Sagging rafters are a sign that a house is in serious trouble. When sagging gets too severe, it can lead to roof collapse, and once the roof goes the house won't last for long. Prevent this unfortunate series of events by bracing your rafters to make them sturdy, and resistant to snow loads and wind. The longer the rafters in your roof, the more important it is to brace them and keep them strong.
Measure the length of one of your rafters and mark the rafter at its midpoint. Tap a small nail into the side of the rafter at that point.
Hang a plumb bob from the nail and mark the point where it touches the joist below the rafter using a pencil.
Measure the distance from the bottom of the rafter to the top of the joist, where the plumb bob is hanging. Measure this same distance horizontally along the joist toward the center of the building and make a mark there.
Measure the diagonal distance between the first mark that you made on the rafter and the second mark you made on the joist. The angle between these two marks is 45 degrees from horizontal, and if your roof is a 45 degree roof, the line between these two points is perpendicular to your rafters.
Cut a piece of 2-by-8 that is slightly longer than your measurement from Step 4. Hold this 2-by-8 up along the same diagonal line so that its ends overlap the rafter and the joist. The 2-by-8 should be perpendicular to the rafter. Make marks on both ends of the 2-by-8 where it meets the edges of the rafter and the joists. Cut the 2-by-8 along these lines with a circular saw.
Place the 2-by-8 back where it was, but this time wedge it between the rafter and the joist so its ends are pressed up against the underside of the rafter and the top edge of the joist.
Screw the 2-by-8 to the rafter and the joist. You can do this by driving screws at an angle through the ends of the 2-by-8, and into the rafter and joist. You also could apply metal joining plates or pieces of wood to the joints between the 2-by-8 and the rafter and joist, and screw the pieces of wood or metal joining plates onto the 2-by-8, rafter and joist to hold them together.
Repeat Steps 1 through 7 for each rafter that you want to brace.