided thermal modification.
How deep is the char?
It depends on the process. Peer-reviewed studies report about 1.5 mm chosen in a Latvian study, an average of 2.5 mm for spruce and 4.5 mm for silver fir in a traditional chimney method, and about 2 mm for mild contact charring of beech. The manufacturer documents we read state no char depth.
Is charred timber cladding fireproof or fire rated?
Not by charring alone. A USDA Forest Products Laboratory study of commercial products found that charring did not systematically improve flammability, and in a Latvian SBI test charring cut the fire growth of spruce by 35 % but it stayed short of Euroclass C, while brushing more than doubled fire growth. Nakamoto Forestry states ASTM E84 Class B for one product and Class A for another, and lists Class A/B/C by finish for its Suyaki range, without a report in the documents we read.
How long does charred cladding last?
No independent long-term field data were found. Nakamoto Forestry states that an unbrushed soot layer can remain intact for 30-50 years and calls yakisugi a 100-year siding material, Yakiwood's website says 80 years or more and its EPD uses a 20-year reference service life. A two-year weathering study reported poor stability of the modified lignin, and a laboratory decay test found no systematic improvement.
Does charred cladding need maintenance?
Manufacturers describe the surface as changing over time. Pioneer Millworks expects flaking of the char layer and says periodic additional finish helps appearance but cannot fully restore the original texture. Nakamoto Forestry says soot erosion and blemishes along edges are unavoidable.
Are there EPDs for charred wood?
Yes, two that we opened: Yakiwood's International EPD System declaration S-P-10564 (per m³) and Nakamoto Zourin's SuMPO declaration JR-BC-20001E-A (per m² of 15 mm board). They use different programmes, units and methods, so their figures are not comparable.
The bottom line on charred timber cladding
Charred timber cladding is a strong visual choice with thin independent evidence behind its performance claims. Treat the char as a finish: ask each supplier for the char depth, the finish, the fire classification report for your build-up, the current EPD and the maintenance regime, and do not substitute the char for any of them.
Related materials
Sources
- Nakamoto Forestry North America (09.2024). Gendai Alkyd Brown spec sheet. Source of the product description, brushing and oil prefinish, ASTM E-84 Class B statement, species, fastening, overage, finish notes and dimensions. Manufacturer document; full text read. PDF (nakamotoforestry.com)
- Nakamoto Forestry, product page "Suyaki" (read 14 October 2026). Source of the unoiled Class A statement, 30–50 year soot-layer and "100-year siding material" statements, finishes, moisture content and acclimation, polyurethane caution and PEFC statement. Manufacturer web page, text retrieved with a fetch tool; claims are the manufacturer's. nakamotoforestry.com
- Nakamoto Forestry, "Fire Ratings & Wildfire Compliance for Nakamoto Forestry Siding" (read 14 October 2026). Table of ASTM E84 classes by product family, CSFM listing status "Pending", note on engineered wall assemblies. Manufacturer web page, text retrieved with a fetch tool; no test report opened. nakamotoforestry.com
- Nakamoto Zourin Co., Ltd (2025). SuMPO EPD, Yakisugi "Shou Sugi Ban" / Suyaki, registration JR-BC-20001E-A, PCR PA-120000-BC-03 (Wood, Wood Materials), functional unit 1 m² (15 mm thick), 6.1 kg/m², verification date 3/24/2025, publication 3/28/2025, expiration 3/23/2030, external verification. Source of GWP and carbon storage figures; the file states it is a self-declared translation. Manufacturer-owned; full text read (3 pages). The SuMPO EPD search (ecoleaf-label.jp) lists the record, checked 14 October 2026. PDF (nakamotoforestry.com) · SuMPO registry entry
- Pioneer Millworks (2026). Shou Sugi Ban Larch, Deep Char, specification sheet (SKU 16401711712). Source of species, finish, dimensions, Janka hardness, weathering and maintenance statements, certifications. Manufacturer document; full text read. PDF (pioneermillworks.com)
- Yakiwood UAB (2023). Environmental Product Declaration, Burnt Wood, International EPD System S-P-10564, EN 15804+A2, declared unit 1 m³ (470 kg/m³), PCR 2019:14 (1.3.1 as printed), published 2023-09-26, valid until 2028-09-25, verifier Prof. Vladimír Kočí (LCA Studio). Manufacturer-owned, third-party verified; full text read. The International EPD System registry record shows status Registered and valid, EPD-IES-0010564:002 (record dated 3 July 2026, same attached PDF), PCR version 1.3.4 and validity to 2028-09-25, checked 14 October 2026. PDF (environdec.com) · registry entry
- Yakiwood, website homepage and FAQ (read 14 October 2026): species, burnt, brushed and oiled finishes, 80-year statement, maintenance statement. Manufacturer web page, text retrieved with a fetch tool; claims are the manufacturer's. yakiwood.eu
- Degmeda UAB, website homepage (read 14 October 2026): species list, EPD badge without a linked declaration, certification claims. Manufacturer web page, text retrieved with a fetch tool; nothing verified. degmeda.eu
- Hasburgh, L., Zelinka, S.L., Bishell, A.B. & Kirker, G.T. (2021). "Durability and Fire Performance of Charred Wood Siding (Shou Sugi Ban)." Forests 12(9):1262. Peer-reviewed; US Forest Service Forest Products Laboratory. Full text read (USDA Treesearch PDF 64332); source of the 28 tested combinations, species groups, cypress result and the note that Japanese cedar was not examined. doi:10.3390/f12091262 · Treesearch record
- Buksans, E., Laiveniece, L. & Lubinskis, V. (2021). "Solid wood surface modification by charring and its impact on reaction to fire performance." Engineering for Rural Development (20th International Scientific Conference, Jelgava), pp. 899–905. Conference paper; full text read. Source of char depths, cone calorimeter and SBI results (EN 13823, spruce) and fire retardant trials. doi:10.22616/erdev.2021.20.tf203
- Sukevicius, L., Grigonis, M. & Zurauskiene, R. (2026). "Charred Wood Cladding System Reaction to Fire: Influence of Wood Type, Surface Covering and Coating." Fire 9(8):323. Peer-reviewed; abstract read via Crossref, full text not opened. doi:10.3390/fire9080323
- Ebner, D.H., Barbu, M.-C., Klaushofer, J. & Čermák, P. (2021). "Surface Modification of Spruce and Fir Sawn-Timber by Charring in the Traditional Japanese Method—Yakisugi." Polymers 13(10):1662. Peer-reviewed; abstract read via Crossref, full text not opened. doi:10.3390/polym13101662
- Ebner, D.H., Barbu, M.-C., Gryc, V. & Čermák, P. (2022). "Surface charring of silver fir wood cladding using an enhanced traditional Japanese Yakisugi method." BioResources 17(2):2031–2042. Peer-reviewed; abstract read via Crossref, full text not opened. doi:10.15376/biores.17.2.2031-2042
- Šeda, V., Machová, D., Dohnal, J., Dömény, J., Zárybnická, L., Oberle, A., Vacenovská, V. & Čermák, P. (2021). "Effect of One-Sided Surface Charring of Beech Wood on Density Profile and Surface Wettability." Applied Sciences 11(9):4086. Peer-reviewed; abstract read via Crossref, full text not opened. doi:10.3390/app11094086
- Kymäläinen, M., Turunen, H., Čermák, P., Hautamäki, S. & Rautkari, L. (2018). "Sorption-Related Characteristics of Surface Charred Spruce Wood." Materials 11(11):2083. Peer-reviewed; abstract read via Crossref, full text not opened. doi:10.3390/ma11112083
- Kymäläinen, M., Turunen, H. & Rautkari, L. (2020). "Effect of Weathering on Surface Functional Groups of Charred Norway Spruce Cladding Panels." Forests 11(12):1373. Peer-reviewed; abstract read via Crossref, full text not opened. doi:10.3390/f11121373
- White, M. (2017). "Charred Timber Cladding: Performance Investigation." Master of Science by Research thesis, Coventry University. Student thesis, not peer-reviewed; abstract on the Coventry research portal read, full text not opened. pureportal.coventry.ac.uk
- Hopkin, D., Spearpoint, M., Kanellopoulos, Y., Gorska, C. & Mayfield, C. (2025). "Multi-Scale Characterisation of the Fire Hazards of Timber Cladding." Fire 8(2):35. Peer-reviewed; tested thermally modified, not charred, timber; abstract read via Crossref, full text not opened. doi:10.3390/fire8020035
- Building Safety Regulator, GOV.UK (22 December 2025). "Fire safety: Fire hazards of timber cladding (Summary and recommendations)." Summary page read (text retrieved with a fetch tool); it does not address charring. gov.uk