04-25-2000, 08:28 AM #1phyrngnFirehouse.com Guest
Lightweight Trusses and Vertical Ventilation
What are your thoughts on vertically ventilating a lightweight truss roof? I have been told NOT to do it, because you'll draw heat up to the gusset plates and cause collapse. I've also been told that vertical ventilation is essential in many cases, including lightweight truss roofs to get the heat out of the structure and prevent heat buildup underneath a greater surface area of the roof/ceiling. Which is right?
04-25-2000, 02:02 PM #2Dalmatian90Firehouse.com Guest
My opinion: If you have fire in the attic with light weight trusses and can safely vent, do so.
The gussets are gonna fail from a lot less heat than that produced by the fire at the vent hole -- so I'd much rather limit the damage and let most of the heat and fire escape in one spot than let it mushroom and destroy all the gussets.
This may also mean on residential roofs like we normally encounter the trusses, get ahead of the fire and vent it to keep it from extending throughout the attic space.
#1 -- 'nother reason Aerial Ladders are great for residential fires...vent the roofs tied into the safety of the aerial...
#2 -- I reckon the only reason vertical ventilation ever fell out of favor was "Well, we don't have enough manpower to do it so Horizontal is good enough" which is usually true...but not that horizontal's best, just cheapest.
04-26-2000, 10:39 AM #3benson911Firehouse.com Guest
I agree with Matt and add this...
Every scenario is different. If the fire needs to be vented vertically, do it. If the lightweight trusses are involved in the fire, do not put anybody on them or under them.
Venting hot smoke, gases and fire through an attic with truss roof construction shouldn't cause a collapse, unless the fire is so intense the trusses become involved in the fire. If your venting causes the attic structure to become involved in fire, then you have a much larger fire problem than just the trusses.
Remember, the gusset plates aren't failing; they're transferring heat to the wood to which they are attached and causing it to pyrolize, burn and take away the gusset plates anchor. Then, the truss fails. This doesn't take long (possibly 5 minutes.)
If you are venting the attic to release the fire, smoke and gasses from an attic fire, then vent from an aerial ladder and attack the fire from the side (gable end) to avoid being caught in a collapse.
Many other scenarios are possible. Size up every fire like it will be your last and worst fire ever. Do not go blindly into a fire unless you DO want it to be your LAST and WORST. Be careful, but be aggressive.
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