Fire stopping of penetrations

Why this is the weakest point in practice

The verification of a fire resisting structure is valid for the condition in which it was tested. As soon as a pipe, a cable bundle or a duct passes through it, what is there is no longer what was tested — unless the seal of the penetration is itself verified.

This is not a theoretical question: the most convenient route for fire and smoke is always the hole.

What determines the right seal

The verification of a fire stopping solution always applies to a specific situation:

• what structure it passes through (wall or floor, which build-up),
• what passes through it (metal pipe, plastic pipe, cable, cable tray, duct),
• at what diameter, and how many,
• with what sealing solution (fire stopping mortar, pillow, collar, wrap, coating).

A plastic pipe and a metal pipe, for example, pose completely different problems: plastic melts and leaves a clear path, so an intumescent collar is needed there to close the resulting void.

A light gauge steel specific

In a light gauge steel wall a penetration is not only a fire question but a vapour control question: the same hole punctures the vapour control layer. So the good solution is not to seal every penetration perfectly, but to have as few penetrations as possible.

That is what the service cavity logic serves: services run on the inner side of the vapour control layer, in a separate plane, and only cross the structure where it is genuinely necessary. Service cavity.

What the documentation needs

• The position and type of penetrations on the drawings.
• The chosen sealing system and its verification.
• After construction, a record of the installed seals — marked, so that what is where can still be identified later.

The last of these is not a formality: at a later alteration this is exactly the information most often missing, and that is when seals get broken without being reinstated.

The point

A fire resisting structure is only as good as its weakest penetration. Good design does not multiply the seals — it reduces the penetrations.