Flat Roofs- Technical Detail

Flat Roofs

Flat Roofs

Flat Roof Design

Build Up Options

Knauf Insulation Flat Roof insulation Rock Mineral Wool.

Warm flat roof single ply membrane

Knauf Insulation Flat Roof GRP TimberDeck

Cold flat roof - insulation between joists

Warm and cold roofs

The position of the thermal insulation in relation to the structural roof deck has created two basic categories of roof design:

  • Warm roofs

  • Cold roofs

A warm roof has the insulation above the structural deck, which is thus kept warm. The waterproof layer is above the insulation and a vapour control layer is placed below the insulation. An inverted/protected membrane roof is a form of warm roof in which the insulation is placed above the waterproof covering (which also acts as a vapour control layer).

A cold roof has the insulation placed below the structural deck, which thus remains cold. This construction relies on a vapour control layer working effectively, and in tandem with a continuous ventilated airspace to disperse the water vapour to atmosphere before it reaches the cold deck and condenses.

Roof loadings

The roof structure must be capable of supporting all of the associated loadings. In addition, the wind pressure acting on the roof should be assessed in accordance with BS EN 1991-1- 4:2005+A1:2010.

Drainage Guidance

Guidance on the number and size of rainwater outlets can be obtained from BS EN 12056- Part 3: 2000. The drainage and edge details should be installed as per the waterproofing.

Single ply

Concrete, metal and timber deck flat roofs can all be designed as flat roofs with a single ply roof covering. Installation of the roof covering should be in accordance with the manufacturers instructions with due consideration being taken of fixing methods and wind uplift.

An integral part of a warm deck flat roofing system is the installation of the vapour control layer on the warm side of the insulation. When insulation is placed over the deck, an insulant with a low rate of thermal expansion should be selected with adequate mechanical strength to resist the compressive and tensile loads during installation and in service.

Differential movement can be avoided by laying a single ply waterproof membrane and mechanically fixing it through the insulation to the deck, using an approved fully bonded system or leaving the membrane loose and covering with ballast. Refer to the Single Ply Roofing Association (SPRA) Design Guide for Single Ply Roofing for advice on alternative fixing methods.

Fixing of insulation/single membrane

Where the insulation is mechanically fixed rather than bonded to the roof deck, the wind uplift capability of the roof finish depends entirely on the mechanical fixings. When mechanically fixed, the roof membrane may be temporarily lifted by wind suction. It is advisable with mechanically fixed membranes to ensure that all insulation slabs or boards are fixed to the structural deck to avoid them moving out of position during windy periods.

When mechanically fixing or fully bonding the membrane the systems should be designed to withstand the wind loadings calculated in accordance with BS EN 1991-14:2005+A1:2010. Suitable mechanical fixings and adhesives are readily available for use with all types of deck.

control layers

The vapour control layer may take the form of a bitumen bonded felt, a nailed felt layer or a reinforced polyethylene sheeting. If in doubt please contact our Technical Support Team on 01744 766666.

Protection of insulation during installation

To prevent moisture entrapment on or in the insulation boards it is essential to protect them during laying and before the application of the roof waterproofing, or to lay the roof covering at the same time as the board.


BS 6229:2003 is the code of practice for flat roofs with continuously supported coverings. It recommends that they are inspected at least twice a year in spring and autumn as a minimum and also after events which may have had a detrimental effect on the roof covering, such as violent storms. Workmanship on flat roofs is covered by BS 8000: Part 4: 1989

Solution Optimiser and Pathfinder

Knauf Insulation Solution Product U-value Range (High) U-value Range (Low)
Warm Flat Roof

Rocksilk® Flat Roof Slab 

0.28 0.10
Cold Flat Roof

Rocksilk® Flat Roof Slab

0.27 0.13
Knauf Insulation Flat Roof insulation Rock Mineral Wool.


Thermal insulation and the flow of heat and water vapour through a construction are inextricably linked. The position of thermal insulation, vapour control layer and the need for ventilation must all be considered together.

The likelihood of condensation occurring will depend on the position of the insulation in the roof construction, the U-value of the roof, the relative humidity and temperature of the internal environment and whether an effective vapour control layer has been provided. To prevent surface condensation, the roof must be kept above the dew point of the internal air with an appropriate thickness of insulation, correctly located. To prevent interstitial condensation, an effective vapour control layer must be installed on the warm side of the insulation.

Contact our Technical Support Team