The strategy for roof ventilation operations will be offensive or defensive. The primary focus of offensive roof ventilation is to create a ventilation opening over or as close to a fire as possible, as safety permits. This type of ventilation is designed to vertically channel a fire and its by-products, limit horizontal extension in a structure, remove heat and smoke from a structure, minimize the potential for flashover/backdraft and increase the safety of fireground operations.
If possible, offensive ventilation should be conducted before defensive ventilation and can be accomplished using natural construction openings such as skylights, scuttle covers, bulkhead doors, etc. or by cutting an opening over the fire area with a power saw and/or an axe.
It is important to ensure the proper location of these openings in relation to the fire area; a misplaced vertical vent opening will quickly pull heat, smoke, and fire toward uninvolved areas.
If offensive roof ventilation operations have been completed, if they cannot be performed for safety reasons, or if fire already has self-vented through the roof, defensive roof ventilation operations should be completed if necessary. The purpose of defensive roof ventilation is to create an opening ahead of a horizontally extending fire to change the horizontal direction and extension of fire, heat, and smoke to a vertical direction, thereby reducing or eliminating the horizontal fire spread.
Defensive roof ventilation is usually accomplished by the strip vent. The strip vent is a long, narrow opening in the roof decking, generally from exterior wall to exterior wall (or fire wall to fire wall) and approximately three feet wide, ahead of the horizontally extending fire. This opening allows you to strategically channel and redirect the fire, slowing horizontal extension and facilitating knockdown.
With a strip vent you are, in effect, “drawing your line in the sand” against the fire. Usually, committing to a strip vent means that you have a well-advanced structural fire that is moving horizontally over a large area at such a rate that you cannot stop it with offensive venting tactics, given on-hand resources. (Editor`s note: The term “defensive” ventilation is convenient because it rightly differentiates between the strip vent and other methods of roof ventilation. However, there is no connection between defensive ventilation and an overall defensive fireground strategy. The strip vent is only “defensive” from the aspect of “letting the attic fire come to you.” Venting of any kind is inherently an aggressive operation. Likewise, effective strip venting requires aggressive, offensive tactics by engine and truck crews operating from interior positions – for example, pulling ceilings and knocking down fire in adjacent occupancies or exposures, etc.)
The strip vent has been used successfully as the primary vent opening in a variety of fire situations in a variety of structure and roof types, but all these successful operations shared common factors: The fire was such that offensive venting either could not be accomplished or had limited effectiveness, fire was in control of a large area of common attic or cockloft space, an aggressive interior attack still could be made from unburned or partially involved portions of the structure, there was a significant attic area over which the fire had not spread, the strip vent could be made in an effective and timely fashion, and a strategic decision by the commanding officer was made to “sacrifice” the heavily involved portion of the structure to save the rest.
Many factors influence the strategic decision to vent defensively. These include building and roof construction, fire conditions, fire load, manpower, building dimensions, firefighter experience and training, and so on. Time is critical. The officer must ask himself, given my resources, the extent of the fire, roof construction, personnel safety, and so forth, can my personnel make a 30- or 50- or 70-foot-long, three-foot-wide opening in the roof in the time it will take the fire to reach that point? What is my “return on investment” for this tactic? Will a sizable vertical vent (offensive) opening as close as possible to the fire area delay fire spread significantly and, most importantly, improve conditions such that personnel can operate effectively and safely in interior positions? The officer in command must weigh many factors. Communication with fireground companies is essential to acquire the needed information on which sound decision making is based.
Consider the principle of distance for time. Strip ventilation can be a time – and personnel – consuming operation. Therefore, if strip ventilation is necessary, place enough distance between the extending fire and the strip operation to allow the strip to be completed before the fire can travel past the strip opening.
Consider timing in strip ventilation operations. Since strip ventilation operations can be a time – and resource – intensive operation and an opening can accelerate the travel of fire toward its location, conduct strip operations as two distinct operations. The first operation is to cut the strip; the second is to open the strip.
Coordinate interior attack operations with strip ventilation operations. To be successful, strip operations also require the ceiling under (or as close as possible to) the strip opening be removed to allow access for a hoseline into the attic to extinguish the attic fire. This operation requires coordination and communication between roof and interior personnel.
Make sure the strip cut is made from wall to wall. If not, fire could pass around the ends, destroying the vent`s effectiveness.
Each strip ventilation method has its advantages and disadvantages. Using a particular method will depend on the type of incident and roof, staffing, individual preference, and your ability – which is developed by training and experience.
By John Mittendorf
John W. Mittendorf is a retired B/C and 30-year veteran of the LAFD. He is the author of the books Ventilation Methods and Techniques and Facing the Promotional Interview.