I'm going to be reroofing an older house with 2x6 rafters and plank 1x sheating. Want to add a layer of 7/16 osb on roof. Longest rafter length is about 14' to the collar tie in a vaulted ceiling (still a 2x6 rafter). 20' wide area. everywhere else we had braced front and back of roof to a wall (truss like) Will this vaulted area carry the small extra load of plywood without sag or any ideas how to stiffen it up keeping in mind the top planes in with the rest of the roof on the front of the house and the bottom side of the 2x6 rafters is drywalled?