Fire spread on the facade

Why the facade is a topic of its own

Compartmentation works inside the building. On the facade, however, fire can get round the floor from outside: flames from a window shoot up the outer face of the wall and ignite through the opening of the storey above.

This phenomenon exists with every structure, but the build-up strongly influences how severe it is. Two factors really matter: how much the materials contribute to the fire, and whether the build-up has a continuous vertical cavity.

The two risks

The combustible layer

If the facade build-up contains organic foam insulation or combustible cladding, it becomes a fire spread route itself. That is why the regulation sets requirements for the reaction to fire class of facade materials — and these get stricter with risk class and building height.

The ventilated cavity

The cavity of a ventilated facade is useful in normal operation: it carries moisture away. In a fire, however, it behaves like a chimney — the hot gas flowing upwards accelerates spread, and it spreads behind the cladding, hidden, where intervention is difficult.

That is why the cavity needs cavity barriers: horizontal elements that close the flow path in a fire.

What this means for a light gauge steel build-up

This is one of the points where fire safety feeds back directly into the choice of build-up.

• Where the regulation does not allow combustible insulation, mineral wool is the answer — the same thermal performance is achievable with it, only careful filling of gaps becomes critical.
• Where a ventilated facade is built, the position of the barriers has to be designed together with the build-up, not inserted afterwards.
• The detailing of window reveals belongs here too: this is where facade, insulation and the edge of the opening meet.

What to know about the order of design

The facade fire spread requirement depends on the height and risk class of the building. If this emerges after the build-up has been fixed, the insulation material has to be changed — which has thermal and thickness consequences, and so feeds back to wall thickness and the floor plan.

The detailed solutions for these requirements are contained in the fire safety technical guideline on protection against fire spread; applying it is voluntary but gives the presumption of compliance.

The point

On the facade, fire safety does not depend on the fire resistance of the structure but on what the build-up is made of and whether it contains a continuous cavity. Those two questions decide what can be built.