Warm Flat Roof - Technical Detail

Flat Roofs

Warm Flat Roof

Warm deck single ply membrane

Rocksilk® Flat Roof Slab

  • Significantly improves acoustic performance and reduces the drumming effect of rainwater on lightweight roofs

  • Compatible with a wide range of single ply membranes - no requirement for separating layer

Rocksilk® Flat Roof Slab is:

  • Non-combustible with a Euroclass A1 reaction to fire rating

  • BBA certified

Knauf Insulation Flat Roof insulation Rock Mineral Wool.


Rocksilk® Flat Roof Slab is a high strength, consistent density, rigid, rock mineral wool slab manufactured using Krimpact technology and specifically designed for use in flat roofing.

Rocksilk® Flat Roof Slab has sufficient compressive strength to be used on warm-deck flat roofs with limited access with concrete, timber or metal decks and subject to light maintenance traffic. If there is a likelihood of more severe traffic occurring, then additional precautions should be considered.

Typical construction

Rocksilk® Flat Roof Slab is suitable for use on all types of roof deck. The slabs are laid over a vapour control membrane and mechanically fixed to the deck. The insulation is overlaid with a single ply membrane which is mechanically fixed.

The advice of the single ply membrane manufacturer should be sought to confirm compatibility with specific membranes.


The vapour control layer should be laid over the structural deck and all joints lapped and sealed. Rocksilk® Flat Roof Slab is then laid in a staggered pattern from one corner of the roof. The orientation of the slabs is unimportant provided the constraints on spanning metal profiled decks are followed. The slabs and membrane should be mechanically fixed to the deck using specialist fixings such as those available from Ejot*.

Typically two fixings equally spaced along the length and in the centre of each slab are required to secure the system in place.



Rocksilk Flat Roof Slab has been tested to the UEAtc Technical Guidelines MOAT No 50: 1992, Thermal insulation systems intended for supporting waterproofing membranes on flat and sloping roofs. It is classified B for mechanical performance – roof only accessible for purpose of maintenance and may be used without any restrictions.

Compressive resistance

Our Rocksilk® Flat Roof Slab range has a compressive strength of 70-90 kPa.

Thermal performance

The thermal conductivity of Rocksilk® Flat Roof Slab is 0.039 W/mK.

Fire performance

Rocksilk® Flat Roof Slab is classified as A1 to BS EN 13501-1, which means that it will not contribute to the development of a fire or flame spread. Rocksilk® Flat Roof Slab has a melting point of over 1000°C. In the event of fire it will emit negligible quantities of smoke and fumes. The heat emission from the product is insignificant.

Typical Section

Typical Specification 

The vapour control layer and Rocksilk® Flat Roof Slab, thickness ......mm to be mechanically fixed to the roof deck.<

The single ply membrane should be applied over the insulation in accordance with the appropriate manufacturer’s recommended specification.

Alternatively, consult the National Building Specifications, Standard version clause/clauses… J42/425……………

Knauf Insulation specification clauses can be downloaded from knaufinsulation.co.uk/nbs

Insulation thickness (mm) Max span (butt along tops) Max span (butt over troughs)
50 170 120
60 180 140
70 200 160
75 210 170
80 220 180
160 220 200


Typical U-values of warm deck single play membrane roofs

U-value tables

Contact our Technical Support Team