Fire Protection of a Refurbished Loft Floor - Technical Detail

Loft Converters

Using Earthwool® Flexible Slab


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

  • Provides thermal and acoustic performance as well as fire resistance

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

Earthwool® 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.


Earthwool® Flexible Slab is a multipurpose
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 Earthwool® 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

Earthwool® 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 Earthwool® 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 Earthwool® 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.
Alternatively, consult the National Building
Specifications, Standard version  clause/clauses P10/250.

Knauf Insulation specification clauses can be downloaded here.


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 Earthwool® 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.

Contact our Technical Support Team

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