How Strong is the MgO Board?

When selecting building materials for a project, strength is often a top priority. But “strength” isn’t just about how much weight a material can hold. It encompasses a range of properties, including resistance to fire, water, impact, and bending. If you’re exploring modern construction options, you’ve likely come across magnesium oxide, or MgO, board. A common question arises: How strong is the MgO board, really?

This article will break down the multifaceted strength of MgO board, comparing it to traditional materials and providing the factual information you need to make an informed decision for your next project.

   

What Exactly is MgO Board?

Before we assess its strength, it’s crucial to understand what it is. Magnesium oxide board is a type of panel product made primarily from magnesium oxide, often combined with a reinforcing material like fiberglass mesh and other mineral compounds. It serves as a versatile sheathing and cladding material, used for everything from exterior walls and soffits to interior tile backing and fire-rated assemblies.

Unlike traditional drywall or cement board, MgO board is known for its unique combination of properties, which contribute directly to its overall strength profile.

   

Breaking Down the Different Types of Strength

To truly understand how strong MgO board is, we need to look at it through several lenses.

   

Structural and Flexural Strength

This is the classic definition of strength: a material’s ability to resist bending or breaking under a load.

Comparison to Drywall: MgO board is significantly stronger and harder than standard drywall. It is more resistant to impact and crushing, meaning it’s less likely to suffer damage during installation or from everyday bumps and dings.

Comparison to Cement Board: In terms of flexural (bending) strength, high-quality MgO board is generally comparable to or can exceed cement backer board. This makes it an excellent choice for substrates where a stiff, stable surface is critical, such as under tile or stone.

Reinforcement: The embedded fiberglass mesh on both sides and within the board provides exceptional tensile strength, preventing it from cracking easily under stress.

In short: For its weight, MgO board offers high structural integrity, making it a reliable choice for both interior and exterior applications where a durable surface is needed.

   

Fire Resistance: The Ultimate Test of “Thermal Strength”

This is arguably where MgO board demonstrates its most impressive form of strength. It is inherently non-combustible.

Fire Ratings: MgO boards typically achieve a Class A (ASTM E84) fire rating, which is the highest possible. Some specific assemblies can provide fire ratings of 2 hours or more.

How it Works: Unlike gypsum drywall, which uses chemically-bound water to resist fire, MgO board is made of mineral compounds that do not burn. When exposed to extreme heat, it does not emit toxic smoke or contribute fuel to a fire. It will simply vitrify (harden further) and remain in place, acting as a critical fire barrier.

Key Advantage: This makes it an incredibly strong choice for fire-rated walls, ceilings, and protective wraps around structural elements in commercial and residential construction.

   

Moisture and Mold Resistance: Strength Against the Elements

Water is a primary enemy of many building materials, causing warping, rot, and mold growth, which severely compromises structural and air-quality integrity.

Impervious Nature: MgO board is highly resistant to water and moisture. It will not rot, warp, or swell when exposed to humidity or direct water contact, unlike wood-based sheathing like OSB.

Mold Prevention: Because it is inorganic and non-porous, it does not provide a food source for mold, mildew, or bacteria. This makes it an exceptionally strong choice for high-humidity areas like bathrooms, kitchens, basements, and for exterior sheathing in wet climates.

Comparison: It outperforms drywall (which turns to mush when wet) and holds its own against cement board in moisture resistance, often at a lighter weight.

   

   

Durability and Impact Resistance

Long-term durability is a key component of strength. How well does the material hold up over time?

Surface Hardness: The surface of MgO board is very hard and dense, resisting scratches, dents, and abrasions better than most other panel products.

Longevity: Being composed of stable minerals, it is not susceptible to degradation from UV light (when used as exterior sheathing under cladding) or pests like termites.

Dimensional Stability: It expands and contracts very minimally with changes in temperature and humidity. This stability prevents the cracks and seams that can appear in other materials over time, ensuring a longer-lasting finish.

   

MgO Board vs. Common Alternatives: A Strength Comparison Table

This table provides a quick, at-a-glance comparison of how MgO board stacks up against other common sheathing and substrate materials.

Property	MgO Board	Standard Drywall	Cement Backer Board	OSB / Plywood
Flexural Strength	High	Low	High	Medium (can vary)
Impact Resistance	High	Very Low	High	Medium
Fire Resistance	Excellent (Non-combustible)	Very Good (Non-combustible)	Excellent (Non-combustible)	Poor (Combustible)
Moisture Resistance	Excellent	Very Poor	Excellent	Poor (will swell/rot)
Mold Resistance	Excellent (Inorganic)	Poor (Organic paper facing)	Excellent	Poor (Organic)
Weight	Medium-Heavy	Light	Very Heavy	Medium
Workability (Cutting)	Good (score & snap, saw)	Excellent (score & snap)	Difficult (requires scoring)	Excellent (saws easily)

   

Practical Applications: Where MgO’s Strength Shines

Understanding its properties, it’s clear why MgO board is chosen for these specific applications:

Exterior Sheathing: Its moisture resistance prevents the rot and mold that can plague OSB, creating a more durable building envelope.

Tile Backer: In showers and wet areas, its strength, stability, and perfect water resistance make it a superior alternative to cement board or waterproof drywall.

Soffits and Fascia: Resistant to moisture, fire, and pests, it provides a long-lasting, low-maintenance solution.

Fire-Rated Assemblies: Used in walls, ceilings, and around structural members to meet and exceed building code requirements for fire separation.

Underlayment: Provides a strong, stable, and silent floor underlayment that resists moisture vapor transmission from concrete slabs.

   

Important Considerations and Potential Weaknesses

No material is perfect, and a balanced view is essential. To claim MgO board is universally “strong” requires acknowledging its potential drawbacks.

Weight: While lighter than cement board, it is heavier than drywall and OSB. This can make handling and installation a two-person job for larger sheets.

Quality Variation: The manufacturing process and recipe can vary between producers. It is crucial to source MgO board from a reputable manufacturer to ensure consistent quality and performance. Early-generation boards had issues with efflorescence and corrosion, but modern, well-made boards have largely mitigated these problems.

Fastening: Its hardness requires pre-drilling for screws near edges, similar to cement board, to prevent cracking.

Breathability: It is a vapor-permeable material, which is generally good for building envelope science, but this must be considered within the overall wall assembly design.

   

Conclusion: A Multifaceted Powerhouse

So, how strong is the MgO board? The answer is: exceptionally strong, but its greatest strengths are not purely structural.

While it provides more than enough flexural and impact strength for its intended uses, its true power lies in its fire resistance, moisture resistance, and durability. It is a material that strengthens the entire building system by protecting against fire, rot, and mold—threats that can compromise a structure’s integrity far more than a lack of load-bearing capacity.

When you consider strength in this holistic sense—as the ability to withstand the tests of time, elements, and disaster—MgO board emerges as one of the strongest and most versatile panel products available to builders and homeowners today. For anyone prioritizing safety, longevity, and resilience in their construction projects, it is a option that deserves serious consideration.