Converting conventional roof truss to Scissors style truss or rafters?
Converting conventional roof truss to Scissors style truss or rafters?
(OP)
I have a residential remodeling client that wants to modify existing conventional 2x4 hem-fir triangular shaped trusses to scissors style trusses to create a vaulted ceiling. They do not want to remove the existing roof deck/roofing to install new trusses due to cost and weather. Is it practical or cost effective to modify these trusses to include new bottom chords and diagonal bracing along with gussets to create the scissors style truss? Or is it more practical and cost effective to design a conventional rafter system with or without a center bearing wall? Here is some existing data and a photo is attached:
Truss horiz. span to bearing walls = 26 feet
Truss vert. height at center = 46 inches
Code live (snow) load is 40PSF above 7000 ft. elevation
Code dead load is 15PSF
Truss members are 2x4 hem-fir, 1968 construction, good condition.
an interior bearing wall (bearing on a slab on grade) is possible below the truss centerline since it is consistent with the proposed floor plan and would be perpendicular to the roof framing.
I've done a search function and found some similar discussion, but the result was to remove most of the roof decking, except near the bearing walls to allow installation of new engineered scissors trusses from the exterior.
Truss horiz. span to bearing walls = 26 feet
Truss vert. height at center = 46 inches
Code live (snow) load is 40PSF above 7000 ft. elevation
Code dead load is 15PSF
Truss members are 2x4 hem-fir, 1968 construction, good condition.
an interior bearing wall (bearing on a slab on grade) is possible below the truss centerline since it is consistent with the proposed floor plan and would be perpendicular to the roof framing.
I've done a search function and found some similar discussion, but the result was to remove most of the roof decking, except near the bearing walls to allow installation of new engineered scissors trusses from the exterior.





RE: Converting conventional roof truss to Scissors style truss or rafters?
Mike McCann
MMC Engineering
RE: Converting conventional roof truss to Scissors style truss or rafters?
Since you will have a center wall - just re-frame it to meet your needs. Make sure you keep the king post (at center-line) intact and secure to new framing or wall.
Done it more than once!!
RE: Converting conventional roof truss to Scissors style truss or rafters?
Is the ceiling removed from wall to wall? If so, you can do a field splice truss as long as you have about a 46 inch wide opening to get each half in the building, but then you would have to redo the existing ducts and wires. You could probably get away with a 36 inch wide opening depending on how the field splice truss is designed.
RE: Converting conventional roof truss to Scissors style truss or rafters?
And you want to watch out for any lateral (side) loads at the top of the exterior walls.
Get somebody who can check this out for you.
RE: Converting conventional roof truss to Scissors style truss or rafters?
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Converting conventional roof truss to Scissors style truss or rafters?
What is a practical way to intall a new ridge beam without removing all of the trusses first? Does it make sense to cut out the vertical truss member temporarily to have room to install the beam under a portion of the top truss chord that could be left in place?
Or how about installing the beam in 1/2 lengths with a mid-span column and footing under a center splice?
Thanks,
Mark
RE: Converting conventional roof truss to Scissors style truss or rafters?
RE: Converting conventional roof truss to Scissors style truss or rafters?