A quick bombardement of information to get you asking questions.....
Critical Thinking and Practical Experience.
“I was once a skeptic but was converted by the two missionaries on either side of my nose.” ~Robert Brault
Brothers from Clark County Fire Dept. (Las Vegas, NV) T17 supporting the Engine on a Single story house fire. Interior crews reported relief following the roof vent. Hoarder like conditions set the stage for a very hot, very difficult "push". Advancement made possible through aggressive topside vent on a pre-engineered trussed roof...
Disclaimer: No Firefighters were injured in the suppression of this fire.
Intelligently aggressive roof operations on pre-engineered systems are a very doable, very viable tactic. Pre-requisites are as follows. You MUST have an operation, understand the construction in your area and know how these roof systems actually behave under fire conditions.
Some critical points: 7/16" and or 1/2" (actually 15/32", as 1/2" is a nonstandard dimension) O.S.B. sheathing over top of pre-engineered, wood truss roofs at 24" O.C. (on-center) spacing are the standard residential, roof decking thickness. On-center spacing, slope or pitch of the roof, as well as dead loads (static forces) supported all determine the decking thickness required to do the job.
Commercial roofs with O.C. truss spacing of 32" on up to 48" will result in OSB sheathing with thicknesses that range from 5/8" (actually 19/32" as 5/8" is nonstandard) up to 3/4" (23/32"), all the way up to 7/8" in rare cases of extreme o.c. spacing. Commercial roof ventilation / operations are a much different animal that require extreme discipline and adherence to our operation and training.
Experience has taught us that the most common residential thickness of 1/2" (15/32") OSB decking will typically fail before the trusses 2"x4" (actually 1.5" x 3.5") or 2"x6" (actually 1.5"x 5.5") top chord once fire has entered the truss space. The decking's membrane or covering thickness and weight can also influence this behavior. By failure we mean it will no longer support working firefighters on top of it; not total burn through. We determine this through strong, disciplined sounding. Use the reach of this tool, however, what the hook tells us must also match what our feet are telling us. Do not abandon your god-given senses in the interest of text book sounding.
Furthermore, in our experience, "Domino effect" is generally a myth in the residential setting. Fire in one portion of the truss space resulting in one, two, or three truss failures will generally not equal TOTAL and COMPLETE roof failure or collapse. We say this with the utmost confidence.
There are several reasons for this:
1) Diaphragm nailing creates a sealed envelope and stiffens the entire roof against "Shear" force.
Shear: are unaligned forces pushing one part of a body in one direction, and another part of the body in the opposite direction.
Example: when wind blows at the side of a peaked roof of a home - the side walls experience a force at their top pushing in the direction of the wind, and their bottom in the opposite direction, from the ground or foundation.
2) lateral bracing through the web and
3) "Supported edges" (ridge or nail blocking) all fight the effects of "Shear" (wind & seismic activity).
This also happens to fight the effects of fire and works in our favor.
In the two following photos you'll notice lateral bracing in the form of a 2" X 4", ran through the "web" and attached to each truss. This in-line member adds continued stiffness and helps resist Shear force. It also helps to tighten up the entire system.
Gusset plates generally do not "pop"off secondary to direct fire impingement (heat). The wood must burn away sufficiently for the plate to detach as witnessed in the attached pictures.
We must also understand the evolution of the "lightweight", pre-engineered truss. We must be familiar with our first due areas construction. Older "corner nailed" (8d nails) gussets versus newer "pressed gusset" plates equal more burn time.
"Travel at RIGHT ANGLES" : The right path of travel (the load bearing walls), strong, disciplined sounding with the appropriate tool and technique along with aggressive diagnostic work with the saw generally keeps us out of trouble. We strive to find the operational yellow line on these roof systems.
We "trade space for time" when truss space involvement exists. If fire has not entered the truss space then we WILL cut over the fire. If no fire has entered the truss space and we know this from strong diagnostic work then there is minimal risk in doing so. This is by definition a contents fire. We know that an opening over the fire will create the fastest relief for interior companies as they advance. The further away from the fire the vent opening is the longer it will take to provide relief or lift to interior companies. Fire involvement and construction type determine it's placement. This is but a snapshot in a much longer conversation. Each of these subtopics will be covered in greater detail at a future date. Our hope was to cause you to question long standing RULES. We hope this short piece will provide you with some momentum towards achieving an educated, balanced truth. Through experience and maintaining an open eye, we've learned that firefighting is never black or white and always circumstantial.
Think critically and challenge long accepted tenets. This will eventually lead to the awareness that knowledge rooted in experience shapes what we value and as a consequence how we know what we know, as well as how we use what we know. Steer clear of extremes and excersice moderation in your views.