My garage roof is 2x6 T&G decking on 4x6 beams that span 14 ft. They rest on the top plate of 2x4 stud walls. The beams are about 5 ft on center. The flat roof material is torch down rubber. The roof sags about 1" in the middle causing leaks and ponds. I need to jack the roof center up and strengthen the beams to eliminate the sag so that rain will not puddle and leak. We are going to put a membrane roof on to finally eliminate leaks. Any suggestions as how to reinforce the roof beams? I was thinking about sandwiching the beams between 2x10 or 2x12's. Not sure how best to attach? screws, bolts ?