80 BUILD October/November 2008

RESEARCH

Relatively small fires can produce enough smoke and hot gases to quickly fill large-area single-storey buildings, concealing the origin of the

fire. This makes fire-fighting more difficult and dangerous, since the fire needs to be located. If the smoke and heat is sufficiently severe, the Fire Service may retreat and conduct an external fire attack. External attacks are often ineffective

as fire hose streams rarely reach the seat of the fire to extinguish it. The result can be more water damage, contaminated run-off and the likely loss of the building and its contents. The temperatures of the hot layer (see Figure 1) trapped beneath the roof may also be high enough to ignite flammable roof materials, or cause distortion or failure of unprotected roof construction.

The Building Act (2004) does not require

building owners to consider owners’ property protection. Consequently, most industrial build-ings have been constructed in the expectation that insurers will cover the fire loss.

Roof venting popular in industrial buildings

To assist Fire Service operations in large build-ings, the New Zealand Building Code Compliance Document for Fire Safety Clause C/AS1 places a limit on the maximum compartment floor area in buildings without sprinklers (typically 1,500 m2). This is designed to limit the total fire load in each fire compartment.

But subdividing large industrial buildings is often undesirable for functional reasons. No subdivision of the building is required according to Clause C/AS1, if at least 15% of the roof area (distributed evenly throughout the firecell) is designed for ‘effective fire venting’. Therefore, ,the roof fire venting option is popular.

Passive system simple

The purpose of fire venting is to allow for passive removal of smoke and heat from the building during a fire. This may:

improve access for the Fire Service to ❚

perform rescue operations and/or locate and control the firereduce the overall severity of the fire on the ❚

building structure via the removal of heat from the building.

A passive fire venting system relies on heating by hot gases to form an opening in the roof, with the buoyancy of hot fire products providing the driving force for removal of the hot gases (see Figure 1).

Effective roof venting during fireAlthough roof fire vents are used in New Zealand to remove smoke and heat from a building during a fire, there are currently no standards to ensure their ‘effectiveness’. A BRANZ research project aims to change this.By Amanda Robbins, BRANZ Senior Fire Research Engineer

Figure 1: Example of a roof fire, with and without roof venting.

A fire starts.

Hot smoke and gases begin to form a hot layer.

The hot layer continues to fill the building.

Smoke completely fills the building. Hot smoke and gases continue to vent through the roof, and the building doesn’t fill with smoke as fast.

The hot layer continues to fill the building – the hot smoke and gases are vented through the roof.

No roof vents Roof vents

BUILD October/November 2008 81

There are advantages with such a system. It is simple, effective in a wide range of fire conditions and independent from any power supply that may be disrupted during a fire.

Experimental work at the Building Research Establishment in the UK during the 1960s investigated roof sheeting materials for passive roof venting, including some material testing (such as for PVC). The results indicated that passive roof venting can be a valuable system in limiting fire spread and maintaining conditions for the seat of the fire to be located more easily. But only limited ‘large-scale’ and ‘full-scale’ testing was carried out, and only a few potentially appropriate roofing materials were tested.

International mechanical and passive systems

Mechanically operated smoke and heat venting systems for fire are established technology overseas, and various codes and standards exist (e.g. AS 2665, AS 2428, NFPA 204, UL 793 and UBC Standard 15-7) that may be suitable as performance standards in New Zealand.

Passive systems, such as dedicated units using drop-out panels, are less common, but these venting systems are also subject to international standards testing and performance criteria.

Definition of effective fire venting

There is no detailed specification or Standard currently referenced in the Building Code

Clause C/AS1 (2005) to ensure that fire venting is ‘effective’ or that appropriate materials are used. As a result, a range of materials are used with unknown passive fire venting performance. New Zealand is the only country using passive fire venting that does not incorporate a dedicated frame or unit, or have a demonstrated level of performance for fire venting purposes. This makes the performance and effectiveness of building designs citing ‘generic fire venting’ questionable.

Detailed guidance is needed on how to assess the effectiveness of roof venting systems, which will lead to appropriate specifications being drawn up.

Are roof panels another option?

Passive fire venting systems utilising the roofing material’s properties and installation of unprotected roof sheeting may be cheaper and easier to install than dedicated fire venting units. If designed appropriately, such systems could potentially be effective for venting fire (and subsequently be competitive with currently established dedicated fire venting systems), but appropriate system testing, performance criteria and design are essential.

Two important issues emerge. These are ensuring:

effective fire venting for future building stock, ❚

designed in accordance with the appropriate regulatory definitions and requirementsthat current building stock with proven ❚

ineffective fire venting (depending on the future definition and performance criteria) is appropriately upgraded for a fire event, or alternative fire-fighting measures are available in the event of fire.

Research is underway

BRANZ completed Stage I of a research project to investigate effective passive roof venting of fires in March 2007. Stage II is currently underway, and focuses primarily on the first issue raised above, as well as determining the full extent of the problem underlying the second point. The approach uses both experimental and modelling work to determine:

a formal definition for ‘effective fire venting’, ❚

including performance criteriawhether passive buoyancy-driven venting ❚

using roof sheeting provides ‘effective fire venting’ and, if so, appropriate test method(s) and criteria to ❚

qualify as ‘effective fire venting’.Results of this research will form a technical basis for recommendations to the Department of Building and Housing. Stage II is nearing completion, and results are expected in November this year.

This research has been funded by the Building Research Levy. Stage I results are in BRANZ study report SR165 available from www.branz.co.nz.