STRIP VENTILATION SYSTEMS
BY JOHN W. MITTENDORF
Your fire department receives an alarm at 1330 hours for a structure fire. You are the officer in command of a truck company responding to the alarm. On arrival, you observe a 60- by 200-foot, one-story minimall (taxpayer) with heavy smoke showing from an occupancy at one end of the structure, and you see dark smoke pushing from roof vents over two units adjacent to the involved store. You quickly conclude that the fire in the end unit has extended into the common attic space and will quickly expose the entire structure.
Your options are limited: You are the only truck company on the first-alarm response–the second-due truck is a mutual-aid response. For the critical initial stages of the operation, it`s only you, your driver, and one firefighter. You direct the firefighter to force entry into several units ahead of the extending attic fire as you and your driver ladder the roof and prepare to ventilate the building.
In this scenario, the truck officer has correctly identified a potential problem. The fire in the end unit has extended into the common attic space and will extend horizontally to the other end of the building unless it is stopped. When the officer and driver reach the roof, strategic objectives will be as important as tactical operations if the truck officer is to be effective within minimal time and staffing constraints.
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 do the following:
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 and 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 (or trench) 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/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 important, 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.
GENERAL ROOF VENTING TACTICS
Prior to a roof-cutting operation, perform/address these tactical and procedural considerations:
Ladder and approach from the uninvolved area. A minimum of two ladders should be raised away from or opposite the location of a fire. This allows personnel to start and return to the strongest portion of the building and their means of egress.
Ladder the strong areas of the building/ roof. Normally, the strongest portions of the building are at the corners. Avoid placing ladders over horizontal openings (that is, windows, doors, etc.). Other areas that offer strength are hips, valleys, and ridges.
Raise the fly of an extension ladder/aerial above a parapet or roof for visibility. If a ladder is a primary means of egress from a roof, make it easy to locate. Therefore, do not limit the extension of a ladder above a roof/parapet to three or four feet.
Deploy properly equipped and adequate personnel. Roof ventilation operations are simplified and safety and accountability increased when a minimum of two firefighters are used. Consider the following equipment the basic minimum necessary to accomplish roof ventilation operations:
complete turnout gear and SCBA;
pickhead axe (used for prying and as a backup for power
pike pole, trash hook, or other suitable tools for removing cut sections of roof decking; and
power saws. Historically, the rotary saw has been widely used as a viable roof ventilation tool although size, weight, and the “gyrosopic” effect of the blade often detract from its effectiveness. In applications other than metal-deck roofs, the modern chain saw has proven to be a superior roof ventilation tool because of its effectiveness and ease of use–which often translates into firefighter safety.
General features that make a chain saw an effective roof ventilation tool are:
minimum four-cubic-inch engine size, adequate for multiple layers of roofing material;
16- to 20-inch sprocket tip guide bar (cooler running chain and reach);
large air cleaner (increased time in smoky conditions);
muffler guard (usually a piece of aluminum on the front of the muffler, which minimizes maintenance and cleanup operations); and
carbide-tipped chain (superior to standard chains). A new carbide chain can successfully cut through 14-gauge steel without ruining the chain).
Read the roof. Before leaving a ladder and walking across a roof, personnel must take the time to observe the roof and any visible conditions. A few considerations are
What is the fire`s location and is fire showing through the roof?
Is the roof stable? Is a portion of the roof sagging? Are there heat blisters?
Does the roof have ventilators, vent pipes, or skylights, and are they issuing smoke?
What are interior/attic conditions? Communicate with interior fire attack crews and the incident commander.
Determine the type of roof. This can be easily accomplished by prefire planning or quickly removing a small piece of composition (or other material) from the roof covering only. This is easily done with an axe or power saw and will reveal the type of roof decking below the roof covering. For example,
corrugated metal indicates a metal-deck built-up roof with open web bar joists,
one- by six-inch sheathing indicates a conventionally constructed roof, and
plywood on a newer building is an excellent indicator of lightweight construction.
Determine the location and extension of fire. Prior to any roof ventilation, you must determine the location and/or extension of fire. With your knowledge of the type of roof, you can quickly determine the feasibility of a strip ventilation operation before leaving the route of egress (ladder). You can determine the location and extension of fire by
visual size-up (what areas of the roof are issuing fire or smoke?) and
small inspection/indicator openings (kerf cut, etc.), which can be cut with an axe or power saw in the roof decking and used to determine the location and extension of fire.
Consider the following four conditions showing from an inspection opening: fire; black, hot, pressurized smoke; white, lazy smoke; or nothing. Always remember to consider the smoke`s pressure, color, and temperature.
Sound the path of travel. Sounding with an axe, pike pole, trash hook, or other suitable tool in front of your intended path of travel will help verify the roof`s safety. Remember, don`t sound with your feet–they are connected to your body.
Work toward the ladder/means of egress. Ventilation cuts should be designed to start in the weakest portion of a roof (toward the fire) and finish in the strongest portion of a roof (away from the fire).
Keep the wind at your back. When possible, ventilation cuts should be planned to keep the wind at the back of personnel.
Cut only as deep as necessary. Unless otherwise necessary, ventilation cuts should be cut through roof decking only. Cuts deeper than roof decking increase the possibility of cutting through structural members.
Predetermine your path(s) of egress. Always know how to safely exit a roof. Generally, exit a roof from the same area you used to walk onto it. Never get cut off by fire from your means of egress.
STRIP VENT TACTICS
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.
The roof must be walkable. The strip vent operation is a power-saw operation. While the strip cut originated in flat-roof applications, experience has shown that it can be used successfully on pitched roofs, provided the pitch does not compromise firefighter safety.
Wood roofs–against the construction. Strip ventilation openings that are cut against the construction will require additional cuts and time to complete compared with strip ventilation openings cut with the construction. To cut against the construction:
Make two parallel cuts about three feet apart across the rafters and section of roof to be ventilated. Depending on staffing and equipment, these cuts may be made singularly or simultaneously.
Make cuts between the rafters every 16 or 24 inches, depending on the spacing of roof members. This produces small sections nailed to single rafters. These panels are easily hinged in the form of louvers or removed.
Remove or hinge the panels. Pry up with a suitable hand tool to remove or hinge the cut sections.
Wood roofs–with the construction (center rafter).
Make two paralllel cuts on either side of a rafter. These cuts should be near the outside rafters.
Make crosscuts between the parallel cuts about every four to six feet. This enhances removing plywood and/or multiple layers of roofing materials.
Remove the cut panels of decking. Each four- to six-foot panel is nailed to the center rafter and is easily removed or louvered.
Wood roofs–with the construction (between the rafters).
Make the first cut along the length of the rafter, as close as possible to the rafter without cutting it.
Mirror this cut along the second rafter, again as close to it as possible.
In this operation, the cut section of decking will fall into the building or attic. Although this method drops material into a building or attic, a strip is quickly completed (particularly when time is a primary concern) without personnel having to manually remove cut sections of decking material. If an attic is not encountered, ensure there are no personnel working below this operation.
Metal-deck roofs. Although the methods necessary to strip ventilate metal-deck roofs are similar to those of wood roofs, initiating a strip ventilation operation in a metal-deck roof is enhanced by a two-step process that consists of removing the insulation or composition covering and then removing the metal decking from the bar joists. This is due to the fact that metal cutting blades are not effective in cutting through the insulation/composition covering. For example, a strip opening cut against the construction can be accomplished as follows:
Using a chain saw or rotary saw with a wood-cutting blade, make two parallel cuts about three feet apart. It is only necessary to cut through the layers of composition and insulation. Let the teeth ride on top of the metal corrugations.
Make crosscuts every four feet between the parallel cuts. This ensures that the cut sections of composition/insulation are easily removed.
If the metal decking under the cut sections is cold, strike the sections to be removed with an axe or similar tool. This will loosen the tar or adhesive bond between the metal corrugations and the layers of composition/insulation, facilitating removal. If the metal decking is warm, the cut sections should be easily removed.
Remove the cut sections and place on the roof away from the fire.
Two parallel cuts are now made through the metal decking with a rotary saw and metal-cutting blade similar to cuts made in the insulation/composition layers.
Crosscuts are then made between the metal bar joists. This process ensures that each of the metal sections is attached to a single bar-joist only and is easily louvered and removed with minimal effort.
Although this task is easily accomplished, it is a time- and blade-consuming operation.
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. n
JOHN W. MITTENDORF is a retired battalion chief and 30-year veteran of the City of Los Angeles (CA) Fire Department. He conducts seminars on fireground operations; is the author of the books Ventilation Methods and Techniques and Facing the Promotional Interview, published by Fire Technology Services; and is a member of the Fire Engineering editorial advisory board.
(Below) The effect of a well-placed and well-performed strip vent is graphically demonstrated in this photo. The fire has traveled from right to left; note the right side of the strip is charred, not the left. Defensive roof ventilation openings should be considered only after offensive roof openings have been completed or have not been able to be completed. (Photos courtesy of author.)
An interesting application of strip venting: The left wing of this E-shaped hotel was well-involved and exposing the center wing. A strip was cut in the roof of the exposed side of the center wing. As fire from the wing of origin began to extend into the attic of the center wing, the strip directed the fire upward and away from the attic, allowing personnel sufficient time to save the rest of the structure.
(Top left) To determine roof decking material, simply cut out a small piece of roof covering. (Top right, bottom left) Small inspection or observation cuts, sometimes called kerf cuts, are a necessary part of roof venting operations, revealing fire extent, severity, and location and other important information.
The “distance-for-time” principle is key to strip vent operations. In this U-shaped hotel, the strip was cut the proper distance from the fire to allow its completion, limiting the fire to the exposed wing and saving the rest of the structure.
(Top left, right) When cutting wood roofs against the construction, the long horizontal cuts across the rafters should penetrate only the decking (not the structural members); short perpendicular cuts should be made between the rafters. This method requires more cutting than with-the-construction strip vents. The cut panels, now attached only to one rafter, may be removed or louvered. (Bottom left, right) For the with-the-construction “center rafter” strip vent, parallel cuts are made near the outside rafters and short perpendicular cuts are made every four to six feet. The sections then are attached only to the center rafter, for quick removal or louvering.
The between-the-rafter strip, with the construction, is a thinner, quick cut–the cut sections fall into the building. Make sure no firefighters are working underneath you when using this method.
For metal-deck roofs, parallel cuts are made through the composition. The roof covering is removed, exposing the decking. Parallel cuts are made through the decking, then crosscuts are made between the metal bar joists. Like the other methods, the deck section is attached only to one structural member and is removed expediently. Strip venting a metal-deck roof is a time- and blade-consuming operation; the 40-foot strip in the photos consumed three metal blades.