Sheathing Solutions: A Guide to Different Construction Boards
Introduction
Calcium silicate boards, fibre cement boards, gypsum boards, plywood, OSB, blended boards, and Portland cement are widely used in construction due to their ability to provide structural support, weather resistance, and a base for exterior finishes. While each type serves a similar function, they differ in composition, properties, and applications. Choosing the right material is essential for ensuring long-term durability, compliance with building regulations, and overall project efficiency. This document outlines the key differences, benefits, and drawbacks of each material, helping builders make informed decisions based on factors such as strength, durability, and moisture resistance.
In modern construction, a variety of boards and materials are employed to meet diverse project requirements. Calcium silicate boards, fibre cement boards, and blended boards that combine elements of both are popular choices alongside traditional options such as plywood, OSB (including fire-rated OSB variants), and gypsum boards. Even Portland cement plays a crucial role, serving as a key ingredient in many composite boards. While these materials share common purposes—providing structural support, weather protection, and a base for exterior finishes—they each have distinct compositions, properties, and applications. This guide explores the key differences, benefits, and drawbacks of each, helping builders select the most suitable solution for their projects.
General Terms
Although sheathing boards can be either structural or non-structural and provide stability to floors, walls, and roof assemblies, for the purpose of this exercise we are concentrating solely on wall sheathing.
In general terms, sheathing is a type of covering or supporting structure – much like a sheath for a blade – that fortifies, protects, and provides additional structural integrity to a building. In the case of sheathing boards (also known as render carrier boards), they not only offer structural support but also function as barriers against climate and weather, while serving as a base for exterior finishes.
Key Requirements of Sheathing Boards to Ensure Durability and Reliable Performance
Strength for Wind Load:
Sheathing boards must withstand high wind pressures, particularly in areas prone to gales and hurricanes. A board should exhibit the same wind load strength on upper floors (up to 50 storeys high) as at ground level. Materials such as plywood, OSB, fibre cement, and calcium silicate boards are commonly used due to their high strength and rigidity.Fastening Capability:
It is essential that a board is designed to allow for secure fastening to the substrate without cracking. This requires a careful balance between strength and flexibility.Weather and Climate Protection:
An effective sheathing board protects the structure against sun, rain, frost, ice, snow, wind, and temperature fluctuations. The boards need to be particularly impermeable to water, providing an effective barrier to moisture ingress.Fire Resistance:
For construction purposes, the boards should be fire-rated to A.Moisture Resistance:
High levels of moisture resistance are essential to prevent mould, rotting, and structural damage. Materials such as plywood, OSB (Oriented Strand Board), and cement fibre boards are commonly used due to their high strength and rigidity.
Types of Sheathing and Render Carrier Boards – Pros and Cons
Plywood & OSB
Plywood:
A versatile and strong board made from thin layers of wood veneer bonded together. It is suitable for high-wind areas but can be prone to warping and moisture issues.OSB (Oriented Strand Board):
Made from compressed wood strands and adhesives, OSB is strong and less expensive than plywood. However, it can swell in moist conditions and is not A-rated for fire resistance.Pros: Versatile, strong, suitable for high winds.
Cons: Not A-rated for fire resistance; moisture performance can be questionable; prone to warping and potential moisture ingress.
Fire-Rated OSB
A variation of OSB with enhanced fire resistance, making it more suitable for fire-prone locations. However, it is still not A-rated and retains the same limitations as standard OSB.
Pros: Suitable for fire-prone locations.
Cons: Still not A-rated; retains the limitations of standard OSB.
Gypsum and Gypsum-Based Boards
Lightweight and cost-effective, gypsum boards offer good mould and moisture resistance, although they are not structural. They require careful handling during installation to avoid moisture ingress.
Pros: Good mould and moisture resistance; lightweight; cost-effective.
Cons: Not structural; low pull-out strength requires prompt sealing and taping; care is needed during fixing.
Calcium Silicate Boards
Composed of lime, silica, and reinforcing fibres, calcium silicate boards are lightweight and resistant to heat, fire, mould, mildew, and pests. They offer excellent thermal insulation but can be brittle and have limited colour options.
Pros: High resistance to heat and fire; excellent thermal insulation; lightweight; resistant to mould, mildew, and pests.
Cons: Lower density compared to fibre cement boards; limited colour options; can be brittle if not handled properly.
Fibre Cement Boards
Manufactured and utilised for over 60 years, fibre cement boards made with Portland cement have a proven track record of withstanding various weather and climate conditions. They offer superior resistance to cracking when fixed with screws or nails to a building substrate. These boards are comparable in strength to OSB Cat 1 and offer high resistance to moisture, mould, and fire. They can also be cut using a saw.
Pros: Proven durability; superior resistance to cracking; high resistance to moisture, mould, and fire; can be cut using a saw.
Cons: Higher density; heavier; may require painting for aesthetic purposes.
Blended Boards (Calcium Silicate and Fibre Cement)
Fibre cement boards blended with calcium silicate offer excellent sheathing performance, provided they are installed with care. They deliver outstanding thermal insulation properties, high fire and moisture resistance, and are versatile and durable.
Pros: Combines the strengths of both materials; excellent thermal insulation; high fire and moisture resistance; versatile and durable.
Cons: The weight may increase costs; excessive exposure to water can affect performance; the extra calcium silicate may make them more prone to cracking when screws are fitted, potentially leading to moisture ingress and subsequent board failure.
Portland Cement
A key ingredient in concrete, mortar, and grout, Portland cement offers high compressive strength, excellent bonding properties, and durability. However, it requires proper curing and has a significant carbon footprint.
Pros: High compressive strength; versatile; excellent bonding properties; durable.
Cons: Requires proper curing; can be affected by environmental conditions; significant carbon footprint.
Best Suited for Sheathing Boards
Based on the data and properties, fibre cement boards are best suited for sheathing applications due to their increased resistance to cracking when fixed with screws or nails to a building substrate, as well as their excellent moisture and fire resistance. This combination ensures a secure fixing to the substrate, providing a robust and durable solution for various construction applications. Fibre cement boards made with Portland cement have been manufactured and utilised for more than 60 years, demonstrating a proven ability to withstand diverse weather and climate conditions.
Conclusion
Calcium silicate boards, fibre cement boards, blended boards, plywood, OSB, gypsum boards, and Portland cement each offer unique advantages and are suitable for different applications, depending on the specific requirements of the project.
Calcium Silicate Boards are known for their high resistance to heat and fire, excellent thermal insulation, and lightweight nature. They are also resistant to mould, mildew, and pests; however, they have a lower density compared to fibre cement boards, limited colour options, and can be brittle if not handled properly.
Fibre Cement Boards offer proven durability and superior resistance to cracking when fixed with screws or nails to a building substrate. They also provide high resistance to moisture, mould, and fire, and can be cut using a saw. Despite these advantages, they are heavier and denser, which may necessitate additional handling and may require painting for aesthetic purposes.
Blended Boards, which combine calcium silicate and fibre cement, harness the strengths of both materials, delivering excellent thermal insulation, high fire and moisture resistance, and overall versatility and durability. Nonetheless, their increased weight may raise costs, and excessive water exposure can affect performance. Furthermore, the added calcium silicate can render them more susceptible to cracking when screws are fitted, potentially leading to moisture ingress and subsequent board failure.
Plywood and OSB are versatile and strong, making them suitable for high-wind areas, but they are not A-rated for fire resistance and can be prone to warping and moisture issues. Fire-rated OSB offers enhanced fire resistance yet still retains some of the limitations of standard OSB.
Gypsum and Gypsum-Based Boards are lightweight, cost-effective, and provide good mould and moisture resistance. However, they are not structural, possess low pull-out strength, and require careful handling to avoid moisture ingress.
Portland Cement is essential in the manufacture of concrete, mortar, and grout, offering high compressive strength, excellent bonding properties, and durability. Its main drawbacks are the need for proper curing and its significant carbon footprint.
Based on the data and properties, fibre cement boards stand out as the best option for sheathing applications, ensuring secure fixation to the substrate and providing a robust, durable solution for various construction needs.
References
UK Government – Construction Products Regulations
Provides guidance on the regulatory framework for construction products, addressing performance requirements such as fire safety and moisture protection.
https://www.gov.uk/guidance/construction-products-regulationsUK Government – Building Regulations Approved Document B: Fire Safety
Details fire safety requirements for buildings, including fire classification and performance criteria for building materials.
https://www.gov.uk/government/publications/fire-safety-approved-document-bEuropean Commission – Construction Products
Outlines regulations and requirements for construction materials across the EU, providing context on assessing building product performance for durability, moisture resistance, and thermal performance.
https://ec.europa.eu/growth/sectors/construction/construction-products_enEuropean Committee for Standardization (CEN)
Develops and publishes European Norms (EN) that set performance standards for a range of construction products, including fire classification (EN 13501) and material performance (e.g. EN 13986 for building boards).
https://www.cen.eu/FEMA – Guidelines for Wind Resistant Construction
Offers guidance on ensuring that building materials and assemblies perform under high wind loads, supporting the discussion on wind load requirements for sheathing boards.
https://www.fema.gov/media-library/assets/documents/905?ver=1400211531135NIST – Building and Fire Research
Provides research findings and guidelines on the performance of building materials, informing best practices for ensuring strength, moisture resistance, and fire safety.
https://www.nist.gov/topics/building-and-fire-research