The rafters are just the skeleton — the roof's tightness is determined by the roof underlay, battens and roof covering. Here are the roof underlay's DUKO classes, ventilation requirements, battens according to roof tile and the details at roof edge, ridge and penetrations where most moisture damage occurs.
Introduction
When the rafters are up, the part of roofing work that determines if the roof stays tight begins: the roof build-up. From top to bottom it is typically roof covering (e.g. tile or concrete tiles), roof battens, spacing strips, underlay and — under the rafters — insulation and vapour barrier. Each layer has a function, and a fault in one layer often shows as moisture somewhere completely different.
Underlay — the roof's second skin
The roof membrane is a water-repellent membrane under the roof covering. It catches flying snow, driving rain, and condensation that penetrate past the roof tiles, and leads the water out to the gutter. Roof membranes are divided into two main types: vapour-open (products that let water vapour escape) and solid/vapour-tight membranes (e.g. boards with roofing felt) that require ventilation both above and below.
The principle is: the lower the roof pitch, the more complex the roof, and the more expensive/difficult a later replacement, the higher class the roof tiles must have. A simple, steep roof on a single-family house can manage with a lower class than a flat, tricky roof on a tall building. The specific class is looked up in DUKO's tool or from the manufacturer — it is not guessed.
Ventilation — why the roof must breathe
A roof must be able to get rid of moisture. Warm, humid indoor air that penetrates into the structure and condensation under the roof tiles must be ventilated away, otherwise moisture collects in timber and insulation causing rot and mould. Therefore most roofs are built with a ventilated air gap, and there must be free air inlet at the eaves and outlet at the ridge.
- 01Between the roof lining and battens, a distance piece (pressure-impregnated) is installed, which lifts the battens clear of the roof lining and creates a ventilated and draining gap — normally at least 25 mm.
- 02With diffusion-tight (fixed) roof linings, ventilation is required on both sides of the roof lining; with diffusion-open roof linings, the upper gap may be sufficient — follow the manufacturer's and DUKO's instructions.
- 03There must be free air intake at the eaves (without birds and insects entering — use bird screen/insect net) and ventilation at the ridge or with roof caps.
- 04Insulation must not pack the ventilation gap — use spacers so the gap remains free all the way to the hip.
Source: BYG-ERFA experience sheets on roof underlay, roof edge and ventilation plus manufacturers' installation guides. The precise gap height and ventilation area depend on roof type, pitch and roof underlay class.
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