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Flat Roofs
For practical and cost reasons, flat roofs are a common type of roof construction for most non-residential buildings.Overview
If a flat roof is your chosen method of construction then a range of installation options follow, which allow you the freedom to insulate not only thermally, but acoustically and with fire protection.
Knauf Insulation Products
Rocksilk Flat Roof Slabs are high strength, consistent density, rigid slabs made from rock wool, manufactured using the new 'Krimpact' technology, specifically designed for use in flat roofing.
Detailed Design Considerations
Warm and Cold Roofs
Thermal insulation and the flow of heat and water vapour through a construction are inextricably linked. The position of thermal insulation, vapour barriers and the need for ventilation must all be considered together.
The likelihood of condensation occurring will depend on the U-value of the construction, the relative humidity and temperature of the internal air and whether an effective vapour barrier 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 barrier must be provided on the warm side of the insulation.
The position of the thermal insulation related 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 covering 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 barrier).
A cold roof has the insulation placed below the structural deck, which thus remains cold. This construction relies on a vapour barrier working effectively in tandem with a continuous flow of ventilation air to disperse the water vapour that reaches the cold deck.
This section only considers warm roofs as follows:
- Warm roof with built-up felt or mastic asphalt roof covering
- Warm roof with single ply membrane
Metal Decks
The profiles of metal deck roofs need to provide sufficient support for the insulation. The greater the thickness of insulation, the greater the span it can accommodate on profiled metal decks. Where the butt joint of Krimpact Flat Roof Insulation slabs occurs along the top of the profiles then the maximum allowable span is (a), as shown in the table. Where the butt joint occurs over the troughs of the profiles then the maximum allowable span is (b), as shown in the table.
| Insulation Thickness (mm) | Max Span 'a' (mm) | Max Span 'b' (mm) |
|---|---|---|
| 140 | 400 | 280 |
| 120 | 360 | 240 |
| 100 | 300 | 200 |
| 70 | 170 | 140 |
| 50 | 170 | 120 |
Warm Roofs - Built-up Felt and Mastic Asphalt
Concrete, timber and metal deck flat roofs can all be designed as warm roofs.
Vapour Control Layers
The vapour control layer may take the form of a bitumen bonded, a nailed felt layer or a reinforced polyethylene sheeting. If in doubt please contact our Technical Advisory Centre.
Bitumen Felt Vapour Control Layers
The deck joints and the deck must be coated with bitumen primer when the vapour barrier is bitumen bonded to the substrate.
Bitumen felt vapour control layers should always be installed as per manufacturers recommendations. Care should be taken to ensure continuity at joints, upstands and roof penetrations. It is important to seal any exposed edges of the boards, e.g. at roof vents, upstands, etc, with waterproofing or hot bitumen in accordance with normal practice. To prevent moisture entrapment on or in the insulation boards it is essential to protect them during laying, before the application of the roof waterproofing, or to lay the roof covering at the same time as the board.
Warm Roofs - Single Ply
When insulation is placed over the deck and under the waterproof membrane (warm roof), it becomes an integral part of the waterproofing. A thermally stable insulant should be selected with adequate mechanical and bond 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 loose and ballasting.
Fixing of Insulation /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 fixing.
When mechanically fixed, the roof membrane may be temporarily lifted by wind suction. The pressure difference between the cavities above and below the boards will quickly equalise.
It is advisable with mechanically fixed membranes, to ensure that all boards are fixed to the structural deck to avoid the boards moving out of place.
When using fully bonded membranes, the insulation boards should be
mechanically fixed to withstand the wind load. The waterproof membrane may then be fully bonded to the insulation.
PVC Membranes
Where extruded polystyrene insulation boards are used, a separating layer must be placed between the PVC membrane and the insulation to prevent plasticiser migration out of the membrane. This may take the form of a glass fibre or polyester fleece with a minimum density of 100 g/m2, or a foil facing to the insulation board. An alternative is to use a membrane with an integrally bonded fleece backing. EPDM and polyolefine membranes do not require a separating layer.
Example Details for Single Ply Roofs
Specification clauses
1. Warm deck - Single ply membrane - Rocksilk Flat Roof Slab
The vapour control membrane and Rocksilk Flat Roof Slab thickness ......mm to be mechanically fixed*/ adhered* to the roof deck. (*delete as required)
The single ply membrane should be applied over the insulation in accordance with the appropriate manufacturer’s recommended specification.
2. Warm Deck - Built-up Felt and Mastic Asphalt - Rocksilk Flat Roof Slab
The vapour control layer and Rocksilk Flat Roof Slab* ......mm thick to be fully bonded, with hot bitumen*/PU adhesive*. (*delete as required)
The waterproof membrane should be applied in accordance with the appropriate manufacturer’s recommended specification.