Retired Los Angeles (CA) Fire Department Battalion Chief John Mittendorf recently gave a Webcast presentation on lightweight roof ventilation (WATCH IT HERE). Below are some questions received during the Webcast and John’s answers to them.
Q. On the picture of the two-story single-family dwelling, would you entertain the idea of vertical ventilation on the fire-involved side, working off an aerial and save doing a defensive cut on the one-story side? What would be your reasons for doing or not doing such ventilations?
A. I am not sure if you are referring to the two-story and then the one-story, or just the one story. I would not advocate doing any roof ventilation on the two-story as it is too involved. In the picture, the two-story is well involved and the fire is beginning to extend into the one-story. So, if there is time and roof strength to cut a vent opening in the one-story and next to the two-story, then I would just cut a defensive opening to stop the extension of fire from extending past that opening. Remember, the common vent opening (4 x 4) will not stop fire from extending around and past that type of opening. Only a strip/trench cut does that, particularly if the opening completely travels from one side of a building to the other side. As in other questions from the Webcast, working from an aerial device is safer than being on a roof, but only if the apparatus can properly access the building/roof.
Q. Can we consider the TIC a reliable tool to locate the fire while conducting roof operations?
A. Generally speaking, yes. However, it all depends on the expertise of the operator. If used properly, it will indicate areas that are warmer/hotter than other areas, and can be used to indicate larger structural members (as they retain more heat than smaller members).
Q. Can I get this video for my station?
A. The video can be found on Google by searching wood truss test burn, or CLICK HERE. The video is 3:57 minutes long.
Q. Has testing been done to determine the time it will take for the roof members to fail in for a fire of a predetermined size to penetrate drywall, a drop ceiling, and a fire-rated drop ceiling?
A. Not that I am aware of.
Q. How come you didn‘t discuss using a roof ladder as a platform to work off of?
A. Although roof ladders can enhance roof operations, I did not spend a lot of time discussing them for several reasons. In no particular order, (1) our time was limited; (2) the slope of the roof in the picture did not require one; (3) if a roof ladder is necessary (other than the pitch of a roof), you probably don’t belong on a particular roof; (4) the implementation of a roof ladder takes a noteworthy amount of time, and that is time that can be spent in finishing the vent operation and getting off the roof; (5) working on a roof instead of a roof ladder is faster and easier (if the slope allows it); and last but not least (6) I have never liked roof ladders unless absolutely necessary because of the time it takes to place them and the difficulty of working off of them. If you know what you are doing and can quickly evaluate whether personnel should be on a roof or not, then roof ladders (in some cases) can add to the time and difficulty of a roof ventilation operation when it is not necessary. However, remember that the pitch of many roofs requires the use of roof ladders, so their use is mandatory on these types of roofs.
Q. I am from a department in the Midwest, and for the last four or five years they have been stressing to ventilate roofs from aerial ladders vs. getting on the roof using ground and roof ladders. It makes perfect sense to me when it is an option. What is your take on this procedure?
A. By all means, working from an aerial device is safer than being on a roof with or without ground and roof ladders. I think a part of this perspective comes from two viewpoints: (1) some officers are more comfortable with being on an aerial device (in all cases), and (2) the growth of lightweight construction makes roof ventilation more dangerous. With that said, you cannot always position your apparatus to get an aerial device to a roof. So if a department believes in vertical ventilation, then it will be necessary to be proficient in roof ventilation without an aerial device if necessary. Training is the key.
Q. Would you ever perform horizontal and vertical ventilation on the same fire?
A. Yes. As an example, horizontal ventilation is easier and can be generally performed in a shorter period of time with less people. So if a basic horizontal ventilation were quickly performed first, and then vertical ventilation performed (and remember, if you want to vertically vent the interior of a structure, it will be necessary to also open the ceiling below your roof opening), the opening in the roof will work better because of the inlet from the horizontal opening and the natural upward flow to the roof opening due to rising heat. In most cases, vertical ventilation works better with horizontal openings (which often come from open doors-windows).
Q. What is your opinion of positive-pressure attack (PPA)?
A. This is a question that could support a lengthy answer all by itself. In short, I am a very strong supporter of positive-pressure ventilation (PPV, of which PPA is a part) and believe that it can be used in any department, but is particularly adept in allowing minimal resources (let’s say a three-person engine company) to simultaneously conduct PPV, interior fire attack, and then a quick primary search. As an example, the officer takes a blower to the front door while doing a 360; the firefighter takes a preconnect to the front door; the engineer gets the water ready; then the officer makes an exhaust opening by the fire; the officer and firefighter position the blower into the open front door and advance the line and knock the fire down. Then they conduct a primary search. Obviously, this does not work on all fires, but it can on many fires. With all of the tests (and practical experience) that have been done (LAFD, North Carolina Bureau of Insurance, U.S. Coast Guard, etc), the categorical statements that PPV enhances a fire and drives fire through a structure are normally false. Lastly, I have used PPV for more than 20 years and believe that it is a great tool when used correctly. Remember that when using PPV, the primary rule that often determines the basic premise of PPV is that an exhaust opening must be created first. Otherwise, either you do not control the flow of air, or it has nowhere to go.
Q. What about the heat collection from the fire below the structural members? Are you saying that direct flame impingement of the roof members is necessary for safety concerns? I have always heard that with lightweight construction, the failure of one member compromises the integrity of all other members because of their reliance on each other (and therefore the entire roof cannot be trusted). Is this not always the case?
A. Two great questions! As we discussed on the Webcast, part of being on a roof is evaluating two critical factors – – is the roof conventional construction or lightweight construction, and where is the fire in relation to you? Let’s assume the roof is lightweight construction. Now, where is the fire in relation to you, and that also includes heat (remember the inspection holes)? As there is a difference in levels of heat-no heat-little heat-moderate heat, these levels in my area of ventilation would not bother me. However, a noteworthy amount of heat would cause me to consider leaving the roof and/or re-evaluating my roof operation. As we discussed, more and more lightweight structural members are using glue, which can degrade and possibly fail in a short period of time, therefore high levels of heat (without direct fire exposure) are becoming more dangerous.
Secondly, the failure of one member of a truss can compromise the integrity of that truss, but does not necessarily mean that it will collapse as that truss is part of the roof sheathing, likely a ceiling, and the amount of dead load imposed on that truss. However, there will come a time when that truss will collapse and is usually a function of a combination of dead and live loads on the truss. Remember that the stability of a bottom chord is more important than the stability of the top chord or webbing. As we discussed in the Webcast, I am not a supporter of the viewpoint that if one or several trusses collapse, then unburned sections of a roof or the entire will collapse. I believe that the evidence of past fires support this and equally applies to lightweight and conventional construction alike. As a quick example, have you ever walked into a building after a fire and seen a portion of collapsed burned roof next to a section of unburned collapsed roof lying on the ground next to each other? I never have.
Q. This was an excellent Webcast. Can I get the article from December 2011 on the Fire Engineering site?
Q. You mentioned a hoseline on the roof. What is your feeling about stretching a hoseline to the roof for vent operations on a regular basis? You said no one on the roof if fire is in a lightweight wooden truss roof due to rapid failure. What about a fire in a commercial building built with a lightweight open web metal truss?
A. (1) Normally hoselines on a roof are not necessary unless a fire in the roof composition happens and starts to extend. Additionally, it takes 2-3 people to put a hoseline on a roof and I think 2-3 people can be better usd elsewhere. So on a regular basis, no. If necessary and you have the people, yes.
(2) In the Webcast, we talked about no one on a lightweight roof for roof operations if fire is in the area of your operation on lightweight construction without being over fire/high heat, then it is a viable operation.
(3) To me, open web metal trusses are nothing more than a metal version of wood lightweight construction as they are both losers. Brannigan says that metal starts to lose its integrity at 800-1,000 F, so metal has its own problems. I think there is little difference between metal and wood lightweight trusses. Remember that cutting ventilation openings in metal deck roofs can be more challenging than cutting ventilation openings in plywood/OSB roofs.
Q. Can you clarify what you mean by the pitch of the wood?
A. Pitch is another word for the “sap” in the wood, which can burn like gasoline. The newer trees have more while the older trees have less.
Q. Is there a key year or time frame when truss construction started to really hit the market?
A. I think a specific answer to this question might be dependent on specific areas of the country. However, on the West coast it was the ’60s. As an example, older truss roofs were built up until the late ’50s, and then panelized roofs took over in the ’60s. I think you will find the 1960s was a watershed for lightweight construction across the country.
Q. Is it feasible to use a Bresnan distributor nozzle through the inspection opening?
A. Yes and no (how is that for a political answer). Yes if you think the Bresnan can quickly put the attic fire out (however, remember it will take you “X” amount of time to get your line and nozzle in place). No if the area around the inspection opening is being subjected to fire as you then have little time left to implement the Bresnan.
Q. Can you discuss the repercussions of accidentally cutting through a pitched truss single member vs. a stick-frame member? Specifically with webbing supporting both the bottom and top chords of a truss assembly. Is it safe to say that there is still a fair amount of strength in the assembly after accidentally doing so?
A. This is an interesting question that is frequently asked in relation to truss construction. As I have cut various trusses with a power saw to see what happens, here is my answer: if you cut through the bottom chord of a truss, that particular truss will surely fail. However, roof operations are not conducted on the bottom portion of trusses. If you cut through the top chord of a truss, nothing will happen because the sheathing (OSB, plywood, etc) around the cut still holds everything together (you have to think about that one for a minute). However, I would not make it a practice to cut through numerous top chords of lightweight trusses. A single cut through a stick frame member is the same as a single cut through a top chord of a lightweight truss.
Q. If you cut a hole in the right side, will you pull the fire over to you? Any suggestions for convincing officers that vertical ventilation is still viable?
A. If a hole is cut on the right side, it will (not might) pull the fire to its location. Although vertical ventilation is a very viable operation, firefighters do not normally convince their officers that it is viable when the officers do not believe so. So, the only practical change comes when you promote to officer, then you can decide how you perform your fireground operations (sad but true).
Q. How do you feel about HVAC units in the attic space? Besides the obvious weight, are there any other problems/issues you have discovered with this?
A. HVAC units in attic spaces can be very dangerous from several perspectives:
- They are hidden from exterior views.
- Their weight is somewhat different than a HVAC unit on a roof as the weight of a roof unit is likely transferred to the exterior walls whereas an attic unit will transfer most of its weight to the ceiling joists which may collapse faster when exposed to fire.
- It is possible that attic HVAC units can be installed as a retro and meet appropriate building codes.
- In summary, I think attic HVAC units are more dangerous that roog HVAC units because they are “out of sight, and out of mind.“
Q. I see how the roof did sag past where the cut was made. When did that sag happen?
A. I wish I could answer that question but I can’t as I did not take the picture.
JOHN MITTENDORF retired from the Los Angeles (CA) Fire Department as a battalion chief after 30 years of service. He is actively engaged in teaching fireground operations in this country and Europe. He is the author of Truck Company Operations, published by PennWell, and received the 2008 Fire Engineering Lifetime Achievement Award.
