Vertical Venting of Private Dwellings
Volunteers Corner
We ended the March Volunteers Corner with some thoughts on horizontal ventilation, a defensive tactic which we’ll discuss more next time. For now, let’s concentrate on vertical venting, an offensive move which will take place earlier.
It does two things: relieves the upper floors of the products of combustion and stops or controls the horizontal movement or mushrooming effect of the fire, stopping smoke, heat, and fire from entering adjoining spaces. In essence, vertical ventilation buys time for victims who might be trapped or next to the fire.
It’s the same principle on which a fire in a fireplace works. Before a fire is started, the owner will open the draft damper in the chimney to create an unobstructed, vertical escape for the products of combustion. Because the chimney is built directly over the fire box, the smoke and convected heat escape harmlessly to the exterior. But if the owner fails to open the damper, the smoke and convected heat will mushroom within the firebox, look for another escape (the hearth opening), and fill the room with smoke.
The exact same process occurs with an uncontrolled fire. The mushrooming will continue until you can open vertically above or the fire supplies its own opening by burning through the structure overhead.
Vertical ventilation is performed by creating an opening or using an existing opening—such as a skylight— to provide a channel from the fire area to the atmosphere. Skylights are becoming very popular in the construction of private dwellings, and will give you not only a good vent but a very fast one.
If the natural light that a skylight provides must pass through another space, such as an attic or cockloft, a shaft is built from the skylight to the room it serves so the light isn’t diffused before it gets there. The sides in this shaft are called returns.
At the bottom of the shaft, there will be shades, blinds, or a piece of glass serving as draft stops or sun screens. If not much smoke escapes after the skylight is removed, the shaft should be probed with a long tool, such as a 10-foot pike pole, and the sun screens pushed out. When probing the shaft, position yourself on the windward side, if possible, and never look into the shaft.
To vent the attic or cockloft using this same hole, the returns should be removed with a pike pole.
Skylights, of course, are a luxury that’s not always present, and we often have to provide our own opening using hand or power tools. Again, the vent hole should be placed directly above the fire. I’ve given ventilation classes many times, and I always ask the students how they know where to cut the hole. Rote replies abound— melting snow, steam from rain, bubbling tar—but rarely will someone say, “I look to see where the fire is.”
I’ve been on roofs that we had to shovel off before a hole could be cut. If we had waited for the snow to melt, I’m sure someone would be questioning our tactics. Likewise, I’ve been on roofs in such driving rain that you would need the heat from a nuclear reactor to make steam. Responses of this sort suggest the student thinks the roof team’s decision-making process starts when the team reaches the roof. Nothing could be farther from the truth; roof size-up starts with your feet on terra firma. Look to see if you can locate the fire before you start your ascent. Then you can concentrate on the mechanics of the vent when you reach the roof.
The first consideration will always be roof stability. The three major factors will be time—how long the fire has been burning: severity of the fire; and construction. All members of the team should be sizing up the roof’s stability as they approach the area to be cut, probing with tools and making sure of their footing.
If the roof’s stable, you’re ready to cut, by hand (with axes) or power (with saws).
If you use an axe, strip the roof covering first, by peeling it off or chopping it. This will expose the sheathing, and if you’re lucky and have good visibility, you might see the nail lines. The nail lines will identify the location of the roof rafters beneath the sheathing. Cutting close to the rafter, your axe will encounter less bounce than it would in the middle of the bay, where the sheathing has less support.
Both plywood and composite board sheathing present problems. They’re hard to cut with an axe, and once cut, they’re difficult to pull. Unlike roof boards, the hole cut in these sheets must be pulled all at once.
Easy job? No way—that’s why they invented power saws. A good axe will make your job easier, but don’t keep your axe razor-sharp. The deeper the axe goes into the wood, the harder it is to get out.
Power saws (either circular or chain) have made our job much easier. They cut much faster than anyone could by hand, and they can cut through the roof covering and sheathing together.
However, they don’t eliminate the problem of pulling the cut section. Many times you’ll find the saw operator cutting sections that are just too large to pull. Don’t fall into this trap. Cut a rectangular hole and keep expanding it. Don’t worry about fire coming out of the hole; most times you can duck under it.
In just about every article I’ve ever read about vertical venting, the author says how big the hole should be. But every one is different. I can’t tell you how big the hole should be, because I don’t know how big the fire is. Cut the hole and keep expanding it, one rafter bay at a time, until water comes out of it. That’s when you know it’s big enough. The thing you don’t want to do is run around the roof making a lot of little holes.
After you’ve cut and pulled the hole, you must remember to push down the ceilings. If you don’t, you will have vented only the space between the roof and the ceiling, and not the space below. Vertical venting is almost always a two-step procedure. When you vent a skylight, the returns must be punched out, and when you cut a hole, the ceilings must be pushed down. There should be no hesitation about initiating vertical ventilation. We don’t wait for handlines and we don’t wait for orders, we just do it! It’s never too early to start vertical venting, because—just like the fire in the fireplace—it should have been done before the fire started.
