Approved Document B
Key Points
- Contains the relevant requirements of Building Act Regulations and guidance on ways of demonstrating compliance
- Intended for the purposes of life safety and includes guidance on the appropriate use of building materials with differing levels of fire performance
- Provides the definitions for non combustibility and limited combustibility products, through the use of the European and National Reaction to Fire classification systems
- Employs the concept of compartmentation as a method of restricting fire spread within a building
- Acknowledges the potential for fire to spread from one compartment to another and provides guidance on prevention, including the use of products of Limited Combustibility in roof constructions which pass across the head of compartment walls
Both non-combustibility* and limited combustibility are defined by, and form an important aspect of the guidance given in Approved Document B: Volume 2 - Buildings other than Dwelling Houses, April 2007 (ADB2).
ADB2 recognises that if a fire penetrates a roof near a compartment wall, there is a risk that it will spread over the roof to the adjoining compartment. A steel deck, whether perforated or un-perforated, is not itself capable of preventing heat being transferred from a fire below the deck to the layers above it.
Specific guidance is given for ‘built-up’ flat roof constructions where the insulation and weatherproof membrane is carried over the head of a compartment wall:
Paragraphs 8.29 and 8.30 of ADB2 state that, where a compartment wall is taken up to meet the underside of this type of roof deck construction, a zone of the roof 1500mm wide on either side of the wall should have a covering of designation AA, AB or AC on a substrate or deck of material of limited combustibility.
This guidance applies to all non-domestic buildings over 15m high and, irrespective of height, buildings such as hospitals, schools, warehouses, factories and shopping centres.
A promise set in stone wool
In practical terms, this means that any insulation used in these zones must be of limited combustibility unless other measures are taken, which are proven to maintain the fire compartmentation. Such measures may include the installation of a fire-resistant ceiling to prevent fire directly attacking the underside of the roof deck or a roof system, proven by fire resistance testing not to compromise the compartment wall.
It should be noted that plastic foam insulation products do not come within the ADB2 definitions of non-combustible or limited combustibility *. Rockwool’s Hardrock dual density roofing boards avoid the need for a fire-resistant ceiling to stop fire spreading over a compartment wall, providing significant cost savings. Where a ceiling is required - in schools, hospitals or commercial property for example - their use can allow the installation of non fire-resistant ceilings (ie suspended ceiling tiles in a grid) directly below the roof structure.
(*) Rockwool insulation products intended for use in the UK construction industry are CE marked, subject to the applicability of this scheme. For Rockwool products carrying the CE mark:
- All un-faced (plain) products are classified A1 in accordance with BS EN 13501-1 and are non-combustible materials as defined by the Building Regulations applicable to all parts of the United Kingdom.
- All products faced with aluminium foil or glass tissue fleeces are classified A1 and are non-combustible materials as defined by UK Building Regulations.
- Rockwool may supply products with other coverings such as chipboard, plasterboard or specialist facings such as open mesh scrim. Whilst these coverings may detract from the overall fire classification of any such composite product, the Rockwool insulation itself remains non-combustible.
