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Fire Resistance of our Concrete Blocks

Historically, buildings formed from masonry have proven time and again to be among the safest and most reliable fire-resistant building methods available. This is true both during the construction phase and for the entire lifetime of the building.

The fire resistance of walls in buildings is the most important of all performance properties in blockwork design. Provision must be made not only for the stability of the structure but also for the safety of the occupants.


All Thomas Armstrong lightweight, dense and Airtec Aerated blocks and achieve the very best possible resistance to fire rating of "Class A1" (BS EN 13501-1:2002). This European Standard supersedes and exceeds the former British Standard BS 6073 "Class 0" spread of flame rating.


Our Dense Concrete Blocks are manufactured from Class 2 aggregates and the Masonry and Lightweight Blocks from Class 1 aggregates.

  • Masonry Fire Detailing

    A new, authoritive detailed guide for safe fire detailing of masonry construction has been published.


    This simple booklet is both a guide to the current requirements of the Building Regulations and a reminder of what has been recognised for centuries, namely that masonry buildings are inherently robust for energy efficiency, fire resistance and good sound insulation.

    Masonry buildings are by their nature and materials resistant to fire. The designer or specifier should remember that this fundamental quality leads to simpler detailing and construction and this simplicity in turn benefits the thermal and acoustic detailing as well as the construction.

    Masonry is an excellent material for a ‘fabric first’ approach. When compared with frame constructions, masonry buildings generally have fewer layers and fewer materials performing separate functions within the wall build-up. However, all buildings contain cavities and some concealed spaces where fire can spread undetected. To minimise the dangers of undetected fires the Building Regulations generally require that cavities are provided with barriers to reduce the area of the void space and to contain any spread of fire. The requirements for masonry buildings are far less onerous than for framed constructions.


    This guide explains the simple measures that are required to comply with the Building Regulations for detailing fire barriers and cavity barriers, in particular Section 8: Compartmentation and Section 9: Concealed spaces (cavities) of Approved Document B Volume 2. The focus is on multi-occupancy residential buildings, but the principles also apply to houses and apartments, download here.

  • Cavity Barriers

    Cavity Barriers and Cavity Barrier closures are required to be installed in and around cavities to resist the spread of smoke and flame.


    In general, no continuous void should exceed eight metres in a single dimension measured in one plane, or 20 metres if the exposed surfaces within the cavity have Class 0 ratings. A selection of barrier materials are available:

    1. asbestos-free board which would give similar protection to that given by 9mm thick asbestos building board or asbestos insulating board (but not asbestos cement),
    2. plastered board at least 12.5mm thick, or
    3. steel at least 3mm thick, or
    4. timber at least 38mm thick, or
    5. wire-reinforced mineral wool blanket at least 50mm thick, or
    6. polythene sleeved mineral wool, or mineral wool slab, in either case under compression when installed in the cavity.


    Useful information is given in Building Research Establishment Digest 214 and 215, Cavity Barriers and Fire Stops, Part 1 & 2.

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