Lightweight wood construction is the most common type of roof construction you’ll find on homes that have been built within the last 30 years or so. Why? Because today’s construction is driven by one factor and one factor alone: money. A lightweight truss can be built quickly at an off-site location, and it relies on a structural system for strength, not surface area.
The weak links of the truss system include the gusset plate (also called gang nails) and the lumber that makes up the top and bottom cords of the truss. Any breach in this system, such as direct fire impingement, contributes to early and rapid failure of the truss involved.
Homes today are filled with synthetic materials that promote a rapid temperature increase in the interior and contribute to flashover conditions. The quickest way to remove gases/materials that contribute to flashover conditions: Cut a well-placed ventilation hole.
When the tones kick off for a fire in one of these homes, ventilation must be rapid and effective; therefore, the location of the vent hole must be precise. Note: The best place to pre-plan how to vertically ventilate a structure is in the comfort of your firehouse, with a whiteboard, a couple of markers and a crew of eager firefighters. This will alleviate any confusion on the fireground and will ensure that important tasks are assigned to all responding crewmembers.
Step By Step
The tactical approach for a crew operating on a lightweight roof should be well orchestrated and, as I mentioned, begins with every member having a predetermined job. When ventilating, emphasis should be placed on three main tasks: 1) ensuring proper ladder placement on the unburned portion of the roof, 2) ventilating over unburned and/or unaffected trusses, and 3) standing on structural roof members that can support a firefighter’s weight.
The following steps allow for efficient size-up and a well organized vent operation.
- Keep it to a minimum: When working on a residential roof, keep the number of firefighters to a minimum to lessen the weight on the roof. Generally, there should be three personnel performing roof ops: a captain, a sawyer and a sounder. The captain should determine the tenability of the roof and where to begin ventilation. The sawyer should be competent with the saw and comfortable with the cut sequence. The sounder must know how to identify structural members and have the confidence to identify warning signs of a catastrophic event, such as a collapse.
- Read the smoke: Confirm that the fire is indeed a room-and-contents fire. Pressurized smoke from the gable ends usually means fire is present in the attic. If attic vents are present, use them as “report cards” to monitor the progress of the fire.
- Use smoke holes: As a ladder company pulls up to a house fire, it’s imperative that the company officer determine whether it’s a room-and-contents or if the fire has extended into the attic; however, fire conditions and therefore fireground tactics always change. If the fire starts out as a room-and-contents fire, personnel will ascend to the roof. At this point, the captain (or first person on the roof) should create a smoke hole. If flames come out of the hole, the fire has progressed to the attic, and you should descend from the roof. Important: In homes with lightweight trusses, heavily-involved attic fires should be deemed too risky for roof operations as early collapse will most likely occur.
- Remember, location, location, LOCATION: The roof crew should operate from the unburned side of the house and work toward cutting the hole directly over the room/contents that are on fire. Listen to the interior crew’s radio reports, as they may give a geographical location of the fire or identify which room is on fire (e.g. kitchen, master bedroom). Reading vent pipes and windows will also give crews an idea about the layout of the house.
- Select the right ladder: Pick the right size ladder, keeping in mind that you want the tip to be visible at least three rungs past the roof. Remember: Two-story houses can be deceiving. A 35′ ground ladder may be the ladder of choice to maintain a proper climbing angle and clear any overhangs that may be present.
- Check overhangs: Note the distance of the overhang to the load-bearing wall. This will ensure crews are walking on load-bearing walls–not the overhang. If smoke indication holes are cut, this must be done on the attic portion of the load-bearing wall to get a true smoke read.
- Locate a truss: This will become the sawyer’s safe area for the remainder of the operation and the center point for which the sawyer will operate their saw. The lead person on the roof can sound out a truss using an axe or a trash hook. They should then mark the truss so the sawyer can make a back cut to confirm the presence of the truss.
- Ascend toward the ridge: The sounder will work up the truss that’s been located at the load-bearing wall while remaining on that structural member. Once the location has been reached, the sounder steps to the neighboring truss (most designs are 2 feet on center) to allow the sawyer to make their ascent. The captain will monitor the roof conditions and radio traffic for the duration of the operation.
- Spread out: To limit the live load on any one given truss, it’s good practice for all three crewmembers operating on the roof to situate themselves over their own structural member. The center truss is reserved for the sawyer, while the outside trusses are occupied by the sounder and captain. The captain should have a good view of the fire side of the structure so they can monitor conditions and progression of the fire.
- Make the cut: While the captain and sounder are positioned on opposite trusses, the sawyer can cut a center rafter louver. If the hole needs to be bigger, extend it downward with construction. This will make it easier to pop out drywall if needed during a room-and-contents fire.
- Climb down: After a hole has been cut successfully, climb down off the roof in the same way you climbed up, ensuring everyone is still on a load-bearing wall. It’s not safe to walk “cross country” on a roof, and only 90-degree turns should be made while on the roof.
Conclusion
Although operating on a roof under any condition is inherently dangerous, a well-coordinated operation can make it safer and more effective. Remember: Never operate on lightweight trusses that are under (or have been under) direct fire impingement.
