Mechanical fixation
General
The traditional method
Warm roof system with metal roof covering
Powerdeck on timber deck with metal roof covering
The traditional method (see fig. 1) for metal roofs is done using the cold roof construction: there is open air ventilation between the insulation and metal roof skin so that outside conditions dominate.
1.-2. Metal roof covering system raised seam
3. Timber deck.
4. Ventilated air cavity
5. POWERDECK
6. Waterproof coating
7. Supporting structure
Ventilation was necessary to prevent/limit surface condensation against the underside of the metal roof skins since they were not resistant to corrosion. Nowadays, most metal roof skin producers have developed a type with a coating on the underside, as a result of which they are 100% corrosion resistant. This means that the cold roof system no longer has to be used, unless there are also major disadvantages associated with this:
• First an extra supporting construction (boarding) is needed. This has major architectural consequences, which in the first instance results in the roof becoming more expensive. Moreover, the boarding system is very complex to construct.
• A second disadvantage is the extra height of the roof construction. A second underlying roof must be made which has an air cavity below it. This makes the roof thicker. This can cause problems for the design in some projects.
• The problem of condensation as a result of nocturnal radiation (undercooling) is the third disadvantage. The undercooling means that on a clear night the metal roof covering becomes colder than the outside air or, even worse, than the dew point of the outside air. This causes the outside air, which is used to ventilate the air cavity, to become a source of moisture rather than a drying flow. This results in the formation of condensation on the underside of the metal roof skins. The lower the slope of the roof, the greater the undercooling and the more pronounced the undercooling condensation.
• Fourthly, the risk of wind currents and air rotation around the heat insulation. Because of this the U value is no longer representative of the insulating capacity.
In short, a cold roof system ventilated with outside air has a number of major disadvantages that can be avoided with a warm roof system.
- Warm roof system with metal roof covering
With a warm roof construction (see fig. 2) all the components of the roof structure are installed directly onto the supporting structure without an air cavity.
• The warm roof system avoids all the disadvantages associated with the cold roof construction ventilated with outside air.
• Warm roof constructions are architecturally much simpler and cheaper than cold roof systems, because of the lack of an extra support structure.
• Moreover, a warm roof is much more compact because there is no air cavity. Because of this there is greater freedom possible with regard to the design.
• The unwelcome condensation due to nocturnal radiation (undercooling) is prevented by not ventilating.
1. Metal roof covering system raised seam
2. Vapour permeable, waterproof foil
3. POWERDECK
4. Waterproof coating
5. Supporting structure
The insulation material for this implementation (warm roof system with metal roof skins) must however meet certain requirements:
• Compressive strength: the insulation must be able to withstand the compressive forces of, on the one hand, the metal roof covering at the attachment points and, on the other hand, walking on the roof during both installation and maintenance.
• Heat resistance: sufficiently high heat resistance (R) or low lambda value (λ), so that a maximum level of insulation is achieved with a minimum thickness.
• Fire resistant insulation.
POWERDECK is a material that meets these requirements, particularly because of:
• high compressive strength (average 1.5 kg/cm2 or 15 ton/m2)
• low λ value (0.024 W/mK: R = 2.5 m2 K/W for a thickness of only 6 cm).
• fire classification A1 according to RD 19.12.1997.
Powerdeck on timber deck with metal roof covering.
System suitable for climate class I, II and III with a slope of min/max 5%/60°.
1. Base layer: timber deck (plywood, panels, …)
2. Vapour control layer: we recommend a class E3 water control layer, for instance polyester reinforced bitumen type APP or SBS with cold or warm bonded seams.
In the case of panels, a separation layer is applied first consisting of an uncoated glass membrane or polyester mat.
3. Powerdeck thermal heat insulation layer dimensions 1200 mm x 2500 mm or 1200 mm x 1000 mm, mechanically attached in advance with 4 screws per board to the timber deck.
4. Mechanical attachment of the insulation board: screw and load distribution plate. These must meet the requirements in accordance with UEAtc class II and have a minimum diameter of 4.8 mm. The backing plates must be compatible with the screws used.
5. A single-layer vapour permeable waterproof foil; strips overlapping by 150 mm.
6. Mechanical attachment of the metal roof covering: specific load distribution plate and sliding cleat.
7. Metal roof covering, fitted with the raised seam technique, secured with suitable attachment cleats.
For quality metal roof covering: see the instructions from the manufacturer concerned.