Metal roof covering wooden support
A. GENERAL
1. The traditional method
2. Warm roof system with metal roof covering
B. ROOF CONSTRUCTION WITH POWERDECK AND METAL ROOF COVERING
1. Powerdeck on timber deck with metal roof covering
POWERDECK - WARM ROOF SYSTEM WITH METAL ROOF COVERING
A. GENERAL
1. The traditional method
The traditional method (see fig. 1) for metal roofs is done according to the cold roof construction: there is outside air ventilation between the insulation and metal roof skin so that outside conditions predominate.
Fig. 1
1. Metal roof covering system raised seam
2. Timber deck
3. Ventilated air cavity
4. POWERDECK
5. Waterproof coating
6. Load bearing 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 now the cold roof system no longer needs to be used because there are also major disadvantages associated with it:
* Firstly, an extra supporting structure (panelling) is necessary. This has major architectural consequences, as a result of which the roof becomes more expensive in the first place. Moreover, the panelling system is very complex to construct.
* A second disadvantage is the extra height of the roof structure. A second underlying roof must be created under which there is an air cavity, which (depending on the pitch) may vary between 30 and 200 mm. This makes the roof thicker. This can cause problems with 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. Because of this the outside air, which is used to ventilate the air cavity, acts as source of moisture and not as a drying flow. This results in the formation of condensation on the underside of the metal roof skins. The smaller the pitch of the roof, the greater the undercooling and the more pronounced the undercooling condensation.
* Fourth there is a chance of wind circulation and air rotation around the insulation. Because of this the U value is no longer representative of the insulating capacity.
In short, the cold roof system ventilated with outside air has a number of major disadvantages that can be avoided with a warm roof system.
2. 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.
* system suitable for climate class I, II and III.
Pitch min./max. 5%/60°.
Fig. 2
1. Metal roof covering system raised seam
2. Vapour permeable, waterproof foil
3. POWERDECK
4. Waterproof coating
5. Load bearing structure
The warm roof system prevents 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 due to the lack of an extra support structure.
* Moreover, a warm roof is much compacter because there is no air cavity. Because of this, greater freedom is possible with regard to the design.
* The unpleasant condensation by nocturnal irradiation (undercooling) is prevented by not ventilating the metal roof covering.
The insulation material for this implementation (warm roof system with metal roof skins) must however satisfy a few requirements:
* Compressive strength: the insulation must resist the forces of the metal roof covering at the attachments on the one hand and walking on the roof during both during installation and maintenance on the other hand.
* Heat resistance: sufficiently high heat resistance (R) or low lambda value (λ), so that with a minimum thickness a maximum insulation level is reached.
* Fire-safe insulation.
POWERDECK is a material that satisfies these requirements, especially because of its:
* high compressive strength (average 1.5 kg/cm2 or 15 ton/m2)
* low λ value (0.024 W/mK): R = 2.5 m2K/W for a thickness of only 60 mm
* fire classification A1 according to RD 19/12/1997
B. ROOF CONSTRUCTION WITH POWERDECK AND METAL ROOF COVERING
1. Powerdeck on timber deck with metal roof covering.
System suitable for interior climate class I, II and III.
1. Base layer: wooden carrier (plywood, panelling, …)
2. Vapour control layer: we recommend a class E3 vapour control layer, for example a polyester reinforced bitumen type APP or SBS with cold seams or hot bonded.
In the case of panelling , a separation layer consisting of an uncoated glass membrane or polyester mat is installed first.
3. Thermal insulation layer Powerdeck format 1200 mm x 2500 mm or 1200 x 1000 mm, mechanically attached in advance with 4 screws per board to the wood base.
4. Mechanical attachment of the insulation board: screw and load distribution plate. These must satisfy 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; overlapping of the strips 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.