What I have is an older garage 20 x 20 that has a 10 or 12 pitch. The rafters were home built and they used 24 inch centers and they used 20 ft. 1 x 6 to span to the rafters and not to the top plate of the wall. The one by sixes are at least a foot above the top plate. What I want to do is install 2x4 running from wall to wall,top plate to top plate so I can insulate, vapor barrier and sheet the ceiling. How would I go about constructing so that I can safely span 20 feet from top plate to top plate ? Tks. Dakota Dave.

Use 2x4's 12 feet long. They will over lap. At the overlap run a 2x4 to rafter on that side of building at about 45 degree angle. Secure with bolts. Do the same on other side.

At location where 2x4 attach to rafter install a collar tie.

At mid span between wall and over lap, install 2x4 perpendicular to ceiling and tied into rafter. Use bolts to secure.

This structure will give u a truss appearance and reduce deflection of rafters caused by added weight.

Chuck