Sawing Off the Limb You Are Standing On

Posted by in Roof, The Unusual, Wood | February 02, 2015

Many of us have heard the expression, “You are sawing off the limb you are standing on.”  Often someone will say this during a friendly debate when the other debater is undermining his own argument by discrediting his own evidence.  This figure of speech conjures up the mental image of someone sitting on a limb of a tree and sawing the limb between himself and the trunk.  We have probably all seen some cartoon character like Wiley Coyote or one of the Three Stooges actually doing this.

This thought came to mind when we were reviewing photographs from a fire scene.  One of the photos showed where some one from the fire department saw cut through the roof deck and the top chord of the roof truss (see featured photo).

I understand that the firefighters need quick access to the attic to determine the extent of the fire, that decisions and actions are made in the heat of the moment (no pun intended) to access areas of the building, and even that the fire poses a greater danger to the firefighters than other perceived dangers, but sometimes the cutting is creating a great danger to themselves by cutting too deeply into the structural members.

I would think that, when cutting through a plywood/OSB sheathed and wood framed roof, one simple practice could reduce the danger to our firefighters and anyone cutting through a wood deck: have the blade set on the circular saw so that it can only penetrate through two layers (four thicknesses) of shingles and a 3/4 inch thick roof deck with an additional depth of an inch or so.  Or if a chain saw or reciprocating saw is being used cut to an equivalent depth.

Cutting the top chord of a truss will usually not cause the roof to collapse in the short run due to the load shifting to the adjacent trusses, however, inadvertently cutting out a section in two adjacent top chords could allow the portion of the deck one is standing on to collapse.

Maybe I’m all wet here with regard to the danger during firefighting and this situation has been studied and safe practices have already been developed by some firefighter’s organization.  If I am, please email me at engineering@prugarinc.com.

And please, friends, be careful when cutting through floor and roof decks when uncertain of the structural components.  If you are not in a fire fighting situation, take the time to check out the structure below.  And if you are in an emergency situation with no luxury of time, please proceed cautiously.  The life saved, may be your own.

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