Sacrificial timber

The term ‘sacrificial timber’ refers to timber in a structural assembly that is intentionally oversized in order to enhance its fire resistance. Timber burns at a regular and measurable rate, so exposed timber sections can be designed with an additional 'sacrificial' element (ie increased width and/or depth) that will char slowly and protect the inner, structural timber from fire damage.

Sacrificial timber can be a design solution if a solid timber member is to be exposed – either fully or partly – or if full fire resistance cannot be provided by plasterboard (or other) fire insulating materials.

The outer surface of a timber element will typically char in a fire, with temperatures in excess of 350°C necessary for flaming to occur. The charred portion of the timber then acts as an insulator and, although some will be irreparably damaged (or ‘sacrificed’), the core timber retains its stability and structural integrity.

The density, and therefore the charring rate, of the timber element will determine the required quantity of sacrificial timber. BS EN 1995-1-2:2004 provides charring rates for different timber materials for the purposes of informing initial design.

Notional charring rates (the charring rates apply to each face exposed to fire):

For example, if a softwood element is exposed on all four of its sides to 20 minutes of fire:

2 x 20 x 0.8 = 32 mm

The width and depth of the element would require 32 mm of sacrificial timber.

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[edit] Related articles on Designing Buildings Wiki

• Fire.
• Fire protection engineering.
• Fire resistance.
• Fire-separating element.
• Fire-stopping.
• Timber frame.
• Timber framed buildings and fire.
• Timber preservation.