Cross-laminated timber turns sawn boards into large structural panels for walls, floors and roofs. Designers use it for prefabricated, load-bearing timber buildings, from houses to tall residential blocks.
This guide sets out what declarations of performance, EPDs, standards pages and independent studies say, and where they stop. Most numbers come from manufacturers, so we label them as such.
What is cross-laminated timber and how is it made?
Layers, adhesives and sizes
Stora Enso describes CLT as a solid wood construction product of at least three bonded single-layer panels arranged at right angles to each other.[1] EN 16351:2021 distinguishes three types: straight or curved panels of timber layers only without large finger joints, straight timber-only panels with large finger joints, and straight panels that mix timber and wood-based panel layers.[5] The boards are strength graded; Stora Enso states strength class C24 with at most 10 % C16, and Mayr-Melnhof declares top layers from C16/T11 to C35/T21 and middle layers with at most 30 % C16.[1][3]
Adhesives differ. Stora Enso lists formaldehyde-free PUR for finger jointing and surface bonding and EPI for edge bonding, together about 4.8 kg of the 470 kg in one cubic metre.[1] Mayr-Melnhof lists PUR (EN 15425) for finger joints and either MUF (EN 301) or PUR for surface bonding, with adhesives at roughly 0.5–1.2 % of the product.[2][3]
Panel sizes
Declared maxima come from the makers, not from the standard. Stora Enso: 16 m × 2.95 m × 0.32 m, with 3.45 m width on request.[1] KLH: up to 16.5 m long, 2.98 m wide at Teufenbach-Katsch and 3.5 m at Bad St. Leonhard, 60–360 mm thick.[4] Mayr-Melnhof's declaration gives up to 18 m × 4 m and 36–360 mm, while its EPD also states delivery up to 16.5 m × 3.5 m.[2][3] Confirm the figure with the plant before detailing.
How does cross-laminated timber perform?
Strength and stiffness
KLH declares, for plate action, a mean modulus of elasticity of 12,000 MPa parallel to the grain of the boards and 450 MPa normal to it, a characteristic bending strength of 24 MPa and a rolling shear strength of 1.2 MPa.[4] Mayr-Melnhof's EPD lists 12,000 MPa and a shear strength of 1.1 MPa for strength class C24/T14.[2] Design values depend on layup, so take them from the ETA annexes of the product you specify. We did not read those annexes.
Heat, air and moisture
All three documents declare λ 0.12 W/(m·K).[1][2][4] By our arithmetic a 100 mm panel alone gives about 0.83 m²K/W, so CLT needs a separate insulation layer in an envelope. KLH classes its panels as tight (class 4 under EN 12207) for air permeability, and Stora Enso states that a 3-layer panel was tested to EN 12114.[1][4] The manufacturers disagree on vapour resistance: KLH declares μ from 300 (dry) to 46 (wet) including joints, Mayr-Melnhof 20 to 50.[2][4] Check which value applies to your hygrothermal calculation.
All three limit use to service classes 1 and 2 of EN 1995-1-1, and panels are delivered at about 12 % moisture.[1][2][4] Check the class definitions against your site conditions, and plan protection during transport and erection. Mayr-Melnhof states that a weatherproofing or wood preservative may be needed for transport and storage.[2]
Sound
KLH refers to annex 6 of its ETA for airborne and impact sound insulation and gives no values in the declaration.[4] We found no sound-insulation figures in the documents we read.
How does cross-laminated timber behave in fire?
The declared reaction-to-fire class is D-s2,d0 in all three documents, and Stora Enso and Mayr-Melnhof add Dfl-s1 for floor use.[1][3][4] The material is combustible, so the question for a designer is how a layered panel chars and whether the bonded layers stay together.
Mayr-Melnhof declares charring rates that depend on the adhesive. For MUF-bonded panels: 0.65 mm/min for floors and roofs and 0.60 mm/min for walls where the top layer burns, rising to 0.76 and 0.71 mm/min when more layers burn. For PUR-bonded panels the top-layer rates are 0.65 and 0.63 mm/min, rising to 1.30 mm/min for floors and roofs and 0.86 mm/min for walls. The declaration footnotes only the floor and roof values for more layers burning (0.76 and 1.30 mm/min) as applying up to 25 mm of char, after which 0.65 mm/min applies up to the next glue line.[3] KLH declares fire resistance from REI 30 to REI 240 depending on panel structure or fire-protection cladding, with parameters in its ETA.[4]
Independent work points the same way. A 2021 review of large-scale tests found that adequate protection can make the CLT's contribution to a fire insignificant, that natural-fire charring rates and zero-strength layers are higher than common guidance, and that the adhesive influences performance, with delamination able to cause secondary flashovers in small enclosures.[9] A 2017 compartment test with exposed CLT on one wall and the ceiling saw the fire self-extinguish after the wood cribs decayed.[10] Both findings come from abstracts. According to the 2017 Brock Commons project factsheet (a promotional document from a provincial forest-products market agency), the project chose full encapsulation of the CLT and glulam with three to four layers of Type X gypsum board.[6] Ask the supplier for the fire test or ETA parameters that match your build-up.
How much carbon does cross-laminated timber hold?
What the EPDs show
Under EN 15804+A2, the tree's carbon uptake counts as a negative biogenic term. Stora Enso's EPD states 207.8 kg of carbon, or 762 kg CO₂e, in one cubic metre. Its A1–A3 GWP-total is −708 kg CO₂e, made of +52.6 fossil, −762 biogenic and +0.9 land-use change. Transport of 634 km on average adds +25.9 kg in module A4. In the EPD's main scenario, incineration with energy recovery, module C3 returns +782 kg CO₂e and module D credits −268.[1]
Mayr-Melnhof's EPD follows the same pattern for panels from its Gaishorn plant: A1–A3 GWP-total −642 (+117 fossil, −760 biogenic, +0.4 land-use change), C3 +766 and D −386 kg CO₂e per m³.[2] Stora Enso's EPD also gives reuse, recycling and landfill scenarios, which change module C3 and D.[1]
Read the number with care
The two EPDs differ in database (ecoinvent 3.8 and MLC 2023.2), data year (2020 and 2021), plants and scenarios, and Mayr-Melnhof does not declare A4 or A5, so do not rank them against each other.[1][2] Both are manufacturer-owned; Stora Enso's is verified by an individual verifier and Mayr-Melnhof's externally.[1][2]
On building level, a 2021 study of 127 frame configurations of 2 to 19 storeys found median whole-life embodied carbon of 119 kgCO₂e/m² for engineered timber frames, 185 for concrete and 228 for steel, with 36 % of the timber emissions, on average, arising after construction. The authors stress that the result distributions overlap, so efficient design still matters.[11] A 2019 cradle-to-gate study of CLT in western Washington found that sourcing lumber locally and using lighter species could cut GWP by up to 14 %.[12] Both are abstract-level readings, and the second concerns hypothetical North American mills.
Which standards and certificates apply?
EN 16351:2021 sets requirements for CLT, covers service classes 1 and 2 and excludes fire-retardant-treated CLT and CLT from reused timber. It replaced the 2015 edition from 15 March 2021.[5] In practice CLT reaches the EU market through a European Technical Assessment based on EAD 130005-00-0304: KLH cites ETA-06/0138 (OIB, 18.01.2021 on its declaration) and Mayr-Melnhof ETA-09/0036 (21.04.2023 on its declaration), both with notified body 1359, and Stora Enso cites ETA-14/0349.[1][3][4] The EOTA database, which states that it lists currently valid ETAs, shows all three: ETA-14/0349 (Stora Enso Wood Products Oy, version 6, issued 13 April 2026), ETA-06/0138 (KLH Massivholz GmbH, version 2, issued 17 January 2021) and ETA-09/0036 (Mayr-Melnhof Holz Holding AG, version 4, issued 20 April 2023), all by OIB under EAD 130005-00-0304, which EOTA titles "Solid wood slab element to be used as a structural element in buildings" (checked 8 October 2026). The two declarations of performance print the dates one day later (18.01.2021 and 21.04.2023). We read the register entries, the manufacturer declarations and a catalogue entry for EN 16351, not the ETA documents or the standard text, so we quote no ETA annex values. Declarations of performance are company statements under the Construction Products Regulation.[15]
Declared values compared
Values as printed in the documents we read. Stora Enso: EPD technical information; KLH: declaration of performance; Mayr-Melnhof: declaration of performance and EPD.
| Property | Stora Enso | KLH | Mayr-Melnhof |
|---|---|---|---|
| Reaction to fire | D-s2,d0; floors Dfl-s1[1] | D-s2,d0[4] | D-s2,d0; Dfl-s1[3] |
| λ | 0.12 W/(m·K)[1] | 0.12 W/(m·K)[4] | 0.12 W/(m·K)[2] |
| μ | Not stated | 300 (dry) to 46 (wet)[4] | 20–50[2] |
| Maximum thickness | 0.32 m[1] | 360 mm[4] | 360 mm[3] |
| Formaldehyde class | Formaldehyde-free adhesives stated[1] | E1[4] | E1[3] |
| Fire resistance data | Not stated in the EPD | REI 30 to REI 240 by build-up[4] | Charring rates 0.60–1.30 mm/min[3] |
Declared values are not test results from one laboratory and are not like-for-like. Use the product's own current declaration of performance for design.
Where is cross-laminated timber used, and what are the limits?
Walls, floors and roofs of timber buildings
Best for: architects · structural designers
Stora Enso lists internal and external walls, floors and roofs, and houses, apartment buildings and industrial and commercial buildings.[1] At Brock Commons the 5-ply floor panels sit on glulam columns on a 2.85 × 4.0 m grid and act as two-way slabs, which removes load-carrying beams.[6]
Exposed interiors
Best for: interior designers
Manufacturers offer non-visible, industrial and visual surface grades, and Stora Enso states that panel faces are always sanded.[1] Exposed surfaces still need the fire strategy and acoustic design of the building. We found no sourced data on finishes. The only indoor-air results we read are the Mayr-Melnhof chamber tests quoted in the fact sheet, which concern the panel and not a finished interior.
Product and industrial designers
Best for: product designers
We found no sourced data on CLT in product or industrial design.
Where has cross-laminated timber been used in real projects?
The three projects below are documented by owner or architect pages. We found no independent performance monitoring for them.
Brock Commons Tallwood House, Vancouver
Architect: Acton Ostry Architects · completed August 2017
An 18-storey student residence, 54 m high, with 17 storeys of mass timber above a concrete podium and concrete stair cores. CLT and glulam came from Structurlam. The project factsheet states the mass timber structure was completed less than 70 days after the prefabricated components arrived, about two months faster than a typical project of this size.[6]
Dalston Works, Hackney, London
Architect: Waugh Thistleton Architects · 2017
A ten-storey scheme of 121 dwellings for Regal Homes. The architect states it is made entirely of CLT, from external, party and core walls to floors and stairs, and that the approach cut construction deliveries by 80 per cent.[7]
Murray Grove, Hackney, London
Architect: Waugh Thistleton Architects · 2009
A nine-storey tower of 29 apartments for Telford Homes, built from prefabricated solid timber and completed within 49 weeks, per the architect's page.[8]
Where to buy cross-laminated timber in Europe
Suppliers selected by our editors and checked against company documents and certificate records where available.
| Supplier | Products | Country | Certifications |
|---|---|---|---|
| Stora Enso | CLT panels | Austria, Sweden, Czech Republic (four mills named in the EPD) | EPD S-P-09949, International EPD System, valid to 2028-10-06; registry shows status Valid. ETA-14/0349 (EOTA database: version 6, 13 April 2026). FSC licence C125195, certificate BV-COC-125195, status valid on the FSC database (8 October 2026); we did not re-confirm which mills are listed as sites. ISO 9001, 14001, 45001, 50001 and PEFC chain-of-custody are company claims.[1][13][15][16] |
| KLH Massivholz | KLH CLT | Austria (Teufenbach-Katsch; Bad St. Leonhard) | DOP_KLH_K_2025_v01_en and DOP_KLH_W_2025_v01_en, 18.08.2025; ETA-06/0138 (EOTA database: version 2, 17 January 2021; 18.01.2021 on the declarations), notified body 1359. We read no KLH EPD.[4][15] |
| Mayr-Melnhof Holz | MM crosslam | Austria (Gaishorn) | IBU EPD-MAY-20240070-IBA1-EN, issued 09.04.2024, valid to 08.04.2029, as listed in the IBU registry; DOP_MMH_201 of 10.04.2025; ETA-09/0036 (EOTA database: version 4, 20 April 2023; 21.04.2023 on the declaration), notified body 1359.[2][3][14][15] |
Certificate numbers are taken from the company documents and registers named in the sources. The KLH declarations and the ISO and PEFC statements were not checked on a certifier's record; confirm them before specifying. Listing policy: editorial selection, not paid.
Frequently asked questions about cross-laminated timber
What is cross-laminated timber made of?
Boards of softwood, mostly spruce, glued face to face in at least three layers with the grain of each layer at right angles to the next. Stora Enso and Mayr-Melnhof state polyurethane (PUR) adhesives; Stora Enso adds emulsion polymer isocyanate (EPI) for edge bonding and Mayr-Melnhof also offers melamine-urea-formaldehyde (MUF) for surface bonding. KLH cites EN 15425 for its adhesives.
What is the thermal conductivity of cross-laminated timber?
All three manufacturer documents we read declare 0.12 W/(m·K). That is a structural value, not an insulation value: a 100 mm panel alone gives about 0.83 m²K/W by our arithmetic, so CLT walls need a separate insulation layer.
Is cross-laminated timber fire resistant?
It is combustible: the declared reaction-to-fire class is D-s2,d0. Fire resistance depends on the build-up. KLH declares REI 30 to REI 240 depending on panel structure and cladding, and Mayr-Melnhof declares charring rates of 0.60 to 0.65 mm/min for MUF-bonded panels where only the top layer burns, and higher rates, up to 1.30 mm/min, where more layers burn. Protection and a fire design for your project are needed.
Is cross-laminated timber carbon negative?
At the factory gate, two EPDs show a negative GWP-total (−708 and −642 kg CO₂e per m³) because of stored biogenic carbon. Both EPDs model that carbon returning at end of life through incineration, so the net depends on what actually happens to the panels.
How large can a CLT panel be?
Declared maxima differ by maker: Stora Enso 16 m × 2.95 m × 0.32 m (3.45 m width on request), KLH up to 16.5 m long and 3.5 m wide at one plant, and Mayr-Melnhof up to 18 m × 4 m in its declaration of performance. Transport limits often decide the real size.
What is the difference between CLT and glulam?
In CLT the layers are crossed and glued into a panel that works in two directions. In glulam the grain of all laminations runs along the length of the beam, according to the Brock Commons project factsheet we read. Our glulam profile covers beams and columns.
Which standards cover cross-laminated timber?
EN 16351:2021 sets requirements for CLT, and the European Assessment Document EAD 130005-00-0304 is the basis for the European Technical Assessments cited by the three makers we read. We did not read the texts of either document.
The bottom line on cross-laminated timber
Cross-laminated timber is a well-documented structural panel with declared D-s2,d0 fire class, λ 0.12 W/(m·K) and verified EPDs that show stored carbon at the factory gate. Its limits are service classes 1 and 2, carbon figures that depend on end-of-life assumptions and fire performance that depends on the build-up. Ask each supplier for the current declaration of performance, the ETA annexes and the fire test that matches your project.
Related materials
Sources
- Stora Enso (2023). Environmental Product Declaration, CLT (Cross Laminated Timber) by Stora Enso, EN 15804+A2, declared unit 1 m³ at 12 % moisture (470 kg/m³), registration S-P-09949, The International EPD System, published 2023-10-06, valid until 2028-10-06; verifier Martin Erlandsson (IVL); data year 2020, ecoinvent 3.8; ETA-14/0349 cited. Source of GWP values, biogenic carbon, panel limits, adhesives, D-s2,d0 and Dfl-s1, λ 0.12. Manufacturer-authored, third-party verified; full text read (copy hosted by the Czech environment agency CENIA). The registry lists a later version (:003, 2024-04-02; see 13). We downloaded that registry file on 8 October 2026 and compared it with this copy: the GWP, biogenic carbon, technical-data and end-of-life figures are the same and only unit labels in the waste table differ. PDF (cenia.gov.cz)
- Mayr-Melnhof Holz Holding AG (2024). Environmental Product Declaration, Cross-laminated timber (MM crosslam), EN 15804+A2, declared unit 1 m³ (470 kg/m³, 11.5 % moisture), EPD-MAY-20240070-IBA1-EN, Institut Bauen und Umwelt (IBU), issued 09.04.2024, valid to 08.04.2029; external verifier Matthias Schulz; LCA by Daxner & Merl; plant Gaishorn, data year 2021. Source of GWP values (modules A1–A3, C, D), adhesives, declared technical data, size limits. Manufacturer-owned, externally verified; full text read. The minus signs of the results table are encoded as soft hyphens in the PDF text layer and were read directly; they agree with the stated sums. The copy downloaded from the IBU registry (see 14) is identical to this file. PDF (mm-holz.com)
- Mayr-Melnhof Holz Holding AG (2025). Leistungserklärung (declaration of performance) Nr. DOP_MMH_201, Brettsperrholz (MM crosslam), dated 10.04.2025, ETA-09/0036 of 21.04.2023, notified body NB 1359 (Holzforschung Austria), label 1359-CPR-0641 as printed. Source of strength-class range, adhesive types, charring rates, D-s2,d0, Dfl-s1, E1. Manufacturer declaration (company claims under CPR); German text read in full. PDF (mm-holz.com)
- KLH Massivholz GmbH (2025). Declaration of Performance Nos. DOP_KLH_K_2025_v01_en (Teufenbach-Katsch) and DOP_KLH_W_2025_v01_en (Bad St. Leonhard), KLH CLT, both signed 18.08.2025; ETA-06/0138 issued 18.01.2021 by OIB, EAD 130005-00-0304, notified body 1359. Source of mechanical values, dimensions, REI range, μ, λ, specific heat, air permeability. Manufacturer declaration (company claims under CPR); full text read. The ETA itself was not read. PDF (klh.at)
- Estonian Centre for Standardisation and Accreditation (EVS), catalogue page for EVS-EN 16351:2021, “Timber structures - Cross laminated timber - Requirements”: valid from 15 March 2021; replaces EN 16351:2015, withdrawn from the same date; three CLT types; service classes 1 and 2; excludes fire-retardant-treated CLT and CLT from reused timber. Independent catalogue page, read 8 October 2026; the standard text was not read. evs.ee
- Forestry Innovation Investment (Government of British Columbia market development agency for forest products), “Brock Commons Tallwood House” project factsheet (2017; file name dated April 2018), hosted by UBC Sustainability: owner University of British Columbia, architect Acton Ostry Architects Inc., Structurlam as CLT and glulam manufacturer, 18 storeys, 54 m, 15,115 m², completed August 2017, 5-ply CLT floors on glulam columns, concrete cores, encapsulation with Type X gypsum board, mass timber erected in under 70 days. Promotional government factsheet, not an owner publication; all 8 pages read; no performance monitoring data in it. PDF (sustain.ubc.ca)
- Waugh Thistleton Architects, project page “Dalston Works”: Hackney, London, 2017, 121 dwellings, client Regal Homes, ten storeys, built entirely of CLT including walls, cores, floors and stairs; states an 80 per cent reduction in construction deliveries. Architect's own page, read as page HTML on 8 October 2026; no carbon figure given. waughthistleton.com
- Waugh Thistleton Architects, project page “Murray Grove”: Hackney, London, completed 2009, client Telford Homes, nine storeys, 29 apartments, prefabricated solid timber, completed within 49 weeks. Architect's own page, read as page HTML on 8 October 2026; no carbon figure given. waughthistleton.com
- Ronquillo, G., Hopkin, D. & Spearpoint, M. (2021). “Review of large-scale fire tests on cross-laminated timber.” Journal of Fire Sciences 39(5):327–369. Independent peer-reviewed review. Abstract read via Crossref (bibliographic details confirmed there); full text not accessed. doi:10.1177/07349041211034460
- Emberley, R., Putynska, C.G., Bolanos, A., Lucherini, A. et al. (2017). “Description of small and large-scale cross laminated timber fire tests.” Fire Safety Journal 91:327–335. Independent peer-reviewed study. Abstract read via the Semantic Scholar API, bibliographic details confirmed on Crossref; full text not accessed. doi:10.1016/j.firesaf.2017.03.024
- Hart, J., D'Amico, B. & Pomponi, F. (2021). “Whole-life embodied carbon in multistory buildings: Steel, concrete and timber structures.” Journal of Industrial Ecology 25(2):403–418. Independent peer-reviewed study of 127 frame configurations of 2 to 19 storeys in engineered timber, concrete and steel. Abstract read via the Semantic Scholar API, bibliographic details confirmed on Crossref; full text not accessed. doi:10.1111/jiec.13139
- Chen, C.X., Pierobon, F. & Ganguly, I. (2019). “Life Cycle Assessment (LCA) of Cross-Laminated Timber (CLT) Produced in Western Washington: The Role of Logistics and Wood Species Mix.” Sustainability 11(5):1278. Independent peer-reviewed cradle-to-gate LCA. Abstract read via Crossref; full text not accessed. doi:10.3390/su11051278
- EPD International, library entry EPD-IES-0009949:003 (S-P-09949), CLT by Stora Enso: status Valid, valid until 2028-10-06, version date 2024-04-02. Registry page read as HTML on 8 October 2026: programme operator EPD International AB, PCR 2019:14 with c-PCR-006; the verifier is not named on the registry page but is named in the EPD PDF (Martin Erlandsson, IVL). The PDF downloadable from the registry is the version compared under 1. environdec.com
- Institut Bauen und Umwelt e.V. (IBU), published-EPD list on epd-online.com, entry for EPD-MAY-20240070-IBA1-EN, “Cross-laminated timber”, Mayr-Melnhof Holz Holding AG (PCR: Solid wood products): issue date 09.04.2024, validation date 08.04.2029. Independent registry entry, read 8 October 2026; the PDF downloadable from it is identical to the manufacturer-hosted copy cited under 2. epd-online.com
- EOTA, ETA database entries (opened 8 October 2026; the database states that it contains currently valid ETAs): ETA 14/0349 “CLT – Cross Laminated Timber”, Stora Enso Wood Products Oy, version 6, issued 13/04/2026; ETA 06/0138 “KLH CLT”, KLH Massivholz GmbH, version 2, issued 17/01/2021; ETA 09/0036 “MM crosslam”, Mayr-Melnhof Holz Holding AG, version 4, issued 20/04/2023; all issued by OIB under EAD 130005-00-0304 (AVCP system 1). The EOTA EAD list titles EAD 130005-00-0304 “Solid wood slab element to be used as a structural element in buildings” (adopted 10/04/2014, cited in OJ 2015/C 226/05). Independent register entries; the assessment documents were not read. 14/0349 · 06/0138 · 09/0036 · EAD list
- FSC, public certificate search (search.fsc.org; opened 8 October 2026), licence FSC-C125195, certificate code BV-COC-125195, holder Stora Enso Oyj Wood Products: status valid (re-checked 8 October 2026), chain-of-custody multi-site certificate; the first results listed sites in Finland, Poland, Estonia, Latvia, Lithuania, Germany, France, Australia and the United States, and we did not re-confirm the Austrian, Swedish or Czech CLT mills there. A chain-of-custody record does not show which products carry the FSC claim. Independent register. search.fsc.org