Roof venting is often utilised in large warehouses to remove smoke in order to reduce damage to a building and its contents, and to maintain access for fire-fighters. In New Zealand, the Compliance Document for the New Zealand Building Code C clauses recommends 15 % opening area for unsprinklered single floor buildings. This opening area is required to be designed for effective fire venting. There is no justification for why 15 % is required, and no definition of how fire venting qualifies as being effective.

Fire Dynamics Simulator (FDS) was used to simulate the performance of various roof venting strategies in two different-sized industrial warehouses (both larger than 1,500 m²) with a 50 MW fire with both a rapid and an extreme t³ growth rate. In particular, roof venting areas of 15 %, 10 %, and 5 % of the floor area were tested with each of the following inlet areas for make-up air: 100 %, 50 %, and 0 % of roof venting area. In each of these cases, the vents were treated as permanently-open holes in the roof.

It was shown that roof venting with 15 % geometric area is ample to provide and maintain tenability for fire-fighters. With sufficient inlet area for make-up air, smaller venting areas could also be employed.

Further simulations were run to test the effect of square-shaped vents that opened simultaneously at 100°C compared with square-shaped vents that opened sequentially at 100°C, 200°C, and 300°C, and strip-shaped vents that opened progressively as each portion of a vent reached activation temperatures of 200°C and 300°C. Vents that opened at 100°C were intended to represent mechanical vents, while vents opening at higher temperatures were intended to represent plastic sky-light or drop-out type vents. The activation temperature proved to be more influential than the opening sequence or shape: there was a significant advantage to be gained by having vents that activated at 100°C as opposed to 200°C or 300°C.

The role of downstands in aiding the effectiveness of roof venting was also investigated, with downstand depths of 10 %, 20 %, and 30 % of the ceiling height being simulated. Downstands were shown to be incredibly useful for exhausting smoke and hot gases, provided their installation was appropriately coordinated with placement of roof venting.

It is concluded that a clear definition of effective fire venting must not only include the area of roof venting, but equally important is the definition of required inlet area for make-up air, as it plays a crucial role in the effectiveness of the specified roof venting area. In addition, the clear aerodynamic area should be specified. This could be achieved by use of a discharge coefficient that describes the proportion of the roof venting area that is clear aerodynamic area for a particular material, vent, and geometric area.

Development of a clear definition of effective fire venting will help to determine how an economic fire protection system can be continued to be used, while going a long way to ensuring predictable and tenable conditions for fire-fighters in New Zealand.