Posted tagged ‘horizontal ventilation’

Rain Roof Construction/Ventilation


Rain Roof Construction

When a three story apartment building with a flat roof begins to leak the owner must fix the leak if he wants to keep his residents. When a building ages to the point that patchwork will no longer suffice, the owner must decide whether to move the third floor tenants to another building or to a lower apartment so that proper repairs can be made to the flat roof. Another decision that can be made is to build a peaked truss roof above the existing flat roof; no tenants will have to be displaced and therefore no money will be lost in rental fees.

These peaked rain roofs pose serious concerns to firefighters, not the least of which are the added void spaces and the increased difficulty in vertical ventilation.

The interior crews should expect to spend some time in a high heat and smoky atmosphere due to the extended time that it will take to ventilate this structure. The Truckies on the roof should be alert to what IS NOT coming out of the ventilation hole (smoke and fire)!

Because of the added void spaces, an extended overhaul should also be expected. Both void spaces (cockloft and attic) should be investigated and cleared of smoke and fire.

From the exterior of the building you can see that the outline of the flat roof is visible. This may be a way to recognize the possibility of a rain roof, however the only way to know absolutely, 100% that there is a rain roof is to pre-fire plan the buildings. Even if you have been told by someone you trust that a certain building has a rain roof, you should get up there and see the construction for yourself. Plan out the types of ventilation and the tactics before the fire, and when the fire happens, surprises are minimized and efficiency increased.

As you can see from the picture above, the load of the rain roof is carried by a truss system. The trusses run from soffit to soffit, in this system the trusses are set approximately four feet apart and are offset. If you look closely the truss system consists of half trusses and alternates from center to the right soffit, the next (four feet away) from center to the left soffit. The trusses are tied together with a lightweight 2x6x8, connected to the trusses with metal joist hangers. Note that these 2×6 members run lengthwise and are spaced about 20 inches on center. The trusses are held off the flat roof by placing them on wood scraps and fastened to the flat roof by metal hurricane straps and screws. The new roof is then sheathed with OSB or plywood, a layer of tar paper, and topped off with asphalt shingles.

When we think about a normal (4×4) ventilation cut, it usually starts with the top cut (to find the trusses/rafters, then the far cut, the bottom cut, lastly the closest cut, the cut is then louvered vertically (peak to soffit). With this construction, if you start with the top cut (looking for the truss/rafter to gauge distance) you will not run into one unless you got lucky enough to plunge close to the truss (remember, they are 8 feet apart). Your next step being the far cut would normally be an easy, unobstructed cut, however with this type of construction you will note the saw bog as it hits the 2×6. If you did not get a chance to pre-fire plan this building, your confusion on the roof will begin to creep in at this time. After completing your cuts, you will notice that the louvers will now run lengthwise.

The cut is completed and your work on the roof is done! Well maybe!

Everything that is done on the fireground is done for a reason. Ventilation is a perfect example of this rule. Generally speaking, ventilation is done to remove heat and smoke to allow the Engine Company to find and extinguish the fire quicker, to give any possible victims a chance of survival, and to reduce smoke and fire damage.

Why do we choose to vertically ventilate as opposed to horizontal/PPV?  We have all been taught that any extensive top floor fire with a possibility of extension into the attic space or cockloft should be vertically ventilated to keep the fire from extending horizontally in the void spaces.

If we have a reason to vertically ventilate, then we should have some expectation of what should happen after we ventilate. In this case we would probably be expecting smoke or fire to immediately ventilate from the hole.

If what we expect does not happen, red flags and alarm bells should be going of in our heads! Something is very wrong and could mean the danger is increasing for everyone on the interior.

I personally have not been placed in this situation (ventilating a rain roof), so I will have to refer to those with the actual experience of operating on this type of roof at a working fire. However, I would immediately begin thinking that horizontal /PPV should be heavily considered.

By the way, before you get to far into the process, communicate to the interior crews and the IC that ventilation will be delayed at best, or that horizontal/PPV is a better tactic to go with.

A few years back there was a fire in an apartment building that had a rain roof. As I understand it Truck 8 was assigned to vertically ventilate. Lt Chapel was on scene and he advised that their main job was to force through a draft stop and extinguish fire in the attic void space. Maybe, he and others that were there will be nice enough to fill us in on the story and provide us with some first hand knowledge.

 Thanks to for the great web site. Be safe!                            -Lt. J. Garcia


Lt. Garcia has a great deal of knowledge in building construction and his trade of fighting fires. He still considers himself a student of the job and pursues being a master of his craft regularly.  Lt. Garcia has been serving for the last 16 years and has spent most of his time in some of the busiest companies on the southside of the city.

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