Structure with rear ventilation
The quality and service life of a roof covering made of zinc-titanium depends firstly on the design and execution of the roof structure as a whole and secondly on the zinc-titanium system itself.
Where the roof is insulated, a system incorporating rear ventilation is the only method recommended by NedZink B.V. for roofs made of NedZink.
If the roof is properly constructed, any corrosion of the zinc roofing materials caused by condensation water from the inside is virtually eliminated.
|From a structural point of view, the design of a roof covered with zinc-titanium is essentially the same as that of a facade. In this Technical Advice we have therefore only discussed roofs which have a pitch of between 3° and 90°. Flat or almost flat roofs with a pitch of less than 3°? should, if possible, not be covered with zinc-titanium unless the surface area of the roof is less than 15 m2, for example over dormer windows and canopy roofs.|
The construction principle is illustrated in fig. 1.
The components are identified beginning on the inside:
- Ceiling – any type, although it must be able to support the insulating material, or have the insulation attached to it.
- Damp control course made of metal or plastic film.
- Supporting rafters – generally made of wood.
- Insulation slabs, matting or foam between the rafters. It is even better if they can be made to run across under the rafters, although a different construction is required for this.
The thickness of the insulating layer depends on the material used and the degree of insulation required.
- Ventilated cavity: thickness depends on pitch of roof (see table).
- Timber boarding: If Zinc-Titanium is to be used, the preferred type of timber is unplaned boards no less than 22 mm thick.
There should be a 5 mm gap between the boards. If the roof pitch exceeds 45°?, the substructure may be of boards with gaps of up to 10 cm.
Nails must be countersunk to prevent contact with the zinc. Ideally, nails should be galvanized.
- NedZink the type selected for this type of roofing.
The principal options are: the roll cap system, the standing seam system, the panel system (diagonal finish) and the NedZink System.
How condensation water is formed
Fig. 2 Temperature curve and water vapour pressure in a ventilated structure
Roofs are stressed not only by mechanical forces but also by the physical aspects of a building. These include fluctuating temperatures, which may vary between – 20° C and + 80° C, and differences in air humidity on the inside and outside. In the commonest situation, where the internal temperature (Ti) is higher than the external temperature (Te), it is generally also found that the air humidity is greater on the inside than the outside (see fig.2). The difference in vapour pressure (Pi – Pe) causes vapour to be transported through the roof structure from the inside to the outside. If the roof is not constructed properly, condensation or frost will be formed on the inside of the relatively cold zinc-titanium.
Damage and prevention
Excessive condensation can be harmful in a number of ways. For example, it can damage the zinc-titanium on the inside as the result of corrosion, or lead to moisture damage on parts of the supporting structure (corrosion, wood rot or mould). Totally sealing the roof on the inside is not the answer: this cannot be done in practice, and in terms of moisture regulation in a building it is generally not desirable.
In order to achieve the desired movement of vapour and to avoid the risk of damage, two main steps must be taken:
- Fit a damp control course on the inside of the thermal insulation
- Include a gap on the external side of the thermal insulation to permit ventilation by external air.
A. Diffusion layer
This layer is necessary:
- To let through just enough vapour, but not too much, to remove excess vapour from the building (eg. vapour produced by the occupants). We recommend the selection of a material which prevents the transmission of vapour, having a minimum value of 10 (diffusion resistance (?) x thickness (s) ).
- To make the structure draught-proof and prevent air from the interior from flowing directly into the ventilation cavity. The cavity is in direct contact with the external air, meaning that an unpleasant current of air could flow into or out of the structure through any cracks or gaps.
The diffusion layer is not needed in all cases, eg. where the structure beneath the ventilation cavity already has a vapour-control value greater than 10. In such cases, however, the ventilation gap and the air apertures must correspond to the values given in the table, and protection must be provided to seal any cracks between the ventilation gap and the interior air space.
B. Ventilation cavity
The ventilation cavity must be open to the external air via ventilation apertures at the lowest and highest points of the facade or roof. The air must be able to flow between apertures without major obstacles. The required dimensions for the gap and the apertures are detailed in the following table:
Min. cross-section of ventilation
apertures (top and bottom) per
m² of roof surface area
3° – 20°
* The exact height of the ventilating cavity is considered per situation.
The advice given above is followed in most building plans. Roof and facade details illustrate clearly how these areas should be built. However, experience has shown that a number of details in a construction project may not be fully thought out, being left largely to the operatives on site. In such cases it often happens that, at certain points, the ventilation apertures are made too small – or sealed up completely – during the construction of joints between roof and facade, between different sections of roof, and on dormer windows, porches etc. We strongly advise therefore that the principles described above in the roofing and facade cladding details should be strictly followed throughout the whole project. This is the only way that the rear ventilation required can be incorporated into the structure.