Fire Protection of a Separating Floor - Technical Detail

Loft Converters

Knauf Insulation FLEXIBLE SLAB

  • Existing ceiling can be retained, avoiding disruption to rooms below

  • Provides thermal and acoustic insulation as well as fire resistance

  • Meets the requirements of Approved Document E for sound resistance of internal floors

Flexible Slabs are high performance non-combustible Rock Mineral Wool slabs with flexible edges on all four sides, manufactured using our revolutionary ECOSE® Technology, and are designed for use in multiple applications.

Flexible Slabs contain no ozone-depleting substances or greenhouse gases.  For further environmental information consult the relevant Environmental Product Declaration 


Knauf Insulation Flexible Slab is a multi-purpose rock mineral wool slab designed to be friction fitted in a wide range of acoustic, thermal and fire resistant applications.


Where the loft space of a two storey house is converted into habitable accommodation, the floor to the new rooms must have a minimum of 30 minutes fire resistance over any part of the escape route directly below.

This is often the case when, for example, the floor of the new room extends over a landing in the stairway enclosure below.  

The solution on this page provides the required period of fire protection without having to upgrade the existing ceiling. 

Typical construction

The use of Knauf Insulation Flexible Slab in a suspended timber floor achieves 60 minutes fire resistance, whilst allowing the existing ceiling to be retained.

To achieve 60 minutes fire resistance: 

  • The floor deck should be minimum 18mm thick tongued and grooved chipboard or timber boarding
  • The joists must be minimum 200mm deep and 37mm wide
  • The wire mesh should be stapled to the sides of the joists at least 40mm above the ceiling level
  • Any ceiling can be used


Fire performance

Knauf Insulation Flexible Slab is classified 
as Euroclass A1 to BS EN 13501- 1 and non-combustible to BS 476: Part 4: 1970 (1984), and Class 1 Surface Spread of Flame to BS 476: Part 7: 1997.


The acoustic performance of an internal floor containing 100mm of Flexible Slab (and 12.50mm standard plasterboard ceiling) will be at least 40dB and therefore, meet the requirements of Approved Document E of the Building Regulations. 

Technical specification

Lay 100mm Flexible Slab on 25mm chicken wire mesh, stapled to sides of joists at least 40mm up from ceiling level and at 600mm centres. The joists should be at least 200 x 37mm and spaced at not more than 600mm centres.  The flooring board to be at least 18mm thick.

Download the Knauf Insulation specification clauses.


After removal of the floor deck, line the voids in the floor with 25mm wire mesh, to form a support tray for the insulation. Staple the wire mesh to the sides of the joists, at least
40mm up from ceiling level.

Cut 100mm Flexible Slabs to size and lay onto the wire mesh support, so that there are no gaps between the insulation and the joists.

Butt joint the insulation slabs so there are no gaps between them. Fix a chipboard floor deck, at least 18mm thick, over the entire floor area.

Exova Test Report: CHILT/AO6041B
Revision B

While Knauf Insulation offer various insulation products into suspended ground floor applications, we would emphasise that site specific conditions should always be  considered on a case-by-case basis and any generic product recommendations must always be subject to an onsite evaluation.

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