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Why are roof trusses narrower than floor trusses

Why are roof trusses narrower than floor trusses

Why are roof trusses narrower than floor trusses

(OP)
I always see roof trusses with the members oriented longways vertical and floor trusses with members oriented longways horizontal.  What is the driving reason for this?  To allow more room for mechanical through the shallower floor truss depth??  Something to do with greater floor loads?

RE: Why are roof trusses narrower than floor trusses

(OP)
ie, why are roof trusses 1.5" wide and floor trusses 3.5" or 5.5" wide?

RE: Why are roof trusses narrower than floor trusses

I think it is more a floor depth issue than anything else, if you were to turn the chords on their sides the trusses would be that much deeper without any significant gain in strength.

It is also easier to nail the floors on a wider truss.

RE: Why are roof trusses narrower than floor trusses

The issue really is panel length.  For floor trusses, given the shallow depth (and need for clearance to provide ductwork) the panels tend to be 1'-3" to 2'6".  The panel length is the distance between webs.  The shorter panel length lends itself to flatwise use because it won't fail in bending plus it gives more surface area to nail decking.  Since floor trusses and roof trusses are both analyzed as frames the chords they see not only axial forces, but bending.  In a roof truss configuration, your panels may be the better part of 8'.  If the chords were oriented flatwise rather than edgewise, you would fail in bending.  Also you get more plate grip at the joints with the edgewise cofiguration.

woodengineer

RE: Why are roof trusses narrower than floor trusses

(OP)
Hey, that makes complete sense.  So if you had a shallow flat roof truss, it can be designed either way?  Would that be an EOR decision?  I ask because we've got ductwork in a shallow flat roof ~30" deep.

RE: Why are roof trusses narrower than floor trusses

our plant can only build up to 24" deep floor trusses (4" wide) but can build up to 14' deep roof trusses (2" wide).  Also, chords have to be parallel with 4" wide trusses because of the jigs that are used to build them.  Roof trusses can be any profile that can be laid out on the table.

I just did a job with 26" deep flat roof trusses (2" wide) with web configuration similar to those you see in floor trusses, ie with a chase for ductwork down the middle.  30" deep should be no problem.

RE: Why are roof trusses narrower than floor trusses

floor machines can't build a slope in 4x2 trusses.

flat roof trusses are normally called "flat", but have at least a 1/4;12 slope built into it.


Another thing to add to <B>woodengineer</b>'s post.... floor loads are greater than roof loading (unless alot of snow load is factored in)... and a 4x sitting on a 5.5" wall give more bearing capacity than a 1.5" truss.

RE: Why are roof trusses narrower than floor trusses

I suspect it has to do with the amount of material used... if it were more economical to have the members 'on the flat' for roof trusses, then they would be on the flat.  The flat orientation also uses less truss plate material.

Dik

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