What is WBDBO? (resistance to fire penetration and fire spread)
WBDBO stands for “weerstand tegen branddoorslag en brandoverslag”, the resistance to fire penetration and fire spread. It is the shortest time, in minutes, during which a fire cannot spread from one space to another. The term comes from Dutch building regulations, now the “Besluit bouwwerken leefomgeving” (Bbl). WBDBO is not a property of a single wall or door, but of every route a fire could take between two spaces.
Penetration and spread
- Fire penetration (branddoorslag): the fire passes straight through an element such as a wall, floor, door or service penetration. The fire resistance of those elements is decisive.
- Fire spread via the outside (brandoverslag): the fire travels through the open air, for example from a window up to the window above or across a roof to the neighbouring building. Distances between openings and the fire resistance of façade and roof parts are decisive.
WBDBO is the shortest of all possible routes. The method of determination is set out in Dutch standard NEN 6068.
| Fire penetration (branddoorslag) | The fire passes straight through an element such as a wall, floor, door or service penetration. The fire resistance of those elements is decisive. |
|---|---|
| Fire spread via the outside (brandoverslag) | The fire travels through the open air, for example from a window up to the window above or across a roof to the neighbouring building. Distances between openings and the fire resistance of façade and roof parts are decisive. |
Use
The Bbl sets WBDBO requirements between fire compartments and towards spaces needing extra protection, such as escape routes. Between fire compartments the basic requirement is 60 minutes, and in certain situations 30 minutes is enough. In the UK the same goal is reached through compartmentation, fire resistance periods of elements and boundary distance rules, rather than a single WBDBO value.
Common weak points
- Service penetrations through compartment walls without a tested fire-stopping system.
- Fire doors that are wedged open or not self-closing.
- Concealed cavities above suspended ceilings or in façades that let fire bypass a separating wall.
- Roof windows and dormers close to a party wall.
- Alterations that create new openings or penetrations.
Related terms
- Fire compartment
- Compartmentalisation
- Fire spread
- Smoke compartment
- Fire resistance
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