Modifying a common wood truss for extra attic space

[JettaLou]

Can a pre-engineered common truss be turned into an attic truss? (45'-0" span, +/-16'-0" high & 2'-0" O.C.) Diagonal chords are 2x6's and interrupt the clear space 5'-0" either side of the centerline of all 13 trusses. I'd like to design a bed/bath or at least storage space with a pull down ladder parallel to the span of the truss.

 

[Ron]

JettaLou...yes, common trusses can be modified, provided you analyze the truss and understand the changes you are making. If you do not already have drawings, an accurate drawing will be necessary.

You will be significantly increasing the load (at least theoretically) in the center of the truss, while removing members from the same area. Expect bottom chord and rafter (top chords) stresses to go up significantly. This will place more stress on the truss plates, which will also likely need supplementation. You will need to provide additional lateral bracing, particularly for the rafters. It is possible you will need to flitch some members for greater strength. Also, watch deflection. You can get adequate strength in members, but still have intolerable deflections in the truss.

Remember to follow the load path all the way to the foundation to make sure you have enough capacity all the way.