MgO Board Fire Rating: Tests and What They Mean

An MgO board fire rating can mean three different results. Reaction to fire (Euroclass A1 or A2 under EN 13501-1, or noncombustible under ASTM E136) describes the board itself. Surface burning (ASTM E84, Class A) describes how flame spreads across it. Fire resistance in minutes or hours (ASTM E119, EN 1364-1) belongs to a tested wall or floor assembly, never to the board alone.

That distinction is where most confusion starts. Product listings often put “A1 non-combustible” and “up to 4-hour fire rating” side by side as if they were the same kind of claim. They are not, and buyers’ fire engineers know it. This guide explains what each test measures, which criteria apply, what decides the result in an MgO board plant, and how a producer gets boards tested for export markets.

What “Fire Rating” Means for MgO Board

The first two are reaction-to-fire properties of the material. The third is fire resistance, a property of a construction: studs, board layers, joints, screws, insulation and the way they are put together. When a buyer asks for the “fire rating” of your MgO board, ask which of the three they mean and which code their project is checked against.

Euroclass A1 and A2: EN 13501-1, EN ISO 1182 and EN ISO 1716

In Europe, reaction-to-fire classes are set by Commission Delegated Regulation (EU) 2016/364, which applies EN 13501-1. The two non-combustible classes are A1 and A2. For products other than floorings, the criteria are:

Source: Regulation (EU) 2016/364, Annex, Table 1. A “substantial component” is a layer of at least 1.0 kg/m² or at least 1.0 mm thick.

Three points matter for a producer:

1. A1 is a heat-content test as much as a furnace test. The PCS limit of 2.0 MJ/kg means the board can carry very little organic matter. Oven-dried wood has a gross heat of combustion of around 20 MJ/kg (USDA Forest Products Laboratory). As our own estimate, assuming the inorganic binder and fillers add no heat, organic filler with that value would use the whole A1 allowance (2.0 MJ/kg) at about 10% of board mass and the A2 allowance (3.0 MJ/kg) at about 15%, before counting resins, fiber-mesh coatings or surface finishes. The lab measures the real value; use the estimate only to see why filler dosing matters.
2. A2 needs a third test. A2 boards must also pass the single burning item test, which brings in smoke (s1 is the best) and flaming droplets (d0 means none). The UK requirement for external walls of relevant buildings is “A2-s1, d0 or A1” (Building Regulations 2010, regulation 7(2)), so an A2 claim without s1, d0 may not qualify there.
3. Layers and finishes are classified too. A decorative or coated face can be a separate component with its own PCS limit. If you add a UV-coated or laminated finish, test that product, not only the plain board.

ASTM E136 and ASTM E84: Noncombustible vs Class A

North American codes ask the same two questions with different tests.

ASTM E136 determines the combustibility of building materials. Specimens are exposed in a furnace at 750 °C (1382 °F), and materials that comply with the criteria in the standard’s Section 15 are typically classified as noncombustible; the standard notes that passing materials “are permitted limited flaming and other indications of combustion.” The acceptance criteria limit temperature rise, flaming and mass loss; check the current edition (ASTM E136-26 at the time of writing) for the exact figures before you plan tests.

ASTM E84 (also published as UL 723) is the “25-ft tunnel” test. A specimen 508 mm wide and 7.32 m long forms the top of the tunnel, and the lab records how far the flame front travels over 10 minutes and how much smoke is produced. The results are a flame spread index (FSI) and a smoke-developed index (SDI). Building codes use three classes: Class A for an FSI no greater than 25, Class B for over 25 up to 75 and Class C for over 75 up to 200, generally combined with a requirement that the SDI be less than 450 (USDA Forest Products Laboratory, Wood Handbook, chapter 18).

The two are not interchangeable. Class A is a surface-burning rating, and some combustible products achieve it. Noncombustible under ASTM E136 is the stricter claim, and it is the one code provisions for noncombustible construction ask for. The International Code Council’s MgO board standard, ICC 1125, reflects this split: it includes surface burning characteristics among its test protocols and has optional provisions for fire-resistance-rated construction and noncombustible recognition.

Fire Resistance Is a System Rating: ASTM E119, EN 1364-1 and EN 13501-2

Fire resistance is measured on full-size walls, floors or roofs in a furnace that follows a standard time–temperature curve.

• ASTM E119 covers walls and partitions, columns, beams, floors and roofs. Its three failure criteria are collapse of the element, passage of flame, and excessive temperature rise on the unexposed side (Wood Handbook, chapter 18). Where required, the fire exposure is followed by a hose stream test. ASTM states that variations in the construction of the specimen, such as size, materials and method of assembly, affect the result, and that the standard gives no information on constructions other than those tested.
• EN 1364-1 tests non-loadbearing walls, and the result is classified under EN 13501-2. For partitions, the classes are E (integrity), EI (integrity and insulation) and EW (integrity and radiation), with times such as EI 15, 30, 45, 60, 90, 120, 180 and 240 minutes (Commission Decision 2000/367/EC).

Because the rating belongs to the assembly, a “2-hour MgO board” does not exist. What exists is a tested wall: for example, a light steel stud of a stated gauge and spacing, one or two layers of board of a stated thickness each side, screws at a stated pitch, joints treated in a stated way and a stated cavity insulation. Change any of these and the result may change.

Published research shows how much the details matter. Rusthi et al. tested light gauge steel frame walls lined with two MgO board types available in Australia in three full-scale fire tests; the tests gave a fire resistance level of 30 minutes, and failure came from loss of board integrity through board cracking or opening of board joints. The authors examined board type, joint configuration and compounds, noggings, screw fastening and cavity insulation as factors (Fire Safety Journal, 2017). A board that is non-combustible can still crack or open at the joints before an hour is up; the wall design decides how long it holds.

For comparison with the gypsum route, see how fire-resistant gypsum board (Type X) is made.

How MgO Board Behaves in a Fire

MgO boards resist fire for two reasons: the binder and most fillers are inorganic, and the binder phases contain chemically bound water that is driven off as the board heats. The same chemistry sets the limits:

• Strength falls with temperature. In a 2025 study of fiber-reinforced magnesium oxychloride composites designed for cladding, compressive strength fell by more than 90% at 600 °C. The main binder phase, phase 5, was entirely converted to magnesium oxide before 600 °C. The material was non-combustible and resisted spalling, with good insulating behavior (Ahmad et al., Construction and Building Materials, 2025). In a wall test, that loss of strength shows up as cracking and joint opening.
• Chloride boards release hydrogen chloride. When magnesium oxychloride phases decompose at high temperature, the chloride leaves as hydrogen chloride and the residue is magnesium oxide (Cole and Demediuk, Australian Journal of Chemistry, 1955). Hydrogen chloride is corrosive and toxic. The EN 13501-1 and ASTM tests above do not classify it, but a fire engineer reviewing an oxychloride board may ask. A magnesium oxysulfate board has much lower chloride than an oxychloride board; check chloride content in the finished board as well as in raw materials.
• Moisture affects behavior in fire. A board that has absorbed water in service is a different product from the one tested. The moisture problems described in MgO board problems are a durability question, but they also matter for whether the board still performs as tested years later.

For the differences between the two binders, see magnesium oxysulfate vs oxychloride board.

What Changes an MgO Board Fire Test Result in the Plant

On our reference design, boards are made with light-burned magnesium oxide, magnesium sulfate solution, fillers such as wood chips, bamboo powder or calcium carbonate powder, and glass-fiber mesh, in thicknesses from 3 to 20 mm. Organic fillers are common in MgO boards, but every percent counts against the PCS limit. Decide the target class before fixing the formula. The full production flow is in the MgO board manufacturing process.

How a Producer Gets MgO Boards Fire-Tested

1. Name the market and the code. EU and UK buyers will ask for EN 13501-1 classification and, for walls, EN 13501-2. US buyers will ask for ASTM E84 and ASTM E136, and for an E119 assembly if the board is in a rated wall. ICC 1125 is the North American classification standard for MgO boards. Buyers elsewhere may reference one of these families or a national standard; ask which report their authority accepts.
2. Freeze the formula and the product range. Test reports cover the product as tested: binder, fillers, mesh, density, thickness and finish. Fix these first, and test the thicknesses and finishes you will actually sell.
3. Sample from production. Specimens should come from normal production after a full cure, not from a laboratory batch. Keep retained samples and the batch records.
4. Use an accredited laboratory, and for CE marking a notified body. A product not covered by a harmonised European standard can be CE marked through a European Technical Assessment issued against a European Assessment Document. Fire protective boards, for example, can be assessed to EAD 350142-00-1106. That EAD assigns the assessment and verification system by class: system 1 (notified product certification body, with factory production control inspection) for A1, A2, B and C products where “a clearly identifiable stage in the production process results in an improvement of the reaction to fire classification”, such as adding fire retardants or limiting organic material; system 3 for other products in those classes; and system 4 for products that do not need testing. Some gypsum plasterboards are classified without further testing under Commission Decision 2006/673/EC. That route exists only where a Commission decision covers the product type and its conditions; check whether one covers your exact product, and otherwise plan on testing your MgO boards.
5. Run assembly tests for fire resistance. An E119 or EN 1364-1 rating needs a full wall built to a stated design. A practical approach is to test one or two standard wall designs with a framing system supplier and publish them; a buyer who builds differently needs its own assessment.
6. Keep it valid. Reports and classifications apply to the product as tested. A change of magnesium oxide source, filler dosage, mesh or coating can invalidate them. Tie your change-control procedure to the test reports.

FAQ

Is MgO board fireproof?

No building board is fireproof. A well-made MgO board can be non-combustible, meeting Euroclass A1 under EN 13501-1 or ASTM E136, so it does not add fuel to a fire. But it loses strength as it heats, and a wall lined with it holds back fire only for the time its tested assembly achieved, for example 30 minutes in the Australian steel-frame wall tests by Rusthi et al.

Is MgO board A1 or A2?

It depends on the formula and the finish. A1 requires a gross heat of combustion of no more than 2.0 MJ/kg for the board and a pass in the EN ISO 1182 furnace test. Boards with more organic filler, coatings or laminates may only reach A2, which also needs the single burning item test. Ask for the classification report, not the brochure.

What is the fire rating of MgO board in hours?

A board has no hourly rating of its own. Hours or minutes come from ASTM E119 or EN 1364-1 tests on a complete wall, floor or roof assembly, and they apply only to that design: stud type and spacing, board thickness and layers, screws, joints and insulation. Ask for the tested assembly drawing behind any claimed rating.

Is Class A under ASTM E84 the same as noncombustible?

No. Class A means a flame spread index of no more than 25 in the ASTM E84 tunnel test, generally with a smoke-developed index below 450. Some combustible materials achieve it. Noncombustible is a separate result under ASTM E136, a furnace test at 750 °C. Many code provisions for noncombustible construction need ASTM E136, not just Class A.

Do MgO boards release toxic gases in a fire?

Chloride-based boards can. When magnesium oxychloride decomposes at high temperature, the chloride is released as hydrogen chloride, which is corrosive and toxic. Magnesium oxysulfate boards have much lower chloride than oxychloride boards. Check the chloride content of the finished board, and expect fire engineers on some projects to ask.

Planning an MgO Board Plant?

If you are planning an MgO board plant for export, fix the target fire classification and markets before the formula and the line. See our MgO board production line for the equipment, MgO board banned? The Denmark story for what buyers now ask, and the MgO board plant setup guide for the full planning sequence.

Sources

• Commission Delegated Regulation (EU) 2016/364 on the classification of the reaction to fire performance of construction products, Annex, Table 1 (as published on legislation.gov.uk): https://www.legislation.gov.uk/eur/2016/364/annex
• Commission Decision 2000/367/EC on the classification of the resistance to fire performance of construction products, Annex (as published on legislation.gov.uk): https://www.legislation.gov.uk/eudn/2000/367/annex
• The Building Regulations 2010, regulation 7 (England): https://www.legislation.gov.uk/uksi/2010/2214/regulation/7
• ASTM E136, Standard Test Method for Assessing Combustibility of Materials Using a Vertical Tube Furnace at 750 °C: https://store.astm.org/e0136-26.html
• ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials: https://store.astm.org/e0119-22.html
• USDA Forest Products Laboratory, Wood Handbook, chapter 18, “Fire Safety of Wood Construction” (FPL-GTR-282): https://www.fpl.fs.usda.gov/documnts/fplgtr/fplgtr282/chapter_18_fpl_gtr282.pdf
• International Code Council, ICC 1125-2026, Standard for Classification of Magnesium Oxide Board and Construction: https://shop.iccsafe.org/icc-1125-2026-standard-for-classification-of-magnesium-oxide-board-and-construction.html
• M. Rusthi, A. Ariyanayagam, M. Mahendran, P. Keerthan, “Fire tests of Magnesium Oxide board lined light gauge steel frame wall systems,” Fire Safety Journal 90, 15–27 (2017): https://researchportal.northumbria.ac.uk/en/publications/fire-tests-of-magnesium-oxide-board-lined-light-gauge-steel-frame/
• F. Ahmad, S. Rawat, R. Yang, L. Zhang, Y. X. Zhang, “Fire resistance and thermal performance of hybrid fibre-reinforced magnesium oxychloride cement-based composites,” Construction and Building Materials 472, 140867 (2025): https://opus.lib.uts.edu.au/handle/10453/189956
• W. F. Cole, T. Demediuk, “X-ray, thermal, and dehydration studies on magnesium oxychlorides,” Australian Journal of Chemistry 8(2), 234–251 (1955): https://doi.org/10.1071/CH9550234
• EOTA, EAD 350142-00-1106, Fire protective board, slab and mat products and kits (copy hosted by Kiwa): https://www.kiwa.com/globalassets/dam/kiwa-netherlands/downloads/ead-350142-00-1106-fire-protective-board-slab-and-mat-products-and-kits2.pdf
• Tecnalia, ETA-23/0206 (4 April 2025), example of a board classified without further testing under Decision 2006/673/EC: https://cms.tecnalia.com/uploads/2025/10/ETA-23-0206-of-04.04.2025-MAXIBOARD-25_EN_firmado.pdf