A timber façade is a rain screen, not a seal. Here are the cladding types, the ventilated cavity with batten spacers, timber durability and use classes (DS/EN 335 and 350), fixing allowing for movement, and the details where moisture otherwise penetrates.
Introduction
A ventilated timber façade is built on a simple principle: the outer timber cladding is a rain screen that stops most water, and behind it a ventilated cavity ensures that any moisture that does get in — or comes from inside — can dry out. The façade should not be hermetically sealed; it must breathe. If wood is allowed to stay wet without drying, it rots.
Clothing types
Timber cladding comes in several patterns, each covering and draining differently. The choice is both aesthetic and technical: how the boards overlap and which way the water is led.
- 01Horizontal on rabbet (overlap): boards are laid horizontally and overlap downward like scales, so water is always led out over the underlying board — a classic, very weather-resistant solution.
- 02Vertical on edge/vertical with barge boards: the boards stand vertically; the joints are covered by overlap or a barge board, so water runs straight down.
- 03Batten-covered cladding (board-on-board): battens with gaps and overlapping trim boards that hide the openings — gives a deep, shadowy facade and good space for movement.
- 04Tongue and groove (shiplap): boards are joined with tongue and groove; suitable, but the joint must be turned so it doesn't trap water — typically tongue and groove cladding is not recommended horizontal outdoors without care, because the groove can collect water.
The ventilated cavity
Behind the cladding are spacers that lift the boards clear of the wind barrier and create a ventilated cavity. Air must be able to flow freely in at the bottom and out at the top, so moisture is carried away. Without free air flow, the cavity becomes a moisture trap.
With vertical cladding, double battens (crossed strips) may be needed so air can pass behind the vertical boards. Insect mesh/bird netting in the openings keeps pests out without blocking air.
Choose wood according to the environment — use and durability classes
Exterior timber is exposed to moisture and biological decay, and must be chosen accordingly. Two standards govern the choice: DS/EN 335 defines use classes for the environment the timber is in, and DS/EN 350 classifies timber species' natural durability against fungal and insect attack.
| Standard | Describes | Relevance for facade |
|---|---|---|
| DS/EN 335 | 5 use classes based on moisture exposure | Facade over terræn = typisk brugsklasse 3 (fugt, ikke jordkontakt) |
| DS/EN 350 | Natural durability of timber species (fungus/insect) | Choose durable heartwood or protect the timber |
| DS/EN 351 / impregnation | Pressure-treated timber | When the species is not durable enough in itself |
Source: DS/EN 335 (use classes) and DS/EN 350 (durability classes), see trae.dk – Durability. A facade is an external, ventilated surface above ground level without permanent soil or water contact — this typically corresponds to use class 3. Choose either a timber species with sufficient natural durability (heartwood) or properly impregnated/protected timber; sapwood of non-durable species without protection has no place on a facade.
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