Overlay And Underlay System

Rockwool warm roof systems used to produce a warm roof construction in buildings with pitched roofs in excess of 15°, providing an effective means of insulating the roof space at rafter level and also offers good acoustic performance.

The Overlay and Underlay boards are vapour permeable and should be used in conjunction with a breather membrane on the outer face, thus preventing the need for ventilation of the roof void. They can be installed directly on a timber rafter construction, or the construction may include profiled steel or timber decking.

Overlay boards may be used without Underlay boards, depending on the U-value requirement. However it is preferable not to use Underlay without Overlay, to avoid any potential for condensation on the tops of the rafters.

System consists of high density rigid Rockwool slab overlay and semi-rigid Rockwool slab underlay.


The Overlay board is very compression resistant; however it is not a structural board suitable for foot traffic, and should not be designed to add to the structural stability of the roof.

Fire performance:

Incombustible to BS 476-4, rated Class 1 surface spread of flame to BS 476-7 and comply with the performance requirements of Class O of the Building Regulations.

Thermal conductivity:

  • Overlay λ: 0.038 W/mK.
  • Underlay λ: 0.034 W/mK.
  • Vapour Resistivity: 7 MNs/g.

General information


Overlay: 1200 x 1000 mm

Underlay: 1000 x 1140 mm

Uniclass 2015

Pr_25_57_06_56 Mineral wool slab insulationPrimary


J21/10 Warm deck roof coating

J21/425 Mineral wool warm deck roof insulation

J31/10 Warm deck roof coating

J31/334 Mineral wool warm deck roof insulation

J41/10 Built-up reinforced bitumen membrane warm deck roof covering

Show more
Specification data - Mineral wool slab insulation

Product Reference

Overlay And Underlay System




50 mm

60 mm

70 mm

80 mm

90 mm

100 mm


100 mm

125 mm

150 mm

Rebated edges

Not required


Standard product features


  • Overlay: 1200 x 1000 mm.
  • Underlay: 1000 x 1140 mm.

Similar products from AIM